process cheese: scientific and technological aspects—a review

Process Cheese:Scientific andTechnological

Aspects—AReview

Rohit Kapoor and Lloyd E. Metzger

ABSTRACT: Process cheese is produced by blending natural cheese in the presence of emulsifying salts and otherdairy and nondairy ingredients followed by heating and continuous mixing to form a homogeneous product with anextended shelf life. Extensive research on the important physicochemical and functional properties associated withprocess cheese and the various physicochemical, technological, and microbiological factors that influence theseproperties has resulted in process cheese being one of the most versatile dairy products with numerous end-useapplications. The present review is an attempt to cover the scientific and technological aspects of process cheese andhighlight and critique some of the important research findings associated with them. The 1st objective of this articleis to extensively describe the physicochemical properties and microstructure, as well as the functional properties, ofprocess cheese and highlight the various analytical techniques used to evaluate these properties. The 2nd objective isto describe the formulation parameters, ingredients, and various processing conditions that influence the functionalproperties of process cheese. This review is primarily targeted at process cheese manufacturers as well as studentsin the field of dairy and food science who may want to learn more about the scientific and technological aspects ofprocess cheese. The review is limited to the relevant research associated with process cheeses as defined by the U.S.Code of Federal Regulations and does not cover imitation and substitute cheeses.

IntroductionProcess cheese is a dairy product which differs from natural

cheese in the fact that process cheese is not made directly frommilk. However, the main ingredient of process cheese is naturalcheese. Process cheese is produced by blending natural cheeseof different ages and degrees of maturity in the presence of emul-sifying salts and other dairy and nondairy ingredients followedby heating and continuous mixing to form a homogeneous prod-uct with an extended shelf life (Meyer 1973; Thomas 1973; Caricand others 1985; Guinee and others 2004). The origin of pro-cess cheese dates back to the early 20th century (Meyer 1973).Contrary to the present status of process cheese, the initial idea ofprocess cheese was to increase the shelf life of natural cheese andfind alternative uses for natural cheese that was difficult to sell.Process cheese was invented in 1911, in Switzerland, by WalterGerber and Fritz Stettler of Gerber and Co. who melted Swisscheese using sodium citrate as the emulsifying salt to produce asmooth, homogeneous product. A few years later, in the UnitedStates, the development of process cheese was brought about byJ. L. Kraft in 1916, when he preserved natural cheese in cansby heating and mixing it in order to increase its shelf life. The

MS 20070698 Submitted 9/11/2007, Accepted 11/28/2007 . Authors are withMidwest Dairy Foods Research Center, Dept. of Food Science and Nutrition,Univ. of Minnesota, St. Paul, MN 55108, U.S.A. Direct inquiries to author Met-zger (E-mail: [email protected]).

development of process cheese with the use of phosphate-basedemulsifying salts in the United States can be attributed to J. L. Kraftand the workers from Phenix Cheese Co. who were awarded nu-merous patents for their work on process cheese between 1916and 1938, as reported by Zehren and Nusbaum (2000), who haveextensively reviewed the history of the development of processcheese in the United States.

Legal definitionIn the United States, process cheese is a generic term used to

describe various categories of cheese as defined by the Code ofFederal Regulations (CFR). According to the CFR, these categoriesdiffer on the basis of the requirements for minimum fat content,maximum moisture content, and minimum final pH, as well asthe quantity and the number of optional ingredients that can beused (21CFR133.169 to 133.180) (FDA 2006). The 3 major cat-egories of process cheese, as described by the CFR, are pasteur-ized process cheese (PC), pasteurized process cheese food (PCF),and pasteurized process cheese spread (PCS). Table 1 summa-rizes the allowed ingredients and compositional specifications ofPC, PCF, and PCS in the United States. In addition to the cate-gories described by the CFR, there is another undefined categorycalled pasteurized process cheese products. This category of pro-cess cheese has a composition similar to the various categories ofprocess cheese; however, ingredients such as milk protein con-centrate that are not allowed in PC, PCF, or PCS are utilized inthe formulation.

194 COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY—Vol. 7, 2008 C© 2008 Institute of Food Technologists

Process cheese—a review . . .

Table 1 --- CFRa definition of the 3 major categories of process cheese in the United States.

Major ingredients and other Moisture FatCategory optional ingredients (and their permitted levels) (% w/w) (% w/w) pH

PCb �Cheese ≤40 ≥30 ≥5.3�Emulsifying agents (≤ 3% (w/w) of the final product)�Acidifying agent�Cream, anhydrous milk fat, dehydrated cream (weight of the fat derived is ≤ 5%(w/w) of the final product)�Water, salt, colors, spices or flavorings, enzyme-modified cheese, mold inhibitors(≤ 0.2% (w/w) or ≤ 0.3% (w/w) of the final product), antisticking agent (≤ 0.03%(w/w) of the final product)

PCFc �Cheese (≥ 51% (w/w) of the final product) ≤44 ≥23 ≥5.0�Other optional ingredients and their permitted levels include all of the ingredientsallowed in PC in addition to milk, skim milk, buttermilk, and cheese whey

PCSd �Cheese (≥ 51% (w/w) of the final product) 44 to 60 ≥20 ≥4.0�Other optional ingredients and their permitted levels include all of the ingredientsallowed in PCF in addition to food gums, sweetening agents, and nisin (≤ 250 ppmof the final product)

aCFR = Code of Federal Regulations (FDA 2006).bPC = pasteurized processed cheese (21CFR133.169).cPCF = pasteurized processed cheese food (21CFR133.173).dPCS = pasteurized processed cheese spread (21CFR133.179).

Production and market trendsThe production of process cheese has remained relatively flat

since 1990, and in 2005, total process cheese (PC, PCF, andPCS) production in the United States was approximately 1014million kg (IDFA 2006). As a relative comparison, total naturalcheese production was approximately 4149 million kg in 2005.Process cheese (243 million kg) was the leader in total super-market cheese sales followed by cheddar (240 million kg) andmozzarella (120 million kg) (IDFA 2006). Supermarket sales ofprocess cheese are primarily in the form of processed slices, andthis form of process cheese accounts for 74% of the total super-market sales. It is also interesting to note that the volume of lowfat/light process cheese increased substantially (22.3%) between2004 and 2005 (IDFA 2006). Although this is currently a smallcategory, low fat/light process cheese appears to have the poten-tial for substantial growth.

Process cheese end-use applicationsand functional properties

The popularity of process cheese can be attributed to its numer-ous end-use applications. According to Sørensen (2001), processcheese is one of the leading cheese varieties in the world thatis used as an ingredient in various food preparations (processedfoods and food service). In the United States, process cheese isproduced and sold in various forms such as loaves, slices, shreds,and spreads and is used as an ingredient in numerous products(Figure 1). The versatility of process cheese can be attributed toits unique functional properties. According to Guinee (2002),“the functional properties of a cheese (when used in a partic-ular food) refer to the performance of the cheese during all stagesof preparation and consumption of the food that would even-tually contribute to the taste as well as the aesthetic appeal ofthat prepared-food.” Guinee (2002), in his review, extensivelydescribes the various functional properties of natural cheese andprocess cheese and their potential applications. Depending onits end-use application, the desired functional properties of pro-cess cheese can be grouped into 2 major categories: unmeltedtexture and melted texture properties. Table 2 and 3 summarizethe important unmelted textural and melted textural properties ofprocess cheese, respectively. In addition to the individual func-tional properties, certain process cheese applications also require

Loaf20.0%

Slice74.0%

Spread4.5%

Shredded/Grated1.1%

Other0.1%

Cubed0.3%

Figure 1 --- Process cheese supermarket sales in theU.S.A. in 2005 based on form (Source: IDFA 2006).

an optimum interaction between both the melted and the un-melted textural properties. For example, the desired functionalproperties of process cheese used to make breaded cheese stickswould not only include high firmness and cohesiveness so thatthe process cheese can be easily cut or shredded when cold, butit would also need to have a normal melt and stretch (so that itsoftens during heating and stretches when consumed) and a high“melted” viscosity (so that the cheese does not ooze out of thebreaded casing during cooking or consumption). Similarly, theappropriate process cheese slice for a toasted sandwich shouldnot only have firmness, cohesiveness, and limited adhesivenessso that it has appropriate machineability during manufacture, butit should also have normal melt during toasting. Consequently, therequired functional properties are unique for each process cheeseproduct form and application. Various researchers have devisednumerous empirical and instrumental techniques to evaluate andquantify the functional properties of process cheese and they arediscussed in the section “Process Cheese Functional Properties.”

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CRFSFS: Comprehensive Reviews in Food Science and Food Safety

Process cheese manufactureFigure 2 indicates a schematic flow chart of process cheese

manufacture. Meyer (1973) and Zehren and Nusbaum (2000)have described in extensive detail the various steps of processcheese manufacture and the different equipment used in eachstep. The major steps in process cheese manufacture can be di-vided into 2 stages:1 Ingredient selection and formulation:

� Selection and grinding of natural cheese (on the basis of age,pH, flavor, and intact casein content)

� Selection of appropriate emulsifying salt� Formulation and computation of other ingredients (in order

to meet the targeted moisture, fat, salt, and pH values of the finalproduct as per government regulations)2 Process cheese processing and storage:

� Cooking (heat and mixing)� Packaging, cooling, and storageIngredient selection and formulation. The 1st stage of process

cheese manufacture involves selection of ingredients and prepa-ration of a formulation. As described in Table 1, in addition tonatural cheese and emulsifying salts, there are various other dairyand nondairy (colors, flavors, spices, food gums, mold inhibitors,and so on) ingredients that are used in process cheese manufac-ture. Different ingredients affect the physicochemical properties,flavor, and the functional properties of process cheese in differ-

Table 2 --- Major unmelted textural properties associated with process cheese along with their definitions, their im-portance in process cheese manufacture, and end-use ability and techniques commonly used to measure them.

Descriptor terms Definitiona Importance Measurement techniques

Firmness Ability of the process cheese (atambient or low temperatures) toshow resistance to deformationwhen subjected to an externalforce.

1. Machineability during“slice-on-slice (SOS)” manufacture(chill belt)

2. Ability to maintain identity whenshredded for preparing shreddedcheese for retail and other foodpreparations

3. Slice identity for cold sandwichfood preparations

1. “Thumb print” test2. “Sliceability” test3. Uniaxial compression test (texture

profile analysis,b Instron): force atmaximum compression

4. Penetrometry: force at maximumpenetration

5. Low temperature dynamic stressrheometryc: elastic modulus

Brittleness/fractureability Tendency of the process cheese tofracture into pieces whensubjected to an external force.

1. Ability to maintain identity whengrated for cheese preparations forsprinkling on foods

2. Optimum “crumbliness” for saladpreparations

1. Uniaxial compression test (textureprofile analysis,b Instron): force atthe 1st peak (point of fracture)during compression

Springiness/resilience Tendency of the process cheese torecover to its original dimensionsupon removal of the applied force.

1. Machineability during SOSmanufacture (chill belt)

2. Ability to maintain identity whenshredded for preparing shreddedcheese for retail and other foodpreparations

1. Uniaxial compression test (textureprofile analysis,b Instron)

2. “Roll” test: a representative sampleof a slice immediately after itcomes off the chill belt/castingrollers is rolled both perpendicularand parallel to the direction of themovement of the chill belt and thepoint at which it fractures is noted.

Adhesiveness/stickiness Tendency of the process cheese toresist separation from a material itcontacts.

1. Machineability during SOSmanufacture (chill belt)

2. Slice separation from other slicesin “slice-on-slice” type product andfrom the wrapper in “individuallywrapped slices (IWS)” type product

3. Stickability to foods in cold cheesedips and cheese spreads

1. Uniaxial compression test (textureprofile analysisb)

2. “Slice separation” test: individualslices are separated (from adjacentslices in the case of a SOS productand from the wrapper in the caseof an IWS product) by hand andtheir stickiness is evaluated.

aAdapted from Guinee (2002). The definitions correspond to the properties of process cheese at ambient and/or cold temperatures.bBreene (1975).cMarchesseau and others (1997), Drake and others (1999), and Piska and Stetina (2003).

ent ways. Moreover, the appropriate selection of natural cheeseand emulsifying salt is also very important in order to produceprocess cheese with desired final properties. Guinee and others(2004) have summarized the main function of certain optional in-gredients on the final properties of process cheese. The effect offormulation parameters and ingredients on process cheese prop-erties is discussed in detail in subsection “Formulation parametersand ingredients.”

Process cheese processing and storage. Following the prepa-ration of a desired formulation, the ingredient blend is processedusing heat and mixing to produce a homogeneous mass, whichis packaged and cooled. Although, the minimum cook temper-ature and time specified by CFR for process cheese is 65.5 ◦Cfor 30 s (FDA 2006), process cheese manufacturers use vari-ous types of cookers with different designs and operating con-ditions to manufacture process cheese. These cookers differ onthe basis of the mode of process cheese production (batch orcontinuous production), the type of mixing and agitating sys-tems involved, and the type and mechanism of heating (in-direct heating or direct steam injection) (Meyer 1973; Bergerand others 1998; Zehren and Nusbaum 2000). Two commontypes of batch cookers use single/twin-screw augers (BlentechCooker, Blentech Corp., Rohnert Park, Calif., U.S.A.) or high-speed cutting blades (Stephan Cooker, Sympak Inc., Mundelein,Ill., U.S.A.). The single/twin-screw auger cookers operate at low

196 COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY—Vol. 7, 2008


mixing speeds ranging from 50 to 150 rpm with product temper-atures ranging from 70 to 90 ◦C with manufacturing times of 3 to7 min. The high-speed cutting blade-type cookers operate at 1500to 3000 rpm at 95 to > 100 ◦C for 2 to 5 min. Recently, an addi-tional cooker called the Rota Therm� continuous cooker (GoldPeg Intl. Pty Ltd., Victoria, Australia) has been developed andis being used extensively for process cheese manufacture. Thiscooker operates at a high mixing speed (600 to 1000 rpm) withtemperatures above 90 ◦C and a residence time of approximately30 to 40 s. Recently, another popular process to manufacture pro-

Table 3 --- Major melted textural properties associated with process cheese along with their definitions, their impor-tance in process cheese manufacture, and end-use ability and techniques commonly used to measure them.

Descriptor terms Definitiona Importance Measurement techniquesb

MeltabilityMelt Tendency of the process

cheese to soften uponheating.

1. Toasted sandwiches, burgers, andso on. Ability to maintain a uniformsoftening with minimal oiling-offwhen used on toasted sandwichesand other heated food preparations

2. Shredded cheese on pizza, inbreaded cheese sticks, cheeseinsets in bratwurst, and burgerpatties

1. Arnott test2. Schreiber melt test3. Dynamic stress rheometry (DSR)4. Melt profile analysis (UW Meltmeter)5. Rapid visco analyzer (RVA)

Viscosity/flow Tendency of the processcheese to spread andflow when completelymelted.

1. Cooker “drop-down” viscosity aftermanufacture

2. Pumpability during manufacture3. Optimum hot-fill ability into loaves

during packaging4. Restricted flow during toasting

(food preparation)

1 Arnott test2. Schreiber melt test3. Tube melt test4. Dynamic stress rheometry (DSR)5. Melt profile analysis (UW Meltmeter)6. Rapid visco analyzer (RVA)

Stretching ability Tendency of the heatedprocess cheese to formstrings when extended.

1. Shredded cheese on pizza, inbreaded cheese sticks

1. Pizza “fork” test: The process cheeseto be tested is shredded and baked ona pizza. The melted cheese after thepizza bake is extended/pulled into astring vertically off the pizza using afork and the length of stretch beforethe string breaks is evaluated.

aAdapted from Guinee (2002).bFor the details of the various tests that measure the melted textural properties of process cheese, please refer to subsection “Techniques for measuring process cheese melted texturalproperties” and Table 5.

Addition of other ingredients - Dairy protein ingredients

- Dairy fat ingredients

- Preservatives

- Coloring agents

- Flavoring agents

- Water

- Salt

-Acidulants

Selection of natural cheese

Grinding

Blending

Processing

Packaging

Cooling

Storage

Addition of emulsifying salts

Figure 2 --- Schematic flow chart ofthe basic steps involved in processcheese manufacture.

cess cheese involves sterilizing the premixes to 130 to 145 ◦C for2 to 3 s (Berger and others 1998). The primary method of heatingutilized in most of the cookers is direct steam injection. Researchhas indicated that processing conditions such as cook time, tem-perature of cooking, extent of agitation (mixing) during cook-ing, and the rate at which the cooked process cheese is cooledhave a significant effect on the functional properties of processcheese. The effect of processing conditions on process cheeseproperties is discussed in detail in the subsection “Processingconditions.”



Due to an array of options in ingredients and formulations, andprocessing conditions, manufacturers have numerous possibili-ties for producing process cheese with different physicochemicalproperties which leads to a variety of flavor, functional properties,and end-use applications as desired by consumers. Therefore, ap-propriate selection of ingredient and processing conditions duringprocess cheese manufacture is very important to produce processcheese with targeted functional properties.

An additional, critical, component of process cheese manufac-ture is the utilization of rapid, at-line techniques for determiningthe fat and moisture content of process cheese. Fat and mois-ture testing are performed to ensure that the composition criteriaidentified in the CFR are met. The most common techniques uti-lized include near infrared spectroscopy and microwave-basedmethodology.

Process Cheese PhysicochemicalProperties and Microstructure

Casein and natural cheeseCaseins are the major group of proteins present in milk and

constitute approximately 2.3% to 3.0% of bovine milk (Eigel andothers 1984; Swaisgood 1992). The 4 major casein moleculesare: α s1-casein, α s2-casein, β-casein, and κ-casein and they arepresent in milk in a ratio of 4:1:4:1, respectively (Walstra 1990;Swaisgood 1992; Wong and others 1996). Caseins, like mostproteins, have hydrophobic sections and hydrophilic sections.Caseins are unique in that they contain covalently attached phos-phate groups and have a flexible hydrated secondary structure(Swaisgood 1992; Wong and others 1996; Farrell and others2002). In their native form, caseins exist in the form of caseinmicelles. The micellar structure of caseins has been extensivelyreviewed (Farrell 1973; Payens and Veerman 1982; Schmidt1982; McMahon and Brown 1984; Ruetimann and Ladish 1987;Walstra 1990; Holt 1992; Holt and Horne 1996; Horne 1998).Although the debate on the micellar configuration of caseinsin milk is still in progress, most of the widely accepted mod-els highlight the same fundamental configuration (Walstra 1990;Holt 1992). A casein micelle is 15 to 20 nm in diameter and iscomposed of around 10000 polypeptide chains with α s1-, α s2-,and β-casein present within the micelle. They are stabilizedby protein–protein hydrophobic interactions and colloidal cal-cium phosphate-mediated cross-links. The κ-casein is mainlypresent on the surface of the micelle with its hydrophobic re-gion embedded in the micelle and the glycosylated hydrophilictail protruding outside (Holt 1992; Horne 1998). The glycosy-lated tail of κ-casein is negatively charged, thereby causing themicelles to repel each other. This phenomenon provides stabilityto the casein micelle, consequently protecting α s (α s1 and α s2)and β-casein components from being exposed to the environ-ment. During the manufacture of natural cheese, rennet is usedto cleave κ-casein at the Phe105 and Met106 position, therebydislodging the glycosylated hydrophilic region (glycomacro pep-tide). Due to this phenomenon, the casein micelles lose theirstability and α s- and β-caseins are exposed to the environment.The phosphoserine residues present on α s-and β-caseins take partin calcium-mediated cross-links, thereby forming a rigid, water-insoluble, cross-linked calcium–paracaseinate phosphate com-plex commonly known as curd (Holt 1992). The fat phase is sus-pended in this calcium–paracaseinate phosphate complex. Ac-cording to Shimp (1985), fat in natural cheese is underemulsifiedand the fat phase, as well as the water phase, is supported bya network of water-insoluble calcium–paracaseinate phosphatecomplex (Figure 3A).

Process cheeseIn contrast to natural cheese, process cheese can be described

as a stable oil-in-water emulsion (Palmer and Sly 1943; Shimp1985; Zehren and Nusbaum 2000). The use of emulsifying saltssuch as disodium phosphate and trisodium citrate in processcheese manufacture aids in improving the emulsification proper-ties of caseins by displacing the calcium phosphate complexes inthe insoluble calcium–paracaseinate phosphate network presentin natural cheese (Ellinger 1972; Gupta and others 1984; Caricand others 1985). This displacement of the calcium phosphatecomplex disrupts the major molecular force that cross-links thevarious monomers of casein in the network. This disruption ofthe calcium phosphate complex in conjunction with heating andmixing leads to hydration and partial dispersion of the calcium–paracaseinate phosphate network. In addition to being hydrated,the partially dispersed calcium–paracaseinate complex interactswith fat via hydrophobic interactions. After manufacture and dur-ing the cooling stage, the partially dispersed caseinate matrixforms “flocs” and the flocs subsequently interact to form a uni-form, closely knit gel network (Zhong and Daubert 2004). Thisphenomenon gives rise to fat emulsified by a uniform closely knitprotein gel network (Heertje and others 1981; Marchesseau andCuq 1995; Ennis and others 1998; Lee and others 2003; Zhongand others 2004). Therefore, process cheese structure essentiallyconsists of a fat phase evenly dispersed (in the form of fat glob-ules, approximately < 1 to about 5 µm in diameter) in a partiallydispersed casein gel network (Figure 3B).

Various techniques have been developed to study andevaluate the physicochemical properties and microstructureof process cheese. Some of these techniques are discussedsubsequently.

Techniques for measuring process cheesephysicochemical properties and microstructure

Microscopic techniques. As a popular saying goes, “Seeing isbelieving”; consequently, over the years, various microscopictechniques have been successfully utilized to study the struc-ture of numerous food and dairy products (Heertje 1993; Kalab1993; Kalab and others 1995). As for the microstructure ofprocess cheese, Caric and others (1985) have extensively re-viewed the microstructural changes that occur when naturalcheese is converted into process cheese. Microstructural stud-ies have described the changes in fat globule size and distribu-tion (Rayan and others 1980) as well as rearrangement of theparacaseinate network (Heertje and others 1981; Heertje 1993;Lee and others 2003) in natural cheese during process cheesemanufacture. Moreover, microscopic techniques have been uti-lized to study the size and distribution of fat globules, the typeof crystals present (a defect in process cheese which will be dis-cussed in a later section), and the characteristics of the protein-based structural network of process cheese. Researchers haveused both low-resolution microscopic techniques such as lightmicroscopy, confocal microscopy, confocal laser-scanning mi-croscopy, and fluorescence microscopy (Modler and others 1989;Sutheerawattananonda and others 1997; Bowland and Foeged-ing 2001; Awad and others 2002; Lee and others 2003) andhigh-resolution techniques such as transmission and scanningelectron microscopy (Rayan and others 1980; Taneya and oth-ers 1980; Heertje and others 1981; Lee and others 1981, 2003;Caric and others 1985; Kalab and others 1987; Modler and oth-ers 1989; Savello and Ernstrom 1989; Tamime and others 1990;Awad and others 2002) to study the effects of various factorssuch as pH, ingredients, emulsifying salts, and processing condi-tions on fat globule distribution and the microstructure of processcheese.



Spectroscopic techniques. Fluorescence spectroscopy has beensuccessfully utilized to evaluate molecular-level interactionsbetween fat and proteins in various food-based emulsions, aswell as monitor structural changes in cheese and Maillard brown-ing in milk and dairy products (Genot and others 1992; Dufourand Riaublanc 1997; Herbert and others 1999; Dufour and oth-ers 2000; Mazerolles and others 2001). Caseins in cheese con-tain the amino acid tryptophan, which is a naturally occurringfluorescent substance. The fluorescent properties of tryptophanin a hydrophobic environment are different from its fluorescentproperties when it is in a hydrophilic environment (Lakowicz1983). Consequently, researchers have utilized fluorescencespectroscopy to measure the spectra of tryptophan and predict themicrostructure of natural cheese as well as process cheese (Karouiand others 2003; Garimella Purna and others 2005). On another

(a)

Casein Calcium phosphate microgranules (crosslinks) Fat Solubilized casein molecules molecules Flocs

(b)

Figure 3 --- Schematic microstructureof (A) natural cheese and (B)process cheese.

note, fluorescence spectroscopy has also been used to evaluateMaillard browning and oxidative stability of process cheese dur-ing storage (Christensen and others 2003).

Analytical techniques involving wet chemistry. Recently, wetchemistry-based techniques have been developed and utilizedto evaluate the physicochemical properties of process cheese(Lee and others 1979; Dimitreli and others 2005). These tech-niques have been used to study the protein-based interactions thatcontribute to the microstructure of process cheese (Marchesseauand Cuq 1995; Marchesseau and others 1997), to study the ef-fect of different emulsifying salts on the mechanism of calciumchelation, and to evaluate the state of calcium in process cheese(Mizuno and Lucey 2005; Shirashoji and others 2006b). The ma-jor protein-based interactions that control the process cheese mi-crostructure are hydrophobic interactions (between the various



caseins as well as between caseins and fat), hydrogen bonds, andcalcium-mediated electrostatic bonds among the caseins. Thebasic principle behind these techniques is utilization of variouschemical dissociating agents to selectively disrupt a particularkind of protein-based interaction in process cheese. This resultsin solubilization of the protein that was involved in the inter-action. The amount of soluble protein is quantified with andwithout the addition of the dissociating agent and is used asan index of the relative importance of each protein-based in-teraction. The various chemical dissociating agents destabilizethe protein-based interactions in different ways. Hydrophobic in-teractions are destabilized by sodium dodecyl sulfate. Hydro-gen bonds are disrupted by urea, whereas ethylenediaminete-traacetate (EDTA) aids in breaking calcium–mediated electrostaticbonds (Marchesseau and Cuq 1995; Lefebvre-Cases and others1998; Keim and Hinrichs 2004).

Recently, an acid-base titration technique has been used toevaluate the calcium chelation ability of different emulsifyingsalts as well as the final state of calcium in process cheeses man-ufactured using different emulsifying salts (Mizuno and Lucey2005; Shirashoji and others 2006b). The basic principle involvedin this technique is that dilute solutions of process cheese aretitrated using an acid followed by a base (at a constant rate)and the buffering capacity of the process cheese solution is mea-sured at different pH values (Hassan and others 2004). Differentcalcium-based complexes and salts present in process cheese af-fect the buffering capacity of the solution in a variety of waysand the data collected are used to identify the state of calciumin the sample (Mizuno and Lucey 2005; Shirashoji and others2006b).

Process Cheese Functional PropertiesThe definitions, the 2 major types, and the importance of the

various functional properties of process cheese have been cov-ered previously in this article (subsection “Process cheese end-useapplications and functional properties”). From a material sciencestandpoint, process cheese can be described as a viscoelasticmaterial since it is neither truly elastic (like an ideal solid) nortruly viscous (like an ideal liquid) (Gunasekaran and Ak 2003).Consequently, according to a rheologist, the functional propertiesof a process cheese are defined as properties that control its de-formation and flow behavior when subjected to external forces.Gunasekaran and Ak (2003), in their book, have provided an ex-tensive description of various rheological and textural propertiesof different cheeses as well as different instrumental techniquesutilized to measure these properties. There have been numerousempirical and instrumental techniques utilized to evaluate thefunctional properties of cheese (Gunasekaran and Ak 2003). Ouraim here is only to highlight and compare the various empiricaland instrumental techniques that have been utilized over theyears to evaluate the functional properties of process cheese inorder to make it easier for readers to interpret the results whenthey come across these techniques in process cheese researchreports.

Once again, depending on its end-use application, processcheese functional properties can be grouped into 2 majorcategories: unmelted textural properties and melted texturalproperties.

Techniques for measuring processcheese unmelted textural properties

Various empirical techniques using customized instruments(Templeton and Sommer 1930; Thomas and others 1970a; Gupta

and others 1984), as well as standard instrumental techniquessuch as textural profile analysis (Gupta and others 1984; Drakeand others 1999; Kapoor and Metzger 2004, 2005) and low-temperature dynamic rheological analysis (Drake and others1999) have been utilized to evaluate process cheese hardness,fracturability, cohesiveness, adhesiveness, gumminess, chewi-ness, slicing ability, and elastic and viscous properties at lowtemperatures. Moreover, different process cheese manufacturersthroughout the United States employ various customized tech-niques to measure the unmelted texture of process cheese suchas firmness, slicing ability, and stickiness/adhesiveness dependingon the resources available at their facilities.

One of the earliest studies on the effect of various parame-ters on the firmness of process cheese was performed by Tem-pleton and Sommer (1930). They compressed a standard sam-ple of process cheese to a specified height and measured theforce exerted by the process cheese in grams, which was in-dicated as the firmness of process cheese. Researchers havealso used different penetrometry techniques to measure the firm-ness of process cheese (Thomas and others 1970a; Kalab andothers 1987; Tamime and others 1990). Thomas and others(1970a) measured the firmness of cheese using a modified balland a cone penetrometer and indicated the firmness of cheeseas the distance traveled by the ball or the cone. In the samestudy, Thomas and others (1970a) also developed instruments tomeasure slicing ability, fracturability, and stickiness of processcheese.

One of the more popular techniques to measure the unmeltedtexture of process cheese is texture profile analysis (TPA). TPAworks by sending a crosshead down a vertical column, causinga “flat” plate to deform a specimen that has been placed on alower plate. The constant crosshead speed leads to both force–time and force–distance curves and the work done to deform thecheese can be calculated (Breene 1975). Various types of equip-ment have been used to perform TPA on process cheese, such asthe Instron Universal Testing machine (Harvey and others 1982;Gupta and others 1984), General Foods Texturometer, and theTA.XT2i texture analyzer (Texture Technologies Corp., Scarsdale,N.Y., U.S.A.) (Drake and others 1999; Awad and others 2002;Kapoor and Metzger 2004, 2005; Prow and Metzger 2005). TPAenables the measurement of a variety of unmelted textural prop-erties of process cheese such as hardness, fracturability, adhesive-ness, springiness, cohesiveness, and gumminess. Breene (1975),Peleg (1976), and Gunasekaran and Ak (2003) have provideddetailed descriptions of TPA and how these parameters are mea-sured. Recently, a low-temperature dynamic rheological analysistechnique called dynamic stress rheometry (DSR) has also beenused to measure and evaluate the unmelted viscoelastic prop-erties of process cheese (Marchesseau and others 1997; Drakeand others 1999; Piska and Stetina 2003). DSR measures the vis-coelastic properties of process cheese. This device determines thestorage modulus (G ′), which is a measure of the elastic propertiesof process cheese, the loss modulus (G ′′), which is a measure ofthe viscous properties of process cheese, and the tan δ (which isG ′′/G ′) (Gunasekaran and Ak 2003). Drake and others (1999) intheir study found a good correlation between G ′, G ′′, and TPAhardness.

Techniques for measuring processcheese melted textural properties

According to Gunasekaran and Ak (2003), melted texturalproperties or meltability of cheese refers to the “ease and extent towhich the cheese will melt and spread/flow upon heating.” Vari-ous empirical and instrumental techniques have been developed



to measure and evaluate the melted texture of process cheese.The earliest method to measure process cheese melted texturewas the Arnott Test (Arnott and others 1957). In this method,process cheese cylinders of specific dimensions are heated inan oven at a specific temperature for a specific time and thepercent decrease in the height of the cylinder was reported asthe melt of process cheese (Arnott and others 1957; Park andRosenau 1984). Olson and Price (1958) developed the Tube MeltTest mainly to measure the melted texture and flow propertiesof process cheese spreads. In this test, a process cheese sampleof specific weight and dimensions is placed in a glass tube thatis heated horizontally in an oven at a specific temperature for aspecific time and the extent of flow is measured to quantify themelted texture of the process cheese. The most popular empiricalmelt test for process cheese is the L.D. Schreiber Melt Test thatwas developed by Schreiber Foods of Green Bay, Wis., U.S.A.(previously known as L.D. Schreiber Co.) (Kosikowski and Mistry1997). Over the years, researchers have modified the SchreiberMelt Test to overcome some of its shortcomings (Bogenrief andOlson 1995; Muthukumarappan and others 1999a). However,the basic principle of the test has not changed. In this test, pro-cess cheese discs of specific dimensions are heated in an oven ata specific temperature for a specific time and the final diameteror area of process cheese after melting is reported as the melt ofthe process cheese (Harvey and others 1982; Park and Rosenau1984; Muthukumarappan and others 1999a). There have beenvarious modifications to the above-mentioned melt tests in termsof the sample dimensions and testing conditions (Park and Rose-nau 1984; Gunasekaran and Ak 2003). Instrumental, rheological-based techniques to measure the melted texture of process cheeseinclude DSR and squeeze flow rheometry. As mentioned previ-ously, DSR is a rapid test to measure the viscoelastic properties ofprocess cheese (Drake and others 1999). Sutheerawattananondaand Bastian (1998) developed a DSR-based method for processcheese that heats the sample and utilizes DSR to measure the G ′,G ′′, and the melting temperature (temperature at which G ′ = G ′′or tan δ = 1) of process cheese. Recently, our laboratory usedDSR and the Tube Melt Test to evaluate the melted texture of pro-cess cheese spreads. We found a good correlation between G ′′at 85 ◦C from DSR and extent of flow as measured from the TubeMelt Test (Prow and Metzger 2005).

Squeeze flow rheometry has also been utilized to measure thevarious melt properties of process cheese (Campanella and others1987). One of the examples of the squeeze flow rheological tech-nique is the UW Meltmeter and Melt Profile Analysis developed atthe Univ. of Wisconsin, Madison, Wis., U.S.A. (Wang and others1998; Muthukumarappan and others 1999b; Gunasekaran andAk 2003). Melt profile analysis measures the softening point (soft-ening time and softening temperature) that defines the ease withwhich the cheese melts (Muthukumarappan and others 1999b)and the melting point (melting time and melting temperature)that indicates both the ease of melting and extent of flow of acheese (Gunasekaran and Ak 2003).

Recently, another instrument was optimized to measure themelted textural properties of process cheese in our laboratory.This instrument is known as a rapid visco analyzer (RVA). TheRVA is a computer-integrated instrument developed by NewportScientific (Warriewood, Australia) to determine the viscous prop-erties of cooked starch, grain, batters, and other foods. The RVAcan measure apparent viscosity over variable conditions of shearand temperature as defined by the operator. Prow (2004) andProw and Metzger (2005) used the RVA to evaluate the meltedtextural properties of process cheese and process cheese spreads.Prow (2004) developed a methodology to continuously measurethe apparent viscosity of process cheese during a heating, hold-

ing, and cooling profile using the RVA and measured the min-imum apparent viscosity (hot apparent viscosity) of the processcheese at the highest temperature as well as the time requiredfor the process cheese to reach an apparent viscosity of 5000 cP(time at 5000 cP) during the cooling stage. According to Prow(2004), the hot apparent viscosity is a measure of how well acheese flows when completely melted; and the time at 5000 cPis a measure of how quickly a melted cheese thickens during cool-ing. There was a good correlation between both the hot apparentviscosity and time at 5000 cP with process cheese melt proper-ties as determined by DSR, the Schreiber Melt Test, and the TubeMelt Test.

Since there are various melt tests, and each melt test measuresa set of different properties (ease of melt, extent of flow, and soon), it becomes difficult to compare results obtained using differ-ent tests. Table 4 is a simplified chart, indicating how the valuesobtained using various melt tests are related. This will help read-ers to quickly comprehend the melted texture of process cheesewhen measured using different melt tests.

As mentioned previously, numerous techniques have been de-veloped to evaluate the functional properties of process cheese.However, one of the major limitations encountered is a poorcorrelation among some of these techniques (Park and Rosenau1984; Gunasekaran and Ak 2003). Consequently, there is still aneed for further research to develop standardized techniques tomeasure the unmelted textural and the melted textural propertiesof process cheese. When this is accomplished, manufacturerscan evaluate the functional properties of their product and ef-fectively communicate the properties of their process cheese toend users.

Factors Controlling Process Cheese Properties

Formulation parameters and ingredientsAs described in subsection “Process cheese manufacture,” the

1st step during process cheese manufacture involves the formu-lation of process cheese using various ingredients. The desiredformulation of process cheese is achieved by appropriate se-lection of natural cheese and other ingredients as allowed bythe CFR. In addition to natural cheese and other ingredients,manufacturers also select the emulsifying salts to be added totheir process cheese formulation. The majority of the manufac-turers in the United States also make appropriate formulationadjustments to their process cheese in order to incorporate re-work (discussed in the subsection “Rework”) into their processcheese.

Typically, process cheese formulations depend on the type ofprocess cheese being manufactured and the type of end-use appli-cations that the process cheese will be targeted for. Various chem-ical and compositional properties of process cheese affect the fi-nal functional properties of process cheese in a variety of ways.Therefore, while formulating a process cheese, manufacturers of-ten try to control the final chemical properties of process cheesethrough appropriate selection of ingredients in order to achieve aprocess cheese formula that will have a specific functional prop-erty after it is manufactured. However, the availability of naturalcheese (type and age), cost, availability of other ingredients, andpresence or absence of rework varies from day to day. Theseare some of the constraints that manufacturers have to deal withwhile formulating their process cheeses in order to achieve a finalproduct with consistent functional properties on a daily basis. Asa result, there are numerous permutations and combinations in-volved in the selection of ingredients for the same process cheeseformula. Traditionally, process cheese makers relied on their ex-



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perience to select the ingredient blend for a specific formula.Over the years, process cheese manufacturers have started utiliz-ing various computer-based formulation programs where they areable to set the desired chemical properties of a process cheese for-mulation and the formulation software determines the ingredientblend that produces the least cost formulation. Least cost formu-lation simply utilizes the cost and composition of an ingredient asa criterion for developing a formulation. As an example, variouswhey- and milkfat-based ingredients (whey powder, dried perme-ate, whey protein concentrate, butter, butter oil, dried cream, andso on) can be used as ingredients in process cheese. However,each of these ingredients has a different composition and cur-rent market price. A least cost formulation program uses the costand composition information from each available ingredient andselects the ingredients used in a formulation based on their im-pact on the overall cost of the formulation. One such formulationprogram that we use in our laboratory is TechwizardTM, whichis an Excel-based formulation software program (Metzger 2003;Kapoor and Metzger 2004) provided by Owl Software (Columbia,Mo., U.S.A.).

As mentioned previously, various chemical and compositionalproperties have an effect on the functional properties of processcheese. These include fat content (Hong 1990), moisture content(Hong 1989; Lee and others 2004), pH (Templeton and Sommer1930, 1932a; Marchesseau and others 1997; Lee and Kloster-meyer 2001), total calcium content (Cavalier-Salou and Cheftel1991), intact casein content (Vakaleris and others 1962; Meyer1973; Piska and Stetina 2003; Garimella Purna and others 2006),lactose content (Templeton and Sommer 1932a, 1934; Thomas1973; Berger and others 1998), and whey protein content (Guptaand Reuter 1992; Thapa and Gupta 1992a, 1992b). Due to theregulations set by the FDA (2006) for moisture and fat contentsfor PC, PCF, and PCS, manufacturers generally keep the moistureand fat contents in their product constant. However, due to theday-to-day variations in inventory of the natural cheese they use,the source and age of natural cheese tend to be different. Thesedifferences in the source and age of natural cheese on a dailybasis lead to variations in the total calcium content, pH, andintact casein content of the process cheese and hence the func-tional properties of the process cheese. In addition to naturalcheese variations that affect the total calcium content, pH, andintact casein content of the final process cheese, the type andamount of emulsifying salts that are added to process cheeseinfluence the state of calcium in process cheese and the pro-cess cheese pH. Moreover, other ingredients (nonfat dried milk,dried whey, whey protein concentrate, and so on) influence theamount of whey protein and lactose in the final process cheese.The variations in the chemical properties of a process cheesethat arise during a process cheese formulation significantly in-fluence its functional properties. The type and amount of re-work that is added to a process cheese formulation also havean effect on the final functional properties of process cheese.Therefore, it is important to control the formulation parametersof process cheese to achieve a product with consistent functionalproperties.

From the discussions above, it is evident that just standardizingthe moisture and fat content of a process cheese formulation doesnot ensure a product with the desired functional properties. It isvery important for the process cheese manufacturers to controland monitor the total calcium content, intact casein content, pH,type and amount of emulsifying salts used, lactose content, wheyprotein content, and type and amount of rework added whileformulating their process cheese in order to produce a processcheese with specific physicochemical and functional properties.The individual influence of these formulation parameters on the



functional properties of process cheese is discussed subsequently.Total calcium content. The total calcium content of a process

cheese not only plays a role during its manufacture but also influ-ences its final functional properties. A high total calcium contentin a process cheese formula leads to difficulty in manufacture ofthe corresponding process cheese, since more calcium needs tobe sequestered from the natural cheese caseins by the emulsifyingsalts added during process cheese manufacture (Sood and others1979; Caric and others 1985; Cavalier-Salou and Cheftel 1991;Zehren and Nusbaum 2000). In a study performed by Cavalier-Salou and Cheftel (1991) on cheese analogs using sodium ca-seinate, they found that as the calcium content of the cheeseanalogs increased, their firmness increased, and their meltabilitydecreased. The major ingredient that contributes to the variationsin the total calcium content in a process cheese formula is nat-ural cheese. It has been observed that when a natural cheesewith high total calcium content is used to make process cheese,the resulting process cheese is firm and less meltable (Olson andothers 1958; Zehren and Nusbaum 2000). The effect of natu-ral cheese on the total calcium content of process cheese andits functional properties is discussed in the subsection “Naturalcheese.”

Intact casein content. Meyer (1973) and Shimp (1985) high-lighted the importance of total intact casein of a process cheeseon the quality of process cheese. Once again, the major ingre-dient that contributes to the intact casein in a process cheeseformula is type and age of natural cheese utilized in the for-mula. The intact casein content of natural cheese is inverselyrelated to the age of the natural cheese. As a natural cheese isripened, its intact casein content decreases (Arnott and others1957; Vakaleris and others 1962; Meyer 1973; Garimella Purnaand others 2006). This occurs because, as the natural cheeseages, the enzymes and residual starter or nonstarter lactic acidbacteria present in the cheese hydrolyze the proteins present innatural cheese into peptides, thereby reducing the amount of ca-sein that is still present in the intact (unhydrolyzed) form. Variousresearchers have indicated the effect of the age of natural cheeseon the functional properties such as body and texture of the re-sulting process cheese (Arnott and others 1957; Vakaleris andothers 1962; Garimella Purna and others 2006). The effect of theage of natural cheese utilized on the functional properties of pro-cess cheese is discussed in the subsection “Natural cheese.” Ina recent study performed in our laboratory, Garimella Purna andothers (2006) manufactured PCF with the same cheddar cheeseat 2, 4, 6, 12, and 18 wk of ripening with 2.0%, 2.5%, and 3.0%trisodium citrate as the emulsifying salt. The results indicated thatas the intact casein content of the natural cheese base used forPCF manufacture decreased, its viscosity immediately after man-ufacture and its firmness decreased whereas its meltability in-creased. However, interestingly, we found that with a decrease inthe intact casein content of the cheddar cheese, the flowabilityof the resulting PCF increased initially up to 12 wk of ripening,but in the case of PCF manufactured with the cheese at 18 wkof ripening, its flowability decreased as compared to 12 wk ofripening. We attribute this change in flow properties of the PCFwith a low level of intact casein to a phenomenon called “over-creaming.” Previous researchers have reported that overcreamingoccurs when the natural cheese used in a formulation is exces-sively ripened (Meyer 1973). The casein in natural cheese thatis excessively ripened is hydrolyzed into small peptides that areeasily hydrated and dispersed during process cheese manufac-ture; and under normal process cheese manufacturing conditionsextensive protein-based interactions occur that lead to a strongprotein network that has restricted flow properties (Meyer 1973;Gerimella Purna and others 2006). The concept of overcream-

ing is discussed in more detail in the “Rework” subsection of thisarticle.

pH. The final pH of a process cheese has been found to havea significant effect on the quality, microstructure, and the typeof protein interactions in the resulting process cheese emulsion(Palmer and Sly 1943; Meyer 1973; Marchesseau and others1997). Various researchers have indicated that the pH range ofa good-quality process cheese should be between 5.4 and 5.8(Palmer and Sly 1943; Marchesseau and others 1997). Accord-ing to Palmer and Sly (1943), the stability of the process cheeseemulsion is decreased when the pH of the process cheese is be-low 5.4 or above 5.8. Marchesseau and others (1997) evaluatedthe microstructure of process cheese manufactured with differ-ent final pH. They found that a process cheese with a lower pH(5.2) had increased protein–protein interactions since the pro-teins were closer to their isoelectric point, thereby promoting theaggregation of proteins leading to a weaker emulsification of thefat phase in the process cheese. At higher process cheese pH(6.1), they found that process cheese had an open structure andtherefore a weaker emulsion. In their study, process cheese witha pH of 5.7 produced a uniform fat emulsion with a closely knitprotein network. Consequently, the final pH of process cheese isan important factor controlling the final structure and thereforethe final functional properties of the process cheese. A previousstudy has shown the effect of the final pH of process cheese on itsfirmness (Templeton and Sommer 1932b). They found that as thefinal pH of the process cheese increased from 5.0 to 6.2, its firm-ness initially increased up to approximately pH 5.8 (where it hadthe highest firmness); however, with a further increase in pH (5.8to 6.2), the firmness began to decrease. Alternatively, previouswork has also indicated a very low correlation between the pro-cess cheese pH and melting properties of process cheese (Arnottand others 1957). The type and level of emulsifying salts (subsec-tion “Emulsifying salts”) (Gupta and others 1984; Shirashoji andothers 2006a) and the type and age of natural cheese (subsec-tion “Natural cheese”) used during process cheese manufacturehave a marked influence on the final pH of the resulting processcheese.

Although previous reports indicate the individual importanceof total calcium, intact casein, and pH on the functional proper-ties of process cheese, they do not indicate the combined effect ofthese factors on the process cheese properties. They also do nothighlight the magnitude and nature by which these factors influ-ence the final properties of process cheese. Currently, a study isbeing performed in our laboratory to evaluate the combined effectof these factors on the functional properties of process cheese.

Emulsifying salts. As discussed previously, emulsifying salt isa major ingredient in process cheese manufacture. Emulsifyingsalts are ionic compounds made up of monovalent cations andpolyvalent anions. The 2 primary functions of emulsifying saltsin process cheese are “calcium sequestering” (to help disruptthe calcium–phosphate-linked protein network present in nat-ural cheese during process cheese manufacture) and “pH ad-justment.” Both of these functions help in hydrating the caseinspresent in natural cheese so that they can easily interact withthe water and fat phases, thereby producing a homogeneous pro-cess cheese emulsion (Ellinger 1972; Meyer 1973; Caric and oth-ers 1985; Guinee and others 2004; Mizuno and Lucey 2005).According to the CFR, there are 13 emulsifying salts that areapproved for use (either alone or in combination) in processcheese manufacture (21CFR133.169 to 133.180). These includemono-, di-, and trisodium phosphates, dipotassium phosphate,sodium hexametaphosphate, sodium acid pyrophosphate, tetra-sodium pyrophosphate, sodium aluminum phosphate, sodiumcitrate, potassium citrate, calcium citrate, sodium tartrate, and



sodium potassium tartrate. The most common emulsifying saltsused for process cheese manufacture in the United States aretrisodium citrate and disodium phosphate. Trisodium citrate isthe preferred emulsifying salt for slice-on-slice process cheesevarieties, whereas disodium phosphate (or appropriate combina-tions of di- and trisodium phosphates) is used in loaf-type pro-cess cheese and process cheese spreads. Sometimes, low levelsof sodium hexametaphosphate are also used along with theseemulsifying salts (in certain applications). Another emulsifyingsalt that has become popular recently is sodium aluminum phos-phate. It is commonly used in rennet casein-based mozzarellatype imitation process cheese varieties since it provides desirablefunctional properties for imitation process cheese that is used toreplace mozzarella on frozen pizzas.

One of the most common questions that arise during the selec-tion of emulsifying salts for process cheese manufacture is theircalcium sequestering ability and their mechanisms of calcium re-moval from caseins. There have been various studies using differ-ent model systems to answer these questions (Cavalier-Salou andCheftel 1991; Guinee and others 2004; Mizuno and Lucey 2005).However, the interaction of emulsifying salts with calcium and ca-sein in process cheese is not fully understood. Important proper-ties of different emulsifying salts and the influence of the type andamount of different emulsifying salts on process cheese proper-ties have been extensively studied (Templeton and Sommer 1936;Ellinger 1972; Meyer 1973; Rayan and others 1980; Thomas andothers 1980b; Gupta and others 1984; Caric and others 1985;Cavalier-Salou and Cheftel 1991; Molins 1991; Berger and oth-ers 1998; Awad and others 2002; Dimitreli and others 2005;Shirashoji and others 2005, 2006a, 2006b). However, due tothe differences in experimental conditions among these studies,including type and age of natural cheese used, differences inprocess cheese formulation and composition, and differences inprocessing conditions used for process cheese manufacture, thesestudies show large variations in their results and, consequently,comparisons and interpretation of the results from these studiescan be difficult. Detailed physicochemical properties of variousemulsifying salts can be found in the literature (Ellinger 1972;Molins 1991; Berger and others 1998; Guinee and others 2004).Gupta and others (1984) extensively studied the effect of the typeand amount of various emulsifying salts on process cheese pH,meltability, hardness, and body and texture. In Table 5, we havecompiled some relevant data from previous research on the im-portant properties of certain selected emulsifying salts along with

Table 5 --- Physicochemical properties of some emulsifying salts and their influence on process cheese (PC) proper-ties.

Physicochemical propertiesa Influence on PC propertiesc

Chemical Formula Solubility, g/100 pH, 1% pH of Hardness MeltabilityEmulsifying salt formula weight, g/mol g H2O at 20 ◦C solution the PC (kg) (mm)

Trisodium citrate (dihydrate) NaH2C6H5O7.2H2O 294 75 8.6 5.9 32 131Monosodium phosphate (monohydrate) NaH2PO4.H2O 138 85b 4.5 5.1 27 NMd

Disodium phosphate (dihydrate) Na2HPO4.2H2O 178 80 9.1 5.8 32 70Trisodium phosphate (dodecahydrate) Na3PO4.12H2O 380 11b 11.9 7.3 26 70Dipotassium phosphate K2HPO4 174 160 8.9 5.9 29 76Sodium hexametaphosphate (NaPO3)n (n = 10 to 15) (102)n 157 6.6 5.2 33 NMSodium aluminum phosphate 9.2 5.9 33 101aData compiled from Mollins (1991), Berger and others (1998), and Guinee and others (2004).bSolubility of the anhydrous salt.cData compiled from Gupta and others (1984). (The values indicated were true for their experimental setup and should be treated as general reference, not as universal results.)�Process cheese (PC) manufactured using 75% young cheddar cheese and 25% aged cheddar cheese with moisture ranging from 38.4% (for PC with sodium aluminum phosphate) to 40.7%(for PC with trisodium phosphate) and fat approximately 31.5%.�The emulsifying salt level for all the PC were 2.2% except for PC with disodium phosphate (which was 2.1%) and PC with trisodium citrate (which was 2.3%).�Hardness values are rounded to the nearest whole number.�Meltability was measured using tube melt test.dNo melt observed.

their influence on process cheese properties. With relevance toTable 5, Rayan and others (1980) manufactured PC (40% mois-ture) with 4 different emulsifying salts at a 2.5% level without pHadjustment. Their results were in agreement with those of Guptaand others (1984) with PC manufactured using trisodium citrateand PC manufactured using sodium aluminum phosphate indicat-ing a higher meltability than the PC manufactured using disodiumphosphate; however, the firmness values of the 3 PC productionswere similar. Thomas and others (1980b) manufactured processcheese (45% moisture) using trisodium citrate, disodium phos-phate, and sodium hexametaphosphate at a 3% addition level(without pH adjustment) and evaluated the emulsion strength (bymeasuring oil separation), firmness, and meltability of all the pro-cess cheeses. They found that the emulsion strengths of all theprocess cheeses were not significantly different. The meltabilityof the process cheeses manufactured using trisodium citrate anddisodium phosphate were not significantly different; however,process cheese manufactured using sodium hexametaphosphatehad a significantly lower meltability. They also found that thefirmness of the process cheese manufactured using trisodium cit-rate was significantly lower than both the process cheese manu-factured using disodium phosphate and sodium hexametaphos-phate. A more recent study also showed the effect of the typeand amount of 3 different emulsifying salts on the hardness andmeltability of process cheese (Shirashoji and others 2005). Shi-rashoji and others (2005) manufactured PC (38% to 39% moistureand 33% fat) with 3 different emulsifying salts, trisodium citrate,disodium phosphate, and sodium hexametaphosphate, at 0.25%,1.5%, and 2.75% of the final process cheese. All of the PC wasadjusted to pH 5.6 to prevent any effect on process cheese func-tional properties due to pH. They found that as the concentra-tion trisodium citrate, disodium phosphate, and sodium hexam-etaphosphate in PC increased its firmness increased and its melta-bility decreased. Also, at 2.75% emulsifying salt concentration,PC made using sodium hexametaphosphate was the most firmand the least meltable followed by PC made using disodium phos-phate and PC made using trisodium citrate. In another study onthe effect of type of emulsifying salts in sliced process cheese, Shi-rashoji and others (2006a) produced sliced process cheeses (46%moisture, 19% protein) using 4 emulsifying salts (trisodium cit-rate, disodium phosphate, sodium hexametaphosphate, and tetra-sodium pyrophosphate, at 2.5% each) without pH adjustment.They evaluated process cheese pH, flowability, and meltability,and created a “texture map” (by performing a large deformation



burst test using a ball-type probe) of all the process cheeses. ThepH of the process cheese made using sodium hexametaphosphatewas significantly lower (pH 5.3) than the other process cheeses(pH 5.9 to 6.0). The meltability and flowability results showedthat process cheese manufactured using tetrasodium pyrophos-phate had the least meltability and lowest flowability followedby process cheese made using sodium hexametaphosphate. Themeltability and flowability of the process cheeses made usingtrisodium citrate and disodium phosphate were similar. The re-sults of the “texture map” indicated that process cheeses made us-ing disodium phosphate and sodium hexametaphosphate tendedtoward a mushy and crumbly texture, whereas tetrasodium py-rophosphate provided the process cheese with a tough and rub-bery texture (Shirashoji and others 2006a). In a study on the influ-ence of the level of trisodium citrate on PCF properties performedin our laboratory, Garimella Purna and others (2006) manufac-tured PCF (44% moisture and 25% fat) using cheddar cheese (at2, 4, 6, 12, and 18 wk of ripening) and trisodium citrate (at 2.0%,2.5%, and 3.0% levels). All the PC batches were processed at85 ◦C for 6 min at 2 different mixing speeds (450 and 1050 rpm).The viscosity was immediately measured after manufacture andthe firmness, RVA hot apparent viscosity (a measure of the flowa-bility of process cheese, Table 4), and RVA time at 5000 cP (ameasure of the meltability of process cheese, Table 4) after PCmanufacture. The results indicated a significant effect of the con-centration of trisodium citrate on the RVA hot apparent viscosityand RVA time at 5000 cP. As the trisodium citrate level of the PCFincreased, both the flow properties and the meltability of the PCFdecreased; however, there was no significant effect of trisodiumcitrate concentration on the firmness of PCF.

Lactose content. The lactose content of a process cheese is an-other critical formulation parameter that needs to be controlledin a process cheese formula since a high level of lactose in pro-cess cheese can lead to the formation of lactose crystals or Mail-lard browning in process cheese. Nonfat dried milk (NDM) anddried whey are the major ingredients that contribute lactose toa process cheese formula. The problem of lactose crystallizationin process cheese due to the addition of NDM or whey pow-der has been addressed by various researchers (Templeton andSommer 1932a, 1934; Thomas 1973; Berger and others 1998).Lactose crystallization in process cheese depends on the maxi-mum concentration of lactose that is soluble in the water phase ofprocess cheese (Templeton and Sommer 1932a; Thomas 1973).The maximum concentration of lactose that is soluble in wateris 17% at 20 ◦C (Templeton and Sommer 1932a; Harper 1992).Hence, as a general guideline, it is important to maintain theamount of lactose in the water phase of process cheese at lessthan 17% in order to avoid lactose crystallization. Therefore,when formulating a process cheese, the manufacturers shouldensure that the final lactose content in the process cheese shouldnot exceed 7.48% for PCF (44% moisture product) and 10.20%for PCS (60% moisture product). Another defect that tends toarise due to the addition of lactose-rich ingredients in processcheese is Maillard browning, which leads to objectionable colorand flavor development (Thomas 1969). Thomas (1969) indicatedthat postmanufacture storage temperature and time as well asthe pH of the process cheese significantly affected the brown-ing of process cheeses. He suggested that process cheese shouldnot be stored at temperatures greater than 35 ◦C for more than6 wk.

Whey protein content. Whey proteins constitute approximately20% of the total proteins in bovine milk (Eigel and others 1984).Approximately 80% of whey proteins in milk are made up of the 2major whey proteins (β-lactoglobulin and α-lactalbumin). One ofthe important characteristics of β-lactoglobulin from a processingstandpoint is the presence of a “reactive” free sulfhydryl group in

its primary structure (Wong and others 1996). Whey proteins arealso highly susceptible to heat treatment and are found to dena-ture between 60 and 70 ◦C. This temperature-induced denatura-tion of β-lactoglobulin exposes the free sulfhydryl group, whichhas the capability of crosslinking with other β-lactoglobulin andκ-casein molecules via disulfide bonds (Sawyer and others 1963;Wong and others 1996). NDM and whey protein concentrate(WPC), if used in a process cheese formula, may contribute to in-creased levels of whey protein in the process cheese. Since wheyproteins can crosslink among themselves as well as with caseinsat high temperatures, a high level of whey proteins in a processcheese formula not only influences its sensory properties but alsomay lead to an increase in the firmness of the final process cheeseand a decrease in its meltability. The influence of whey proteinincorporation in process cheese on its functional and sensoryproperties has been extensively studied (Gupta and Reuter 1992;Thapa and Gupta 1992a; Ido and others 1993; Abd El-Salam andothers 1996; Al-Khamy and others 1997; Fayed and Metwally1999; Mleko and Foegeding 2000, 2001; Laye and others 2004).Gupta and Reuter (1992) ultrafiltered whey to produce a liquidconcentrate (26% total solids) with 20% protein and 5.8% lactosewhich was utilized as an ingredient to replace 20% of the solidsin a PCF formula. They determined that addition of up to approx-imately 8% whey protein in the final PCF with average moisturecontent of 47% did not have an effect on the overall acceptabil-ity score of process cheese. However, in another study, Thapaand Gupta (1992a) have indicated that PCF (42% to 43% mois-ture, 2.5% emulsifying salt) containing WPC (at approximatelythe same final whey protein level) was firmer than PCF with noadded WPC. Abd-El-Salam and others (1996) studied the effect ofliquid WPC (28% total solids, 15% whey protein) addition (0%,20%, and 40% of the final blend) on the compositional and therheological properties of processed cheese spread (57% moisture,3% emulsifying salt). In their study, addition of WPC increasedthe moisture by 0.8%, lactose by 2.5%, and pH by 0.3 in the finalcheese spreads made from 40% WPC (6% whey protein in the fi-nal product) as compared to cheese spreads with no WPC added.They found an improved meltability (possibly due to the increasein moisture content), flavor, and other sensory properties of thePCS as the amount of WPC in the cheese spreads increased. In arecent study with a rennet casein process cheese model system(17% casein, 24% fat, 2% emulsifying salt), Mleko and Foeged-ing (2000) found that up to a maximum of 2% casein can bereplaced with whey protein. However, they did find a slight in-crease in the firmness and a decrease in the meltability of theprocess cheese. They proposed that the heat-induced disulfide in-teractions involving the free sulfhydryl groups of β-lactoglobulinshave a marked influence on the firmness and the melting proper-ties of the process cheeses. Additional work by these researchersinvolved replacement of casein proteins with polymerized wheyproteins in a rennet casein-based model process cheese system(Mleko and Foegeding 2000, 2001). They produced polymerizedcross-linked whey proteins by heating whey protein dispersionsto induce the disulfide cross-links between the whey proteinsand added these polymerized whey proteins to a model processcheese system. Their work showed that, as the level of polymer-ized whey proteins was increased, there was an increase in thefirmness and a decrease in the meltability of the process cheeseanalogs produced. However, they did conclude that replacementof 4% rennet casein (in a 17% rennet casein-based process cheeseanalog system) with 2% whey protein polymers could still pro-duce process cheese analogs of identical texture and meltability(Mleko and Foegeding 2001).

Rework. “Rework” is a term used to describe process cheeseproduced in a manufacturing facility that cannot be sold for a va-riety of reasons. The type of rework ranges from process cheese



lost during production line changeovers; shavings and edge trim-mings removed during slice line operations; residual processcheese that is removed from the cookers, lines, hoppers, andpackaging machines, also referred to as “hot melt” (Kalab andothers 1987); and process cheese that has been rejected by qual-ity control due to improper weight, packaging, or on the basisof a quality defect (Kichline and Scharpf 1969; Zehren and Nus-baum 2000). Since rework poses economic challenges to manu-facturers, it is incorporated into a fresh process cheese blend andreprocessed. According to Lauck (1972), the amount of reworkproduced in a process cheese manufacturing facility ranges from2% to 15% of the total process cheese produced. Since reworkhas already undergone the emulsification process and also con-tains emulsifying salts, the addition of rework to a fresh blendduring process cheese manufacture can cause difficulty duringprocessing and also affects the final functional properties of pro-cess cheese (Kichline and Scharpf 1969; Lauck 1972; Kalab andothers 1987). In general, the addition of rework tends to decreasethe meltability and produce a firmer process cheese (Kalab andothers 1987). Kichline and Sharpf (1969) specify that the max-imum amount of rework that can be added to process cheesewithout any undesirable effects on its properties is 4% of the to-tal cheese blend used for process cheese manufacture. Kalab andothers (1987) found that the types of rework, as well as the amountused, have an effect on the final properties of PCF (43% moisture,24% fat, pH 5.5 to 5.7, and 2.7% added emulsifying salt in theform of trisodium citrate). They studied the effect of type of reworkon the apparent viscosity of PCF immediately after manufacture,its firmness, and its meltability. In their study, they used 3 sourcesof rework at 20% of the final PCF: fresh rework (quickly frozenPCF emulsion right after manufacture), regular PCF slices fromolder processing runs, and overcooked process cheese mass tosimulate process cheese that had undergone excessive processing(which they referred to as “hot melt”). The “hot melt” was used at2 levels: 10% and 20% of the final product. Their results indicatedthat when any kind of rework was used in PCF blends, the manu-factured PCF was more viscous coming out of the cooker and hadhigher firmness and lower meltability relative to the control. Theyalso found that the effect of the type of rework on the increase inapparent viscosity after manufacture and the firmness of the PCFwas fresh rework < PCF slices from older process cheese runs <hot melt when used at 10% of the total blend < hot melt whenused at 20% of the total blend. A similar trend was observed forthe decrease in meltability of the PCF with the PCF with hot meltat 20% showing no melt. The microstructural results from theirstudy showed that hot melt samples had a denser protein matrixand were overemulsified when compared to fresh rework sam-ples (Kalab and others 1987). This phenomenon where excessivecooking of process cheese can increase the interactions amongthe caseins to such an extent that they attain a thick pudding-likeconsistency is also referred to as “overcreaming” (Meyer 1973).

In Europe, “precooked cheese” or rework obtained at differ-ent times during process cheese manufacture is sometimes usedfor enhancing the cooking and functional properties of processcheese (Meyer 1973; Berger and others 1998). According toMeyer (1973), fresh rework, which has a weakly dispersed (hy-drated) protein structure (also referred to as “long structure”), canbe effectively used to stabilize a process cheese that might showa tendency to overcream under normal cooking conditions. Nor-mal rework with optimum protein dispersion and emulsificationcan be used from 2% to 30% in process cheeses where the cream-ing action is desired. However, Meyer (1973) cautions that hotmelt should not be used at more than 1% of the process cheeseblend in order to avoid adverse effects on the functional prop-erties of the process cheese. In the United States, it is a generalpractice to collect shavings and edge trimming from slice-on-slice

production lines and immediately use them as “fresh rework” atapproximately 10% of the final blend.

The importance of controlling various formulation factors ina process cheese formula has been described previously. Alsodiscussed previously are some of the important ingredients thataffect these formulation parameters in a process cheese formula(for example, natural cheese affects the total calcium, intact ca-sein, and pH; nonfat dried milk and whey ingredients affect thelactose content and whey protein level). Since the manufactur-ers use these ingredients to control the above-mentioned processcheese formulation parameters, the effect of natural cheese andother ingredients on process cheese properties is discussed sub-sequently.

Effect of ingredients. Natural cheese. Natural cheese is oneof the most important ingredients used in process cheese. Asdiscussed previously, natural cheese has a marked influenceon total calcium, intact casein, and pH and, hence, the finalfunctional properties of process cheese. Depending on the coun-try of manufacture, availability, and market demand, varioustypes of natural cheeses such as cheddar, Swiss, Gouda, andso on, are used to manufacture process cheese (Meyer 1973).However, in the United States, cheddar cheese is the majortype of natural cheese used for process cheese manufacture.Depending on the type of process cheese manufactured, theamount of natural cheese in a process cheese formula variesfrom 51% to > 80% of the final process cheese (FDA 2006).Consequently, the characteristics of natural cheese utilized tomanufacture process cheese have a major influence on processcheese characteristics and appropriate selection of naturalcheese is critical in order to achieve a process cheese with thedesired chemical and functional characteristics. The naturalcheese used in a process cheese formulation is generally selectedon the basis of type, flavor, maturity, consistency, texture, andpH (Zehren and Nusbaum 2000). Process cheese manufacturers,through years of experience, have realized and mastered the artof selecting the appropriate blend of young and aged naturalcheese in order to achieve process cheese with desired flavorand textural properties (Meyer 1973; Thomas 1973). Numerousresearchers have highlighted the importance of natural cheesecharacteristics on functional properties such as unmelted textureand meltability of process cheese (Barker 1947; Meyer 1973;Thomas 1973; Caric and others 1985; Shimp 1985; Zehren andNusbaum 2000). The physicochemical characteristics of naturalcheese that influence the process cheese properties includepH, calcium and phosphorus contents, and age, or the amountof intact casein present in the natural cheese (Templeton andSommer 1930; Barker 1947; Olson and others 1958; Vakalerisand others 1962; Meyer 1973; Thomas 1973; Harvey and others1982; Zehren and Nusbaum 2000; Kapoor and others 2007).

The importance of natural cheese pH on process cheese prop-erties has been highlighted in a study performed by Olson andothers (1958) in which they manufactured cheddar cheeses withmodified manufacturing protocol so as to produce 2 cheddarcheese treatments with different final pH levels. The 2 cheddarcheeses were then used to manufacture PCS (at 10, 30, 60, 90,and 150 d of ripening), which were analyzed for unmelted textureusing penetrometry and meltability using the tube melt test. Theirresults indicated that even after the final pH of the PCS was ad-justed to 5.4 to 5.5, the PCS batches made using cheddar cheesewith higher pH were harder and less meltable when compared tothe PCS made using cheddar cheese with normal pH at all stagesof ripening. However, the observed effect of natural cheese pHon process cheese properties was also related to the changes inthe state and amount of calcium and phosphorus caused by thedifferences in natural cheese pH (Olson and others 1958; Zehrenand Nusbaum 2000). The effect of natural cheese pH on the state



and amount of calcium and phosphorus in natural cheese is dis-cussed subsequently.

Minor changes in the manufacturing protocols during natu-ral cheese manufacture, such as set pH, drain pH, and level ofsalting (salt-to-moisture ratio), can significantly change the stateand amount of calcium and phosphorus in natural cheese (Dolbyand others 1937; Czulak and others 1969; Upreti and Metzger2006). In a study on low-fat mozzarella cheese, Metzger and oth-ers (2000) studied the effect of cheese milk set pH on the amountand state of calcium in the final cheese. It was found that as thepH of the cheese milk before rennet addition during cheese man-ufacture (set pH) was reduced to 6.0 or 5.8 (using acetic acid), thetotal calcium of the final cheeses decreased by 11% (set pH 6.0)or 23% (set pH 5.8), respectively, when compared to the controlcheese that was set at a pH of 6.5. Czulak and others (1969) high-lighted the effect of drain pH of cheddar cheese on the calciumcontent and the final pH of the cheese. They found that as thepH of the curd during whey drainage was decreased from 6.14to 5.75, there was a 27% reduction in the total calcium contentof the cheese curd at the time of whey separation.

As discussed in the subsection “Intact casein content,” the in-tact casein content of natural cheese is inversely related to theage of the natural cheese. As a natural cheese is ripened, its intactcasein content decreases (Fenelon and Guinee 2000; GarimellaPurna and others 2006). Researchers have described the effect ofthe age of natural cheese on the functional properties of processcheese (Templeton and Sommer 1930; Arnott and others 1957;Olson and others 1958; Vakaleris and others 1962; Piska andStetina 2003; Garimella Purna and others 2006). All these stud-ies consistently indicated that as the age of natural cheese usedin process cheese manufacture increased, the unmelted firmnessof the resulting process cheese decreased (Templeton and Som-mer 1930; Olson and others 1958; Vakaleris and others 1962;Piska and Stetina 2003; Garimella Purna and others 2006) andthe meltability of the resulting process cheese increased (Olsonand others 1958; Vakaleris and others 1962; Garimella Purna andothers 2006).

Currently, another major thrust in the natural cheese indus-try is the utilization of concentrated milk to manufacture naturalcheeses in order to increase the throughput of cheese plants. Thetype of concentration technique and the extent to which milk hasbeen concentrated also influence the final pH, calcium content,and degree of proteolysis in the natural cheese (Sutherland andJameson 1981; Anderson and others 1993; Acharya and Mistry2004; Nair and others 2004). Consequently, utilization of thesenatural cheeses (manufactured using concentrated milk) as aningredient in process cheese can also influence the functionalproperties of the process cheese manufactured using this naturalcheese as an ingredient (Acharya and Mistry 2005). Acharya andMistry (2005) manufactured PC using 5 different cheddar cheesetreatments that were manufactured from cheese milk concen-trated with different concentration techniques at different levels.The treatments were cheddar cheeses manufactured using control(normal cheese milk), ultrafiltered milk (concentration factor of1.5×), ultrafiltered milk (concentration factor of 2.0×), vacuum-condensed milk (concentration factor of 1.5×), and vacuum-condensed milk (concentration factor of 2.0×). They found that,as the concentration factor of the milk utilized to manufacturecheddar cheese was increased to 1.5×, the calcium content ofthe cheddar cheese manufactured increased by 10% when themilk was ultrafiltered and by 4% when the milk was vacuum-condensed. Moreover, when the concentration factor of the milkutilized to manufacture cheddar cheese was increased to 2.0×,the calcium content of the cheddar cheese manufactured in-creased by 18% when the milk was ultrafiltered and by 13%when the milk was vacuum-condensed. Moreover, they found

that the degree of protein hydrolysis in the cheddar cheeses wasalso affected by both the method and the level of concentration.When these natural cheeses were used to make PC, the hard-ness of PC increased and the melt and the flow properties of PCdecreased, as the level of concentration of the cheese milk tomanufacture the natural cheese increased (in both the concen-tration techniques). Consequently, utilization of natural cheesesmanufactured using concentrated milk as an ingredient in pro-cess cheese can also influence the functional properties of theprocess cheese manufactured.

Sometimes, it is also common for process cheese manufacturersto freeze natural cheese that will eventually be used for processcheese manufacture (Zehren and Nusbaum 2000). Thomas andothers (1980a) studied the effect of frozen natural cheese on pro-cess cheese functional properties. They manufactured 3 processcheese treatments utilizing the same natural cheese (regular orfrozen). The natural cheese was split into 3 parts. One part wasripened for 6.5 mo at 10 ◦C, the 2nd part was ripened for 6.5 moand then frozen at −20 ◦C for 3 mo before being used to makeprocess cheese, the 3rd part was ripened for 3.5 mo followed byfreezing at −20 ◦C for 3 mo and further repining for 3 mo. Theyfound that when frozen natural cheese (in all the freezing treat-ments) was used to manufacture process cheese, the firmness ofthe resulting process cheese was higher and its meltability waslower when compared to the process cheese manufactured usingthe same natural cheese which was not frozen.

Nonfat dried milk/dried whey/whey protein concentrate. Thelevels of dairy-based ingredients other than natural cheese usedin PC are specifically defined by the CFR (Table 1). Ingredientssuch as NDM and whey-based dairy ingredients such as liquidwhey, whey powder, and WPC can be used in PCF and PCS (FDA2006). Since the addition of these ingredients to process cheeseformulation helps to reduce the cost of the product, manufac-turers often try to maximize the addition of NDM and whey-based dairy ingredients in their products. The amounts of theseingredients typically added to process cheese are not known tocause significant changes in process cheese properties. However,since commercial NDM and dried sweet whey have an approx-imate lactose concentration of 50% and 75%, respectively, andcommercial NDM and WPC have a significant amount of wheyproteins, 2 important formulation factors need to be taken intoaccount when using NDM and other whey-based ingredients inPCF and PCS manufacture. These factors are the level of lactoseand the level of whey protein (as discussed in subsections onlactose content and on whey protein content) provided by theseingredients in the final process cheese.

Food gums/hydrocolloids. The CFR allows gums or hydrocol-loids to be used in PCS at levels not exceeding 0.8% of the finishedproduct (Table 1). These include carob bean gum, gum karaya,gum tragacanth, guar gum, gelatin, sodium carboxymethylcellu-lose (cellulose gum), carrageenan, oat gum, algin (sodium algi-nate), propylene glycol alginate, or xanthan gum singly or in com-bination. Gums do not directly affect any of the above-mentionedformulation parameters in the process cheese; however, sincePCS has a high moisture content (up to 60%), the major func-tion of gums in PCS is to bind water and to provide appropriateviscosity/thickening to the product and improve its mouthfeel.Therefore, gums in a PCS formula have an effect on melted tex-tural properties. Zehren and Nusbaum (2000) indicate that theselection of gums depends on various factors. These include easeof dispersibility, solubility, hydration behavior, moisture holdingability, cook viscosity, compatibility with milk proteins and othercompounds present in process cheese, and optimum working pHrange. Phillips and Williams (2000) provide extensive informa-tion on the properties and uses of different gums used in the foodindustry. However, there is a lack of available literature dealing



with the effects of various gums on the final properties of PCS.Another important area where the use of gums is gaining popu-larity is in low-fat and reduced-fat process cheese and imitationprocess cheese varieties (Brummel and Lee 1990; Swenson andothers 2000).

As described in the subsection “Process cheese manufacture,”after the preparation of a desired formulation, the ingredient blendis processed using heat and mixing to produce a homogeneousmass, which is packaged and cooled. Although effective monitor-ing and control of the formulation parameters are critical to ensurethe production of process cheese with specific functional proper-ties, appropriate selection of the process cheese cook conditionsis also very important, since differences in processing conditionsduring process cheese manufacture have a major influence on thefunctional properties of process cheese. The effect of processingconditions on process cheese functional properties is discussedsubsequently.

Processing conditionsProcessing parameters such as cook temperature (Lee and oth-

ers 1981; Hong 1989; Berger and others 1998; Glenn and others2003), cook time (Rayan and others 1980; Bowland and Foeged-ing 2001; Glenn and others 2003; Shirashoji and others 2006b),the amount of mixing provided during manufacture (Glenn andothers 2003; Garimella Purna and others 2006), and the rateat which the process cheese is cooled (Piska and Stetina 2003;Zhong and others 2004) also play a major role in controllingthe emulsion formation and the resulting functional properties ofprocess cheese.

Glenn and others (2003) at North Carolina State Univ. per-formed an extensive study to evaluate the effect of processing con-ditions on process cheese meltability. They used 5 mixing speed/cook temperature combinations (50 rpm/74 ◦C, 50 rpm/86 ◦C,100 rpm/80 ◦C, 150 rpm/74 ◦C, and 150 rpm/86 ◦C) to manufac-ture processed cheddar cheese. Process cheese at each mixingspeed/cook temperature combination was processed for 1, 5, 10,15, 25, and 35 min. The Schreiber melt test was performed onall the process cheeses to measure their meltability. They calcu-lated the time–temperature effect (thermal history) and the time–shear effect (strain history) for each of the 30 mixing speed–cooktemperature–cook time combinations that were used to manufac-ture process cheeses (as discussed previously) and finally corre-lated the meltability of the process cheese with the thermal historyand strain history. The thermal history of process cheeses rangedfrom 24 MJ·s/kg for the process cheese manufactured at 74 ◦Cfor 1 min to 886 MJ·s/kg for the process cheese manufacturedat 86 ◦C for 35 min. The strain history of process cheeses rangedfrom 807 for the process cheese manufactured at 50 rpm for 1 minto 84776 for the process cheese manufactured at 150 rpm for35 min. They found that as the thermal history and the strain his-tory of process cheese increased its meltability decreased, whichindicates that an increase in cook temperature, cook time, andmixing speed during manufacture produces process cheese withlower meltability.

The individual effects of processing temperature, processingtime, mixing speed during manufacture, and rate of cooling ofprocess cheese after manufacture on the functional properties ofprocess cheese are discussed subsequently.

Processing temperature. Although the minimum cook temper-ature and time specified by CFR for process cheese is 65.5 ◦Cfor 30 s (FDA 2006), process cheese manufacturers use varioustypes of cookers with different designs and operating conditionsto manufacture process cheese. Consequently, the cook temper-atures utilized range from 70 to > 100 ◦C depending on thecooker design and the variety of process cheese manufactured.Lee and others (1981) manufactured processed Emmental cheese

at 4 different cook temperatures (80, 100, 120, and 140 ◦C) andsubsequently analyzed the firmness (using penetrometry) and themicrostructure of the product. They found that, as the cook tem-perature during process cheese manufacture increased, the firm-ness of the process cheese increased and strength of the processcheese emulsion increased.

Processing time. Rayan and others (1980) manufactured PC(40% moisture) with 4 different emulsifying salts (trisodiumcitrate, disodium phosphate, tetrasodium pyrophosphate, andsodium aluminum phosphate) at a 2.5% level. They used a pro-cessing temperature of 82 ◦C with processing times of 6, 11, 16,26, and 46 min and subsequently measured the meltability, firm-ness, degree of elasticity, and microstructure of all the PC batches.They found that as the processing time of the PC increased, therewas a significant increase in the firmness and degree of elasticityand a significant decrease in their meltability. The results heldtrue for all 4 emulsifying salts. The microstructural results fromtheir study showed that as the processing time of the PC increased,there was a decrease in the size of fat globules, thereby indicatinga stronger emulsification in PC with increasing processing time.In another study on PC manufactured at 80 ◦C for 10, 20, and 30min using cheddar cheese and 2.75% trisodium citrate (38.5% to40.1% moisture, 32% fat), Shirashoji and others (2006b) showedthat as the processing time of the PC increased, its firmness in-creased and its meltability decreased.

Mixing speed during processing. In a study performed in our lab-oratory, Garimella Purna and others (2006) manufactured PCF(44% moisture and 25% fat) using the same cheddar cheesebase (at 2, 4, 6, 12, and 18 wk of ripening) and trisodium cit-rate (at 2.0%, 2.5%, and 3.0% levels). All the PC batches wereprocessed at 85 ◦C for 6 min at 2 mixing speeds (450 and1050 rpm). We found a significant effect of the mixing speedon the viscosity immediately after manufacture, on firmness, onflow properties, and on the meltability of the PCF at all natu-ral cheese ripening and all trisodium citrate levels. As the mix-ing speed during PCF manufactured increased, there was an in-crease in the viscosity immediately after manufacture and thefirmness of the PCF, and there was a decrease in the flow prop-erties and meltability of the PCF. In a subsequent study on theeffect of mixing speed on the microstructure of PCF, we per-formed cryoscanning electron microscopy on the PCF (with 2.5%trisodium citrate) manufactured by Garimella Purna and others(2006). Figure 4 indicates the cryoscanning electron microscopyimages and the distribution in fat globule diameters of the 2 PCFbatches. Figure 5 shows the number of fat globules/100 µm2

and mean fat globule diameters of the 2 PCF batches. The re-sults clearly indicate that at high mixing speed, the PCF showeda larger number of fat globules/100 µm2, a lower mean fatglobule diameter, and a more uniform distribution in fat glob-ule diameter as compared to PCF manufactured at low mixingspeeds.

Rate of cooling after manufacture. Piska and Stetina (2003)manufactured PCS-type products using a blend of Dutch-typehard and semihard cheeses. These PCS products cooled at 2 dif-ferent rates: slow cooling (where the PCS reached a tempera-ture of 20 ◦C in approximately 50 h) and fast cooling (where thePCS reached a temperature of 20 ◦C in less than 1 h and a tem-perature of 5 ◦C in 2 h). They found that PCS that was cooledslowly was significantly firmer and had significantly higher ad-hesiveness and gumminess. In another study on commercial PCsamples, Zhong and others (2004) continuously measured theG ′ (storage modulus) of the PC which was subjected to differentcooling rates (0.025, 0.05, 0.1, and 0.5 ◦C/min). They found thatas the rate of cooling of PC decreased, the G ′ increased, indicat-ing that PC cooled at a slower rate was firmer. In the same study,they cooled 5-pound PC loaves at the same rate using 2 types of



coolers (free convection and forced convection) and measuredthe slicing ability and meltability of the PC. They theorized thatsince the surface of the loaf would cool faster (irrespective ofthe cooler type) when compared to the center, there should bedifferences in the functional properties of the PC samples fromthe surface and the center. However, when they calculated thecooling rate at different locations in the 5-pound loaf under theircooler conditions, they found that the cooling rates (under theirconditions) of the surface and the center of the loaf were not verydifferent. They also found no trend between the sampling loca-tion of the PC sample in the loaf and the slicing ability and themeltability of the PC. However, they found a significant effect ofthe type of cooler used on the functional properties of the PCafter cooling. PC samples cooled in the forced convection coolerwere less firm and more meltable than the PC cooled in the freeconvection cooler.

Effective control of the formulation and processing parametersduring the manufacture of process cheese ensures the productionof a good-quality process cheese that is free of defects. However,there still are some occasions when certain chemical, textural, ormicrobiological defects arise in the final process cheese, whichmake the product unfit for sale. In the following 2 sections, wediscuss some of the possible defects that are associated with pro-cess cheese.

Defects in Process Cheese

Defects related to appearanceCrystal development. Researchers have shown various in-

stances of different crystals in natural cheeses. These range fromtyrosine, calcium lactate, and lactose to various salts of cal-cium phosphate and calcium citrate (reviewed by Caric andothers 1985; Guinee and others 2004). Concentrating on pro-cess cheese, researchers over the years have shown instancesand even identified different types of crystals using microscopy,Fourier transform infrared spectroscopy, and x-ray diffraction(Sommer 1930; Scharpf and Michnick 1967; Scharpf and Kichline1968, 1969; Rayan and others 1980; Uhlmann and others 1983;Klostermeyer and others 1984; Caric and others 1985; Pom-

Figure 4 --- Cryoscanningelectron microscopy imagesof 2 PCF samples (with 2.5%trisodium citrate)manufactured at (a) 1050rpm; and (b) 450 rpm withfat globules indicated as Fand protein matrix indicatedas P. Also indicated is thedistribution in fat globulediameter (a1) PCFmanufactured at 1050 rpm;(b1) PCF manufactured at450 rpm.

mert and others 1988). The types of crystals identified in pro-cess cheese ranged from salts of calcium tartrate (Sommer 1930;Leather 1947) (although tartrate crystals are not commonly foundanymore since tartrate-based emulsifying salts are not typicallyused today), calcium citrate (Morris and others 1969; Scharpf andKichline 1969), tertiary complexes of sodium and calcium citrates(Klostermeyer and others 1984), various salts of sodium and cal-cium phosphates (Scharpf and Kichline 1968; Pommert and oth-ers 1988), and, sometimes, calcium salts of free fatty acids (Besterand Venter 1986). Other types of crystals that can occur in pro-cess cheese are lactose crystals (discussed previously) (Templetonand Sommer 1932a).

It has already been discussed previously (subsection “Lactosecontent”) that lactose crystallization in process cheese can beavoided by maintaining the level of lactose below its maximumsolubility level in the water phase of process cheese. However,major sources of crystal development in process cheese are due tothe use of emulsifying salts such as various salts of phosphates andcitrates that lead to the formation of various complexes/salts thatmight have a lower solubility than the regular emulsifying salts.The solubility of these complexes/salts is further influenced by pHof the process cheese or storage conditions. Scharpf and Kichline(1968) developed a multiple regression model and an equationinvolving prediction of crystal formation in process cheese (40%to 42% moisture). They showed that crystal formation was di-rectly dependent on the pH and the amount of total phosphorusin process cheese (measured as % P2O5 content and involvingthe sum total of natural phosphorus present in the process cheeseand phosphorus coming from the emulsifying salts). They showedthat the chances of crystal formation increased with increasingpH as well as increasing phosphorus content. The equation hasbeen shown to be effective in predicting crystal formation inprocess cheese made using various phosphate-based emulsifyingsalts (Zehren and Nusbaum 2000). Scharpf and Kichline (1968)also showed that crystal formation accelerated when the processcheese surface was directly exposed to cold air. Therefore, carefulselection of emulsifying salts during process cheese manufactureand proper storage conditions of the final process cheese are im-portant to prevent crystal development (Berger and others 1998).Manufacturing equipment such as casting lines and slice cooling



belts have also been associated with promoting crystal develop-ment in process cheese as they can act by providing nucleationsites for crystallization when not cleaned properly and regularly(Berger and others 1998).

Color defects. Browning. As discussed previously (subsection“Lactose content”), various factors influence browning (Maillardreaction) in process cheese. Browning in process cheese is initi-ated when ingredients with high lactose content are used duringprocess cheese manufacture (Thomas 1969, 1973). Additionally,high process cheese final pH (> 5.9) (Thomas 1969) and highstorage temperatures (35 to 37 ◦C) (Thomas 1969; Kristensen andothers 2001) have been found to accelerate browning in processcheese. Various natural cheese compositional factors also have aninfluence on the browning of process cheese manufactured fromit. Bley and others (1985) found a high correlation between thesalt-to-moisture ratio as well as the levels of residual galactose andlactose of cheddar cheese on the browning of the process cheesemanufactured from it. Therefore, careful selection of ingredients

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Figure 5 --- Effect of mixing speedon the microstructural attributes ofPCF manufactured with 2.5%trisodium citrate.

and optimum final pH during process cheese manufacture, alongwith desirable postmanufacture storage conditions, needs to beadhered to in order to prevent browning defects in the processcheese.

Pink discoloration. Zehren and Nusbaum (2000) indicated thepresence of pink discoloration in process cheeses that were ei-ther artificially colored with annatto or used a natural cheese,which had annatto color added to it. They further indicated thatuse of alkaline extracts of annatto in process cheese showed ahigher propensity to cause pink discoloration in process cheese.Shumaker and Wendorff (1998) evaluated the effect of process-ing temperature, type and amount of emulsifying salt, amountof colored natural cheese in the process cheese blends, and thetype of annatto colorant used on pink discoloration of processcheese. They found that with an increase in the amount of col-ored natural cheese in the blend there was an increase in the pinkdiscoloration of the resulting process cheese. They also found thatwhen the ratio of aged cheese (uncolored) was increased in the



natural cheese blend (uncolored) during process cheese manu-facture with added annatto colorant, there was an increase in thepink discoloration. Shumaker and Wendorff (1998) also foundthat annatto emulsion-based colorants were more susceptible topink discoloration than annatto suspension-based colorants.

Functional defects and defects in body and textureFunctional defects of a process cheese are not defects in the true

sense. Functional defects in a process cheese can be referred toas the inability of the process cheese to demonstrate and performthe desired functional properties and end-use behavior for whichit has been manufactured. These can range from very high to verylow meltability, very high to very low firmness, presence of stick-iness, and so on. These properties can be adjusted to the desiredlevels by appropriate control of various formulation and process-ing parameters in process cheese (section “Factors controllingprocess cheese properties”). In addition to these functional de-fects, process cheese is also prone to defects in body and texture,including brittle, crumbly, and grainy texture, and oil separation.

Brittle, crumbly, and grainy texture. This type of defect generallyarises when the final pH of the process cheese is too low (<5.4)(Gupta and others 1984; Berger and others 1998; Shirashoji andothers 2006a). Microstructural studies by Marchesseau and oth-ers (1997) and Shirashoji and others (2006a) have indicated thatat low process cheese final pH the proteins are closer to theirisoelectric point and, hence, the net negative charge on theprotein decreases. This causes the proteins to shrink and thereare increased protein–protein interactions (due to the absenceof charge-based repulsive forces between protein molecules).Therefore, the proteins aggregate among themselves, leading toa weaker process cheese emulsion with a crumbly and grainytexture.

Oil separation. This defect arises due to improper emulsion for-mation of the cooked process cheese. Improper emulsification ofthe process cheese can occur due to a variety of reasons, includ-ing too low or too high a level of emulsifying salts (subsection“Emulsifying salts”), low final pH of process cheese (subsection“pH”), low level of intact casein in the process cheese (use ofa highly aged natural cheese in the process cheese blend) (sub-section “Intact casein content”), or inadequate or very extensiveprocessing temperature and/or time during process cheese man-ufacture (Meyer 1973).

Microbiology of Process CheeseGlass and Doyle (2005), from the Food Research Inst. (Univ.

of Wisconsin), have published an extensive review on the safetyconcerns associated with process cheese along with the variousformulation and physicochemical factors that help control thegrowth of pathogenic microorganisms in process cheese.

Causative agents (microorganisms)Process cheese shows very low susceptibility to microbial

spoilage (Warburton and others 1986; Glass and others 1998). Inspite of this, process cheese varieties have been associated withcertain microbiological safety concerns. Improper packaging andstorage of process cheese can lead to mold growth (Meyer 1973).However, the above-mentioned problem can be easily overcomeby adding mold inhibitors such as sorbates and propionates inprocess cheese (Table 1). More critical spoilage microorganismsthat can lead to microbiological safety concerns in process cheeseinclude pathogenic sporeformers such as Clostridium spp. andBacillus spp. and postpasteurization pathogenic bacteria suchas Listeria monocytogenes, Salmonella spp., Staphylococcus au-reus, and E. coli O157:H7 (Glass and Doyle 2005). Research hasindicated that the most common microorganisms associated with

process cheese are of the genus Clostridium (Kautter and others1979; Sinha and Sinha 1988; Glass and Doyle 2005). A few out-breaks involving botulism from the consumption of canned PCShave been reported over the years (Glass and Doyle 2005). Allthe PCS associated with these outbreaks were found to have highwater activity (about 0.96 to 0.97) and high pH (about 5.7 to 5.8),which might have led to the production of toxins produced byClostridium botulinum in the PCS during storage (Briozzo andothers 1983; Glass and Doyle 2005).

Factors that control microbiological spoilageCanned PCS-type products can be classified as low-acid

canned foods (21CFR113). Therefore, anaerobic sporeformerssuch as Clostridium spp. are a major concern. According to theregulations set forth by the CFR, all low-acid canned foods need tobe subjected to a process to render it commercially sterile. There-fore, these foods either need to be subjected to heat-processingso that there is a 12-log reduction in the botulinal spores or thereneeds to be appropriate formulation changes as well as adjust-ments in pH and water activity in these products in order to inhibitthe growth of microbes and toxin production. However, there is amajor undesirable effect on the microstructure and the functionalproperties of process cheese when they are heated to sterilizationtemperatures (such as 121 ◦C for 2.5 to 3 min, which is the min-imum heat process to inactivate C. botulinum spores in a food)(Glass and Doyle 2005). Therefore, appropriate formulation ad-justments during PCS manufacture, as well as appropriate controlof pH and water activity of the final PCS, have been found to in-hibit the growth, survival, and toxin production of Clostridiumspp. in PCS (Tanaka and others 1979, 1986; Somers and Taylor1987; Roberts and Zottola 1993; Eckner and others 1994; terSteeg and others 1995; ter Steeg and Cuppers 1995; Plockovaand others 1996; Loessner and others 1997; Glass and Doyle2005).

Effect of pH and water activity. Researchers have shown the im-portance of final pH and water activity of process cheese on thegrowth and toxin production of C. botulinum in process cheese(Tanaka and others 1986; ter Steeg and others 1995; ter Steeg andCuppers 1995; Glass and Doyle 2005). Water activity values ofPCS typically range from 0.94 to 0.96, which is lower than thewater activity (about 0.97) that supports the growth of nonpro-teolytic C. botulinum. Tanaka and others (1986) indicated thatthere was no botulinal toxin produced in PCS with water activ-ity of less than 0.944; however, PCS with water activity above0.957 showed toxin production. Between water activity values0.944 and 0.957, the toxin production in PCS was dependent onmoisture content, pH, NaCl concentration, and disodium phos-phate concentration. A lower pH has been found to prevent mi-crobial spoilage, toxin production (Tanaka and others 1986; terSteeg and others 1995; ter Steeg and Cuppers 1995) and has beenfound to enhance the inhibitory activity of sorbic acid, which isan allowed preservative (mold inhibitor) in PCS (Glass and Doyle2005).

Effect of emulsifying salts and NaCl. Numerous researchers haveshown the inhibitory effect of phosphate-based emulsifying saltson the growth of various microbes and their antibotulinal effectsin PCS (Tanaka and others 1979, 1986; Eckner and others 1994;ter Steeg and others 1995; ter Steeg and Cuppers 1995; Loess-ner and others 1997). Tanaka and others (1986) extensively stud-ied and modeled the influence of moisture, pH, disodium phos-phate, and NaCl level on toxin production in PCS. They preparedpredictive models involving the influence of pH and total per-centage of NaCl + disodium phosphate on toxin production inPCS with various moisture levels (51%, 52%, 54%, 56%, 58%,and 60% moisture). They found that a lower pH and higher lev-els of NaCl + disodium phosphate produced safer PCS. These



predictive models are extensively used by the industry to predictthe safety of PCS. Loessner and others (1997) showed the effec-tiveness of addition of long-chain phosphates at levels of 0.5% to1.0% to prevent the growth of C. tyrobutyricum in PCS. Trisodiumcitrate has been found to be less effective than disodium phos-phate in preventing bacterial spoilage in PCS (Tanaka and others1979; ter Steeg and others 1995).

Effect of other additives. Addition of lactic acid has been foundto prevent development of botulinal toxin in PCS (Glass andDoyle 2005), whereas the addition of 0.13% to 0.26% potas-sium sorbate (mold inhibitor/preservative are allowed for use inprocess cheese at levels of ≤ 0.2% of the final product [Table 1])has been reported to delay botulinal growth and toxin produc-tion in cured and uncured meat and poultry products (Glass andDoyle 2005). Nisin, a bacteriocin produced by certain strains ofLactococcus lactis, is an approved additive in PCS at levels be-low 250 ppm. Nisin has also been found to be effective againstgrowth of spoilage microorganisms and toxin production in PCS(Somers and Taylor 1987; Roberts and Zottola 1993; Plockovaand others 1996).

ConclusionsSince the invention of process cheese about 95 y ago, its re-

search has come a long way. Research has led to the developmentof process cheese as a food product and transformed it into one ofthe most versatile dairy products owing to its numerous end-useapplications. Numerous varieties of process cheese can be foundin the marketplace, including various forms (slices, loaves, shreds,and spreads) designed for different applications (normal melt, re-stricted melt, and so on). Additionally, numerous categories ofprocess cheese, such as reduced fat and reduced sodium, areavailable for consumers who prefer them.

Over the years, process cheese research has been multifacetedand has identified the critical factors that control the characteris-tics of process cheese and has developed various empirical, rhe-ological, and microstructural techniques that are used to evaluatethe important functional properties of process cheese. This accu-mulation of research has allowed the process cheese industry toproduce a customizable product targeted for a variety of end-useapplications.

There have, however, been certain drawbacks associated withprocess cheese research conducted to date. A significant portionof process cheese research remains as trade secrets within theindustry and, consequently, has not been published. Moreover,the majority of the published studies on different formulation andprocessing factors that influence process cheese functional prop-erties have either been performed on model systems or on prod-ucts outside the standard of identity regulations. Consequently, itis difficult to extend the results from these studies directly to PC,PCF, and PCS.

Although significant progress has been made on the mea-surement of process cheese functional properties, the availabletesting techniques still have some limitations. In general, thereare rheological-based methods that provide critical and accu-rate data, but they require expensive equipment and are time-consuming to perform. In contrast, the available empirical-basedmethods provide crude results but are simple to perform and donot require expensive equipment. Consequently, there is still anunmet need for accurate, rapid, and cost-effective techniques formeasuring process cheese functional properties. A related unmetneed is for techniques that provide real-time online data duringthe production process. In an ideal situation, online testing meth-ods would be used to measure the functional properties of pro-cess cheese during the production process and such data wouldbe used to make adjustments in the formulation or manufactur-

ing procedure in real time to ensure that process cheese with thetargeted functional properties is produced.

Additionally, the majority of process cheese research con-ducted to date has been focused on identifying the formulationand processing parameters that have an impact on process cheesefunctionality. Consequently, there is limited understanding of themolecular level interactions in process cheese and how these in-teractions are related to the physicochemical and manufacturingconditions employed in process cheese manufacture.

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