j.1750-3841.2011.02054.x

E:FoodEngineering&PhysicalProperties

Physical Properties and MicrostructuralChanges during Soaking of Individual Cornand Quinoa Breakfast FlakesWenceslao T. Medina, Andres A. de la Llera, Juan L. Condori, and Jose M. Aguilera

Abstract: The importance of breakfast cereal flakes (BCF) in Western diets deserves an understanding of changes intheir mechanical properties and microstructure that occur during soaking in a liquid (that is, milk or water) prior toconsumption. The maximum rupture force (RF) of 2 types of breakfast flaked products (BFP)corn flakes (CF) andquinoa flakes (QF)were measured directly while immersed in milk with 2% of fat content (milk 2%) or distilled waterfor different periods of time between 5 and 300 s. Under similar soaking conditions, QF presented higher RF valuesthan CF. Soaked flakes were freeze-dried and their cross section and surface examined by scanning electron microscopy.Three consecutive periods (fast, gradual, and slow reduction of RF) were associated with changes in the microstructure offlakes. These changes were more pronounced in distilled water than in milk 2%, probably because the fat and other solidsin milk become deposited on the flakes surface hindering liquid infiltration. Structural and textural modifications wereprimarily ascribable to the plasticizing effect of water that softened the carbohydrate/protein matrix, inducing partialcollapse of the porous structure and eventually disintegration of the whole piece through deep cracks.

Keywords: breakfast foods, microstructure, milk soaking, quinoa

IntroductionRolled, extruded, and puffed cereals are some of the most com-

monly consumed breakfast foods. Their nutritional value and af-fordable price make them a popular food product: as reported bySchwartz and others (2008), between one quarter and one-halfof American children ages 4 to 18 y regularly consume cereal forbreakfast.Despite that wheat and maize flour are the most commonly used

raw materials (Guy and others 2001), other grains are beginning tobe used, such as the Andean quinoa with the objective to improvethe nutritional value of extruded flakes.Quinoa (Chenopodium quinoa Willd) is a pseudocereal origi-

nated in the Andean region near Lake Titicaca in Peru and Bo-livia (Jacobsen and Mujica 2002). Since 3000 BC, quinoa seedshave served as a chief food source for the Andean population be-cause of its high nutritional value (Ruales and Nair 1992; De laTorre-Herrera 2003). Today, the main producers and exporters ofquinoa seeds in the world are Bolivia and Peru, although seedsare cultivated in other areas, including the savannah near Bogotain Colombia and in the Ecuadorian central mountain region. Itscultivation in Peru is mainly confined to 3 agro-climatic areas,namely inter-Andean valleys and highlands, arid mountain zones,

MS 20100981 Submitted 9/1/2010, Accepted 1/1/2011. Authors Medina andAguilera are with Dept. of Chemical and Bioprocesses Engineering, Pontificia Univ.Catolica de Chile, P.O. Box 306, Santiago, Chile. Author de la Llera is with Schoolof Engineering and Applied Sciences, Faculty of Arts and Sciences, Harvard Univ., 29Oxford St., Cambridge, MA 02138, U.S.A. Author Condori is with El AltiplanoSAC Co., Taparachi Industrial Park D-15, Juliaca, San Roman, Puno-Peru. AuthorMedina is also with School of Agroindustries, Faculty of Agrarian Sciences, Univ.Nacional del Altiplano de Puno, P.O. Box 291, Puno-Peru. Direct inquiries to authorMedina (E-mail: [email protected]).

and coastal regions (Jacobsen and Mujica 2002). European coun-tries, such as Spain, are studying its adaptation to Mediterraneanclimates, and others, such as Denmark, Finland, and England areinterested in its cultivation (Vilche and others 2003).Quinoa seeds are processed in the same way as rice and wheat

by grinding seeds into flour to prepare bread, cakes, extrudedproducts, and fermented drinks (Ando and others 2002; Repo-Carrasco and others 2003; Bhargava and others 2006). The nu-tritional value of the quinoa seeds, however, is superior to thatof other cereal seeds (Berdanier and others 2008). Quinoa seedshave a greater protein content (15 g/100 g) than that of cerealseeds, such as wheat (12 g/100 g), barley (10 g/100 g), corn (9.4g/100 g), or polished rice (8.6 g/100 g) (Coulter and Lorenz 1990;Fleming and Galwey 1995; Galwey 1995). They have a rich car-bohydrate content in the order of 69 g/100 g (Sungsoo and others1999) and are particularly rich in essential amino acidsespeciallylysine, tryptophan, and cysteine (Ruales and Nair 1992)andminerals, such as Ca, Fe, Mn, Mg, Cu, and K (Konishi and others2004).Most consumption methods of breakfast cereal flakes (BCF) in-

volve mixing the flakes with milk of varying fat contents. Reportson frequency of food consumption of the 3rd Natl. Health andNutrition Examination Survey in the United States of Americaindicate that consumers eat breakfast cereals mixed with cold milkand hot milk approximately 8.7 and 4.3 times per week, respec-tively (Ganji and Kafai 2004). In both conditions, liquid uptakeby breakfast cereals in the bowl is a relevant factor for consump-tion and acceptability, as it influences the texture and integrityof flakes. These changes could be expressed as a reduction in theforce needed to disintegrate the flake as the soaking proceeds, achange that may be ascribable to alterations of its microstructure.As a pseudocereal, quinoa could not be formally included in

the group of breakfast cereal. For this reason, quinoa flakes (QF)

C 2011 Institute of Food Technologists RE254 Journal of Food Science Vol. 76, Nr. 3, 2011 doi: 10.1111/j.1750-3841.2011.02054.x

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Breakfast flakes microstructure and physical properties . . .

and corn flakes (CF) will be referred as breakfast flaked products(BFP) when compared to BCF.A BCF is solid foam consisting of a carbohydrate/protein ma-

trix and entrapped air cells and its mechanical properties dependon compositional, physical, and processing parameters leading tothe arrangement of molecules and elements into distinct micro andmacro structures (Aguilera 2005). Concomitant with the uptake ofliquid by a BCF is a softening of the carbohydrate/protein matrixinduced by the plasticizing effect of water. This process has beendescribed by Nelson and Labuza (1993) as a transition from a glassybrittle state to a soft rubbery state that reduces the mechanical re-sistance. According to Pittia and Sacchetti (2008), intermolecularinteractions in the flakes matrix could be weakened by the plasti-cizer, leading to the solubilization of some components, and to adecrease in mechanical integrity.Although experimenting with individual cereal flakes or flaked

products is tedious and subject to their intrinsic variability, it isa better way to understand the liquid uptake phenomenon thanusing bulk samples (Nuebel and Peleg 1993). Working on indi-vidual samples minimizes flake-to-flake interactions and providea more uniform exposure to the liquid that is relevant when mi-crostructural analysis is to be performed later. The approach ofanalyzing individual food units in spite of their structural variabil-ity has been successfully validated in deep-fat frying of potato stripswhere the relative contributions of surface and structural oil to to-tal oil uptake during frying are now well documented (Bouchonand Aguilera 2001). Medina and others (2010) studied the uptakeof tritiated liquids by individual BCF showing that measurementsin bulk overestimate the amount of liquid absorbed by the massof flakes. Most of the difference corresponds to interstitial liquidoccluded between the flakes that do not drain before weighing themass of flakes. Mechanical testing of individual flakes measures theactual force needed to produce the fracture facilitating the under-standing of a relationship between mechanical properties and theirmicrostructure.The objective of this study was to assess the changes in mechan-

ical properties (that is, maximum rupture force) of QF and CF,a common breakfast cereal, during soaking in water and in milkand to analyze the microstructure of original and soaked flakes.In turn, these microstructural changes were to be associated tochanges in the flakes mechanical properties through the analysisof their initial cross-section porosity and the maximum ruptureforce-soaking time curves.

Material and Methods

MaterialsTwo types of BFP were employed in this study: CF (purchased

from a local supermarket) and QF produced in Altiplano SAC Co.in Caracoto, JuliacaPuno, Peru. According to themanufacturers,the main ingredients were milled corn, sugar, malt favoring, andhigh fructose corn syrup for CF and pearled quinoa, sugar, pow-dered milk, carob syrup, and vegetable oil for QF. Liquid mediafor soaking included distilled water and milk (Soprole, Santiago,Chile) with 20 g/L of fat content (milk 2%) prepared by mixingwhole (31 g/L fat content) and skim milk (15 g/L fat content).QF were produced by single screw extrusion (die tempera-

tures between 156 and 160 C, screw speed between 350 and400 rpm, die pressure of 121.6 kPa). The extruded pellets wereflaked between 2 rollers and sweetened with a sugar-based syrupin a tumbling mixer, dried to remove excess moisture, and finallystored in high-density polypropylene bags. The sweetening step

is a common process in quinoa flake elaboration to improve theacceptability of product for consumers (Repo and Kameko 2004).QF used in this study, unlike CF, were effectively sugar coated.

Chemical analysisTable 1 shows the results of a proximal chemical analysis of

QF and CF. Crude protein content was determined by the Kjel-dahl method using 6.25 as the conversion factor (AOAC, 955.041995), the lipid content was determined by the petroleum etherextraction method (AOAC, 945.16 1995), the ash content was de-termined by the dry-ashing method (AOAC, 923.03 1995), andthe carbohydrate content was calculated by the difference method,that is, subtracting the percent crude protein, crude fiber, crudefat, and ash from 100% dry matter.Chemical analysis shows that QF had higher protein content

than CF, while CF had greater carbohydrate content than QF.

Moisture content, surface area, and weight of flakesIn order to reduce the variability among flakes, 130 individual

CF and QF of similar surface area were carefully selected, weighed,and photographed. The area of one surface of dry flakes (referredto as top surface) was determined by image acquisition and imageprocessing. A CCD color camera (Cool Snap Pro Color, Photo-metrics Roper Div., Inc., Tucson, Ariz., U.S.A.) with a resolutionof 1392 1040 pixels was used to record images. Samples wereilluminated by a standard ring light (2.95, Advanced illumina-tion, Inc., Rochester, Vt., U.S.A.). The camera was mounted ona stable support over a base of 28 33 cm. All components ofthe image acquisition system were placed inside a black chamber.The top surface area of each flake was calculated from the digitalimages using a program developed in MatLab 6.0 R (Mathworks,Inc., Natick, Mass., U.S.A.).The original weight of all individual flakes was determined with

an analytical balance Sartorius (type 1465, Goettingen, Germany)with a precision of 1 mg. The initial moisture of flakes was deter-mined in a Sartorius moisture balance (type WMA6005-f94121AG, Goettingen) at 130 C until constant weight and expressed aspercent of dry basis (% db) (ISO 1985).

Initial cross-section porosity (es), cell-wall material density(S), and density of flake (f )Initial cross-section porosity of flakes was determined by image

analysis of samples of each type of flake fixed in chemical solutionsand embedded in paraffin (Varela and others 2008). For prepara-tion, samples were fixed in a solution of 5% formaldehyde, 5%acetic acid, and 90% ethanol 70% during 48 h at room temper-ature. Samples were then dehydrated in a graded water/ethanolseries (50%, 70%, 95%, and 100%) and embedded in paraffin.Three 18 m-thick microslices were obtained with a Jung Kmanual microtome (Jung Co., Heidelberg, Germany), collected

Table 1Proximal analysis of CF and QF components (d.b.%).

Breakfast flaked product (BFP)

Corn flakes Quinoa flakes(CF) (QF)

Crude protein 6.27 0.04a 9.64 0.08bOil 0 0a 5.87 0bCrude fiber 0.21 0.06a 1.26 0.08bAsh 2.09 0.01a 1.99 0.01aCarbohydrate 91.43 0.06a 81.24 0.13bDifferent letters in each row indicate a statistically significant difference (P < 0.05) amongthe values (Fishers LSD).

Vol. 76, Nr. 3, 2011 Journal of Food Science E255



on a clean glass slide and treated with an albumin/glycerin so-lution to aid adhesion. Slides were placed on a warming tray(40 C, 3 h) to allow the microslices to flatten, the water to evap-orate and the microslices to adhere to the slide. The microsliceswere deparaffinized in xylene (3 baths, 10 min each bath) andhydrated using an ethanol series (100%, 95%, 70%, and 50%,5 min each step, ending with water for 1 min). The microsliceswere stained with a safranin O solution during 30 min (0.5 gsafranin, 50 mL ethanol at 95%, and 50 mL distilled water) andthen washed in water to remove all excess of stain from the tissue.Subsequently, a dehydration process in ethanol (50%, 70%, 95%,and 100%, 5 min each step) was applied. Finally, flake microslices

were stained for 3 min with fast green (0.1 g fast green, 10 mLabsolute ethanol, and 10 mL eugenol). To improve the color con-trast, the section tissues were washed twice (10 min each) with aeugenol solution (50 mL eugenol, 25 mL absolute ethanol, and25 mL xylene). Sections were finally cleared in 3 changes of 100%xylene (10 min each) and air dried. A coverslip was mounted withpermount to make a permanent preparation. This stain methodwas conducted for the superficial and transversal flake porositydetermination.Photomicrographs were taken using a CCD color camera (Cool

Snap Pro Color) with a resolution of 1392 1040 pixels mountedon a stereomicroscope (Olympus SZX7, Optical Co. Ltd., Tokyo,

Figure 1Scheme of the procedure used fordetermination of the cross-section porosity (es).

Figure 2Experimental setup of penetrating probe to test flake mechanical properties.

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Japan). The magnification used was of 2 with a field of view of3.2 2.4 cm. Two slices per treatment were analyzed performinga scanning through of all samples. Color images were transformedfrom an RGB format to a black and white format using a mixture-modeling-based segmentation procedure reported by Demirkayaand others (2009). The threshold value was generated accordingto the results of the histogram analysis and remained constant.As seen in the white and black format of the processed image(see Figure 1), the large contrast between the white backgroundand the black object made image segmentation easier. The ratiobetween the total area of air cells to the cross section area wasdefined as the cross-section porosity (es) (Equation 1).

e s = Total pore areaCross or trasnversal section area . (1)

The cell-wall material density of a flake (S in g cm3) wasdetermined with a Helium picnometer (Accupyc 1330 series N2441, Micrometrics Instruments Inc., Norcross, Ga., U.S.A.) witha volume module of 12.03 cm3 and employing approximately2.90 g of QF and approximately 1.27 g of CF. The density ofthe flake ( f in g cm3) was determined through the relation-ship between the individual weight and its volume (cm3). Vol-ume was determined by multiplying the average thickness of theindividual flake by its top surface area. Average thickness wasobtained by measuring thickness of individual flake in 4 differ-ent points of samples using a vernier caliper with 0.01 mm ofaccuracy.

From soaking process to microstructure analysisMicrostructure analysis was conducted on a set of 26 flakes of

each type of BFP that underwent a controlled soaking process.Each individual flake was placed in a 10-cm3 tube with perforatedbase that was located in a cup of 1 aluminum tray, where 15 cm3

of either milk or water had been previously added. The tray of4 cups was placed in a stainless steel vessel (26.5 I19.4I3.5 cm)containing cool water. The temperature of the liquid medium wasmaintained constant at 5 0.2 C by circulating a refrigerant fluid

through the jacket in the bottom and sides of the stainless steelvessel holding the tray and samples. In each run, 4 flakes (2 QFand 2 CF) were simultaneously soaked and removed sequentiallyat various time intervals: 5, 10, 15, 30, 45, 60, 90, 120, 150,180,240, and 300 s.

Scanning electron microscopy (SEM)Each type of flake was placed into a ceramic bowl and liquid

nitrogen was poured on top to rapidly freeze the sample, thusminimizing adverse effects on the microstructure. Samples werestored in an ultralow freezer (Nuaire, Glacier 9668, Plymouth,Minn., U.S.A.) at 86 C until freeze-dried in a LABCONCOfreeze dryer (FreeZone Plus 4.5 Liters, Kansas City, Mo., U.S.A.)for 12 h and later photographed with the scanning electronicmicroscope (SEM). This procedure was repeated twice for eachtype of flake and time considered.The surface and internal structure of CF and QF soaked for

different times were observed using a scanning microscope modelJSM-5300 (SEM JEOL, Tokyo, Japan). The cross sections of flakeswere exposed by free fracture and observed simultaneously withthe outer surface. Flake pieces were mounted onto SEM stubsusing especial resins, sputter-coated with gold-palladium, andobserved under a voltage of 30 kV. At least 2 specimens wereexamined for each type of flake. Micrographs were taken at mag-nifications of 50 , 75 , and 100 , and selected images arereported.

Measuring flake maximum rupture force (RF)A set of 65 flakes of each type of BFP were employed in the

determination of flake maximum rupture force (RF) as a functionof soaking time. RF is the maximum force value registered in theforce-time diagram obtained during texture analysis. Mechanicalproperties were evaluated in 5 individual flakes at 13 selectedsoaking times (0, 5, 10, 15, 30, 45, 60, 90, 120, 150, 180, 240,and 300 s) in a TA-TX2 Texture Analyzer (Stable Micro Systems,Haslemere, Surrey, U.K.). Force was applied while the flake wasresting and immersed in the test liquid on a specially constructeddevice with a constant liquid temperature of 5 0.2 C, obtained

Weight (g)0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45

Surfa

ce a

rea

(cm2 )

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4QFCF

Figure 3Top surface area and weightrelationship of 130 selected flakes of cornflakes (CF) and quinoa flakes (QF).




by circulating a refrigerant fluid through its jacket (Figure 2).Cold temperature was chosen in light of the high frequency ofcold breakfast cereal consumption (Ganji and Kafai 2004). Thepenetrating device consisted of 9 2.00-mm dia pins uniformlyattached to a circular plate (2.5 cm in diameter) and was used tokeep the flake submerged in the liquid for a given soaking time.Then, the device descended at 0.5 mm/s (speed) so that the pinspenetrated the flake while advancing through holes in the bottom(see Figure 2).The equipment was calibrated with a load cell of 5 kg. Since

the compression force was applied with the 9 plungers acting onthe top surface of each flake, the relation between flake weightand surface area was considered as normalization criteria for RFand was expressed as newtons per unit of weight per area of flake(N g1 cm2).

Table 2Physical features of CF and QF. Values in parenthesisare the coefficients of variation (CV).

Breakfast flaked products (BFP)


Moisture content (d.b.%) 4.78 0.05 (0.01)a 3.01 0.08 (0.03)bTop surface area (cm2) 2.295 0.305 (0.133)a 2.497 0.221 (0.089)bWeight (g) 0.120 0.016 (0.133)a 0.321 0.036 (0.112)bDifferent letters in each row indicate a statistically significant difference (P < 0.05) amongthe values (Fishers LSD).

Table 3Initial cross-section porosity (es), cell-wall material den-sity (S), density of flake ( f ), thickness, and initial RF of bothBFP.

Breakfast flaked products (BFP)


Initial cross-section porosity 0.37 0.05a 0.28 0.01bCell-wall material density (g cm3) 1.200 0.014a 1.385 0.010bDensity of flake (g cm3) 0.57 0.08a 0.74 0.09bThickness (mm) 1.03 0.14a 2.14 0.31bInitial RF (N g1 cm2) 245.41 50.18a 603.74 114.17bDifferent letters in each row indicate a statistically significant difference (P < 0.05) amongthe values (Fishers LSD).

Mathematical fitting of RFThe Equation proposed by Vega-Galvez and others (2009) for

the rehydration of aloe leaves was adapted to fit the RF valueswith soaking time (Equation 2):

RF = exp((1 + t )a

b

), (2)

where RF is the maximum rupture force of flakes at time t, anda and b are constants. The fitting process was carried out usingthe function tool cftool in MatLab 6.0 R (Mathworks, Inc., Natick,Mass., U.S.A.).

Statistical analysisA 1-way analysis of variance was performed on top surface area,

weight, moisture content, initial es, S, f , thickness, and initialRF of both BFP using the Statgraphics 4.0 package (Bitstream,Cambridge, Mass., U.S.A.), to assess differences between thosecharacteristics and properties at P < 0.05.The goodness-of-fit of the adapted equation used to model

the experimental data was evaluated by means of coefficient ofdetermination (R2) and root-mean square (RMS) values, whichwere calculated as shown in Equation 3.

RMS = 1

N

N1

(Vo Va

Vo

)2, (3)

where N is the number of data points, Vo is the observed value,and Va is the adjusted value.

Results and Discussion

Moisture content, top surface area, and weightdeterminationsInitial average moisture content, average top surface area, and

average weight of CF and QF are shown in Table 2. Measurementsof top surface area and weight with coefficients of variation (CV)

Soaking time (s)0 30 60 90 120 150 180 210 240 270 300

RF (N

g-

1 cm

2 )

0

50

100

150

200

250

300

350

400

450

500

550

600

650CF WaterCF MilkQF WaterQF Milk

Figure 4Experimental and fitted rupture force(RF) curves compared with soaking time forquinoa flakes (QF) and corn flakes (CF) indistilled water (W) and milk 2% (M). Dashedlines and continuous lines correspond to thefitting of experimental data for M and W,respectively, with the Vega-Galvez modifiedequation.


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values under 0.15 reflect the relative dispersion of the BFP samplesselected (Ramrez and others 2009) and are about half of those forindividual flakes chosen randomly from commercial samples. Ascatter plot of top surface area and weight for all samples used inthe study are shown in Figure 3; even after visual selection, therewas some variability in size and weight of flakes used.Significant differences (P < 0.05) were found in moisture con-

tent, top surface area, and weight between 2 BFP. The initialaverage moisture content of all flakes was in a range expected forcommercial breakfast cereals (Bailey and others 1995; Sacchettiand others 2005). QF had a larger average top surface area andaverage weight than CF.

Initial cross-section porosity (es), cell-wall material density(S), density of flake (f )The initial cross-section porosity (es), cell-wall material density

(S), density of flake ( f ), flakes thickness, and initial rupture force(RF) of both BFP are shown in Table 3. Significant differences(P < 0.05) were found in all indicated properties between bothproducts. QF samples exhibited a lower es value (0.28) than CF(0.37) probably because of differences in compaction of its solid

Table 4Empirical model parameters and statistical goodness.

Vega-Galvez adapted modelparameters

Flake-liquid aa B (g cm2 N1)b R2 RMS

CFmilk 0.04025 0.1838 0.91 0.11CFwater 0.04427 0.1835 0.94 0.15QFmilk 0.0268 0.1572 0.93 0.08QFwater 0.03176 0.1571 0.92 0.10aa = dimensionless parameter; bB = (g cm2 N1)CF = corn flakes; QF = quinoa flakes.

matrix and shape of air cells: these were elongated in QF andalmost round in CF. The relative high es value of CF could berelated to its low f (0.57 g cm3) as foam structures with alarge proportion of air cells are associated with low values of solidmaterial per unit of volume. A high f value for QF was probablythe result of sucrose as an ingredient of the product (Ryu andothers 1993; Barrett and others 1995; Fan and others 1996). QFhad a larger S value and flake thickness (1.39 g cm3, 2.1 mm,respectively) than CF (1.20 g cm3, 1 mm, respectively).The higher initial RF value of QF compared to that of CF

could reflect the differences in matrix structure of the flakes. Ingeneral, CF had thinner cell walls than QF because corn flour wasthe main ingredient in its formulation. Singh and others (2009)reported that when snacks are produced only with corn flour, theircell walls are very thin.In some manner, RF could be the measure of the strength of cell

wall and affect its texture and crispness (Chen and others 2010).Dogan and Karwe (2003) indicated that chemical changes duringquinoa extrusion cooking influencing the development of textureand mechanical properties and that breaking strength is affectedby strength of cell wall and are influenced by starch gelatinizationand protein denaturation. Furthermore, the same authors reportedthat quinoa extruded products with smaller pore size had strongerstructures.

Variation of RF with soaking timeThe curves of RF compared with soaking time for both BFP

are shown in Figure 4. Experimental and fitted curves could bedivided into 3 periods of RF reduction with soaking time. In the1st period (initial 30 s), a drastic reduction in RF occurred; a 2ndperiod between 30 and 60 s exhibited a gradual transition to the3rd period where, after 90 s, RF decreased only slightly.

Figure 5SEM photomicrographs of corn flakes (CF) and quinoa flakes (QF) prior to soaking. The cross section and surface of both breakfast flakedproduct (BFP) are shown in the left and right sides, respectively. Features: external surface (ES); large air cell (LAC); closed air cell (CAC); dense solidmatrix (DSM); and surface pore (SP).




The maximum RF values were observed at time zero (that is,dry flakes prior to soaking in a liquid medium). The initial RF forQF was larger (603.74 N g1 cm2) than that of CF (approximately245 N g1 cm2). The RF values for CF were within normal limitswhen compared to those reported for corn flakes by Sacchettiand others (2003), Takeuchi and others (2005), and Gondek andLewicki (2006). RF data for QF are not reported previously inliterature.As the soaking time increased, the liquid uptake by the car-

bohydrate/protein matrix caused the loss of the initial hard andbrittle texture and a reduction in RF. Curves of RF comparedwith soaking time for CF and QF exhibited a similar trend, asshown in Figure 4. During the first 30 s of soaking, samples ex-perienced a rapid drop in their RF. In CF, this drastic decreasein rupture force occurred mostly in the first 10 s, reaching a rel-

ative minimum of 120 N g1 cm2 in milk 2% and 115 N g1cm2 in distilled water. After this strong RF reduction, there was aslight increase in rupture force values in the form of a shoulder inthe smoothly decreasing RF curve. This increase occurs in QF at45 s and between 10 and 30 s for CF. According to Bourne (1982)it could be a measure of the force needed to extrude the samplethrough the holes situated at the bottom of the anchored support.Lewicki and others (2004) explain this increase in force as a resultof partial plasticization of walls of air cells, which may increase thecohesion between the pins of the device and the flakes materialand a greater force for fracture. After passing these relative maxi-mums, all curves tapered off gently until reaching a fairly constantvalue as the soaking time approached 300 s. The final gradualreduction in RF is associated to the disintegration of the soft wetflakes.

Figure 6Cross section and surface SEM photomicrographs of corn flakes (CF) and quinoa flakes (QF) after soaking. (A1) CF cross section after 5 s inwater; (A2) CF cross section after 5 s in milk; (A3) CF cross section after 90 s in water; (A4) CF cross section after 90 s in milk; (A5) CF cross sectionafter 300 s in water; (A6) CF cross section after 300 s in milk. (B1) QF cross section after 5 s in water; (B2) QF cross section after 5 s in milk; (B3) QFcross section after 90 s in water; (B4) QF cross section after 90 s in milk; (B5) QF cross section after 300 s in water; (B6) QF cross section after 300 s inmilk. (C1) CF surface after 5 s in water; (C2) CF surface after 5 s in milk; (C3) CF surface after 90 s in water; (C4) CF surface after 90 s in milk; (C5) CFsurface after 300 s in water; (C6) CF surface after 300 s in milk. (D1) QF surface after 5 s in water; (D2) QF surface after 5 s in milk; (D3) QF surfaceafter 90 s in water; (D4) QF surface after 90 s in milk; (D5) QF surface after 300 s in water; (D6) QF surface after 300 s in milk. Features: externalsurface (ES); large air cell (LAC); closed air cell (CAC); dense solid matrix (DSM); and surface pore (SP). (Continued)


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These experimental results concur with what is reported in theliterature. Takeuchi and others (2005) studied textural features of3 commercial breakfast cereals by soaking flakes in whole milk(3.25% fat) and determining maximum rupture force in bulk witha texture analyzer. Maximum rupture force of cereals type A (cornstarch flake), B (rice starch flake), and C (wheat starch flake) weremeasured in the proportion of 10 g of cereals for every 100 g ofmilk 2%. Samples of cereals A, B, and C had a maximum ruptureforce before plasticization of 43, 22, and 118 N and of 22, 4, and104 N after plasticization, respectively. Prior to normalization bysurface area and weight, the maximum rupture force of QF andCF occurred before liquid immersion and were 91.5 and 12.9N, respectively. For flakes soaked in water, the lower maximumrupture force for QF (33.6 N) and CF (2.4 N) occurred after 180and 300 s of immersion, respectively. With flakes soaked in milk,the lower maximum rupture force for QF (34.5 N) and CF (3.4N) occurred after 45 and 180 s of immersion, accordingly.A similar pattern had been observed in the behavior of me-

chanical properties of cereals studied individually and in bulk. Asthe time of immersion increased, the matrix softened and became

plasticized. A fast reduction in rupture force was observed duringthe first seconds of immersion and after this period the mechanicalproperties of the flakes remained fairly constant probably becauseflakes had reached their maximum hydration capacity (Sacchettiand others 2003; Martnez-Navarrete and others 2004; Takeuchiand others 2005). Takeuchi and others (2005) explained that therelationship between textural crispness (that is, rupture force) andwater activity (that is, related to soaking time) is sigmoid and maybe studied in terms of 3 regions, each one described by a straightline. These regions represent chronologically the mechanical prop-erties of the BFP before, during, and after the plasticization of thecarbohydrate/protein matrix.Modeling the effect of soaking on the mechanical properties

of BFP is difficult due to the heterogeneous structure of thesesolid foams, the many mechanisms of liquid uptake that may takeplace at 1 time, for example, diffusion, capillary imbibition, andconvection through air cells (Shittu and others 2004; Planinicand others 2005), and the unknown effects of water plasticizationon the product structure. Then, the empirical model of Vega-Galvez and others (2009), was adopted to fit the flake rupture

Figure 6Continued




force-soaking time data. The different parameters of the equationand goodness-of-fit of the model are shown in Table 4.

R2 provides a measure of how well outcomes are likely to bepredicted by the model, where a value of 1 suggests a perfect fit.R2 values for the 4 cases considered in Table 4 were above 0.9.The RMS is a frequently used measure of the differences betweenvalues predicted by a model (the adapted Vega-Galvez model, inthis case) and the set of data (specifically, the RF values for QFand CF). Small RMS values (that is, close to zero) suggest that themodel closely fitted the data and vice versa.Data and models showed that the RF for BFP was lower when

the flake was immersed in water than in milk. Sacchetti and others(2003) and Medina and others (2010) have suggested that duringimmersion in whole milk it is possible that a layer of lipids andcasein micelles is deposited on the flakes surface, which hindersliquid transfer into the interior, thus, retarding softening of thematrix. In the case of QF, it is suggested that the sugar coatingis probably responsible for the slower transition during the initialpart of the soaking period as it occurred with the quantificationof milk absorption in frosted flakes (Medina and others 2010).

Considering that texture is the sensory and functional mani-festation of the structural, mechanical, and surface properties offoods detected through the senses of vision, hearing, touch, andkinesthetic (Szczesniak 2002), it is important to study the texturalperception of BFP by understanding the dynamics of the productin the consumers mouth as determined by their initial structure(Szczesniak 2002; Lenfant and others 2009). This could be helpfulin the formulation or process evaluation of specific targeted andinnovative textures.Sensorial firmness could be related to textural parameters ex-

tracted from force-deformation curves. Ravi and others (2007)found a correlation coefficient of 0.66 (P 0.001) between sen-sorial firmness and mechanical properties when values of RF tocorn balls were between 13.1 to 14.1 N and between 26.8 to 65.9N for puffed rice. Probably one acceptable correlation of mechan-ical properties with CF and QF sensorial firmness will be founddeveloping one adequate methodology taking into account thatnonnormalized RF for CF and QF are 12.9 and 91.5 N at thebeginning of the soaking process and 3.5 and 34.5 N at the end,respectively.

Figure 6Continued


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Microstructural examinationMicrographs of the surface and cross section of flakes prior to

soaking are shown in Figure 5. The cross-sections of CF revealedan internal porous structure composed of several rounded air cellsof different dimensions as reported also by Gondek and Lewicki(2006). Some large air cells protruded almost into the surface andwere separated from it by a thin layer while other smaller air cellswere embedded within a thick and dense solid matrix. Air cellsseemed to be distributed within this continuous matrix. Air cellsin QF were elongated in shape, possibly due to the rolling effectafter extrusion, giving the impression that the matrix was formedby stacked layers. The air cells were reduced in number com-pared to CF consistent with the higher density exhibited by theseflakes. However, in both types of BFP closed and interconnectedair cells were apparent. CF exhibited a surface with rugosity ap-preciable at least at 2 scalesseveral hundred microns and a fewmicrons, where a few small pores and fissures were present.QF displayed a continuous surface with high rugosity, crackedin appearance, yet lacking any pore openings of significant size(possibly covered by the sugar coating). Protrusions on the sur-face of QF were more evident, sharper, and larger than thosein CF.

Microstructure and RF relationshipsRapid freezing and freeze-drying is a good method to observe

microstructural changes in flakes that have been soaked. Soakingin water is advantageous since it precludes observing the depositsof milk solids on the surface although it exposes flakes to a largerproportion of water. The rapid initial decrease in RF suggests thata fast plasticization of the matrix occurs by liquid uptake throughpores and cracks on the outer surface of the flakes. Figure 6 showscross section and surface photomicrographs of freeze-dried flakesof both BFP at 5, 90, and 300 s of soaking and in both liquid media.It is possible to observe how their surfaces and internal microstruc-tures changed according to the time and liquid employed. At shorttimes (5 s), the surface and inner structures show minor structuralchanges (compare with Figure 6), although a considerable drop inRF had occurred (Figure 5). This means that the liquid rapidlyimbibed and softened the matrix, although the overall flake struc-ture was still preserved (Figure 6, A1, A2, B1, B2, C1, C2, D1,and D2). Some surface pores are evident particularly in the water-soaked samples (Figure 6, C3 and D3) that may become exposedby dissolution of soluble material. Along the soaking process, itwas possible to observe a progressive disintegration of the densesolid matrix from the outer layers to the inner portions. After 90 s

Figure 6Continued




of soaking pores and cracks become evident on the surface offlakes (Figure 6, C5, C6, D5, and D6). Air cells become obliter-ated toward the end of soaking and the matrix exhibited signs ofcollapse and aggregation (Figure 6, A3, A5, A6, B3, B5, and B6).Changes on the flakes surfaces are more notorious, especially atthe end of soaking process where large and deep cracks revealeddisintegration of the piece (more evident for flakes immersed inwater than for those immersed in milk 2%) (Figure 6, C5, C6,D5, and D6).Several mechanisms have been proposed to interpret the ki-

netics of liquid uptake by food and biological materials. Duringhydration of grains and in the presence of a dense cellular struc-ture, it appears that Fickean diffusion predominates throughoutthe process (Engels and others 1986; Deshpande and others 1994;Calzetta Resio and others 2005). Weerts and others (2003) haveproposed that capillary flow may prevail during the rehydration ofporous freeze-dried tea leaves. Takahashi and others (1997) haveshown that water imbibition into a fibrous matrix is 1st drivenby capillary flow and then followed by liquid diffusion into thefibers. Other authors suggest that rehydration may depend on thedegree of swelling or structural disruption of the products matrix(Krokida and Marinos-Kouris 2003; Saguy and others 2005). Mi-crostructural evidence in Figure 6 suggests that at long times ofsoaking the protein/carbohydrate matrix undergoes a continuoussoftening (perhaps including dissolution of soluble components)by imbibition of the liquid and eventually it collapses, leading tothe disintegration of the flake structure.

ConclusionsReduction in the RF of selected BFP with soaking time in

distilled water and milk with 2% fat was related to changes inmicrostructural features. Superficial liquid absorption softened thecarbohydrate/protein matrix reducing the initial mechanical prop-erties of flakes. This reduction in RF occurred apparently in 3periods. First, superficial liquid imbibitions and liquid infiltrationsthrough surface pores or defects of flakes cause a drastic reductionof RF, occurring primarily in the first 30 s of soaking. SEMmicro-graphs suggest that changes in the flakes structure occur mostlyat the surface. A 2nd period of RF reduction is more gradualand occurred in the next 50 s. It is surmised that as the internalair cells became filled with liquid the thin walls separating themstarted to collapse forming large irregular cavities. In a 3rd period,the reduction in RF was minimal and the whole flake started todisintegrate along deep cracks.

AcknowledgmentsFinancing from CONICYT-Chile under fellowship support

Doctoral Thesis N 24091107 to WTME and DICTUC underproject 40030 = 5180125 is appreciated. Also WTME acknowl-edges support by CONICYT-Chile for his doctoral fellowship.We also thank the collaboration of Miss Carolina Daz, under-graduate student of UTEM Chile in practice with DICTUC andare thankful for Dr. Cristian Ramrezs comments.

NomenclatureBCF Breakfast cereal flakesBFP Breakfast flaked productsCV Coefficient of variationCF Corn flakesQF Quinoa flakesRF Maximum rupture force

RMS Root-mean squareW Distilled waterM Milk 2%

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