Colloids and Surfaces B: Biointerfaces 42 (2005) 1–8

Melting characteristics of fat present on the surface of industrialspray-dried dairy powders

Esther H. -J. Kima, Xiao Dong Chena, ∗, David Pearceb

a Department of Chemical and Materials Engineering, The University of Auckland, Private Bag 92019, Auckland, New Zealandb Fonterra Research Centre, Palmerston North, New Zealand

Received 29 November 2004; accepted 19 January 2005

Abstract

The melting characteristics of the fat present on the surface (surface free-fat) of two industrial spray-dried dairy powders (cream powderand whole milk powder) were investigated in comparison with those of other milk fat fractions present in the powder, such as free-fat fromthe interior of the powder particle (inner free-fat) and encapsulated fat. The melting characteristics of the milk fat fractions were studiedb surfacef ongt glyceris ccumulatea g time wase rovement.©

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y fatty acid composition, melting profile and solid fat content profile. The results indicated that all milk fat fractions includingree-fat contained various triglycerides with melting points ranging from−40 to +40◦C. However, some fractionation was observed amhe different milk fat fractions. The free-fat fractions (surface free-fat and inner free-fat) had a greater proportion of high-melting tridepecies than the encapsulated fat. Furthermore, the high-melting triglyceride species present in the free-fat fractions were slightly adt the surface of powder. This phenomenon was observed in both cream powder and whole milk powder and its effect on wettinstablished. This indicates that manipulation of the surface fat content during drying operation may hold the key to functionality imp2005 Elsevier B.V. All rights reserved.

eywords:Fatty acid composition; Melting behavior; GC–MS; DSC; Milk fat; Surface free-fat; Inner free-fat; Encapsulated fat

. Introduction

Spray-dried dairy powders are common ingredients inany food and dairy industries. Many powder properties im-ortant in its storage, handling and final application (e.g. wet-

ability, dispersibility, flowability and oxidative stability) arexpected to be largely determined by the surface compositionf the powder. If one of the milk components is preferen-

ially present on the powder surface, these properties may behanged dramatically. Of special importance is the amountf fat on the powder surface. The presence of fat renders theowder surface hydrophobic decreasing wettability and dis-ersibility [1]. Fat on the surface acts as a bridge between

he particles reducing flowability[2,3]. It is also readily sus-eptible to oxidation and the development of rancidity[4,5].nderstanding the mechanism behind the formation of pow-

∗ Corresponding author. Tel.: +64 93737599x87004; fax: +64 93737463.E-mail address:d.chen@auckland.ac.nz (X.D. Chen).

der surface composition and the ability to control the surcomposition will be, therefore, highly useful in product quity improvement and new product development.

In order to understand the mechanism behind the fotion of powder surface composition, a detailed knowledgthe surface composition of spray-dried dairy powders is fineeded. In our previous publication[6], the surface composition of various industrial spray-dried dairy powders winvestigated by means of electron spectroscopy for chemanalysis (ESCA), a method providing direct chemical aysis of the outermost surface layer (∼10 nm). It was founthat the surface composition of powders is significantlyferent from their bulk composition. Particularly pronounis the accumulation of fat on the surface of fat-containdairy powders, such as cream powder and whole milk pder. The top 10 nm surface layer of these powders is mmade of milk fat (99 wt.% for cream powder, 98 wt.%whole milk powder)[6]. This observation indicates that tsurface-related properties of these powders will be gr

927-7765/$ – see front matter © 2005 Elsevier B.V. All rights reserved.

oi:10.1016/j.colsurfb.2005.01.004

2 E.H. -J. Kim et al. / Colloids and Surfaces B: Biointerfaces 42 (2005) 1–8

determined by the melting characteristics of the fat presenton the powder surface. Milk fat is a heterogeneous mixture ofvarious triglycerides with a melting range from−40 to 40◦C[7]. In the spray-drying process, the milk fat is encapsulatedby protein and lactose matrix (so called ‘encapsulated fat’),and only part of the fat can be extracted by organic solvents(so called ‘free-fat’). The free-fat is the fat that is extractedusing solvent within a pre-fixed time and thus originates notonly from the powder surface, but also comes from the in-terior of the powder particle[8]. Different triglycerides canhave different encapsulation efficiency and the distribution oftriglycerides in the powder particle may not be uniform. Inorder to further investigate the surface formation mechanismof fat-containing dairy powders, the melting characteristicsof the fat present on both the powder surface and the bulkpowder, therefore, are of great interest.

The effects of different fat phases on the encapsulationefficiency (level of surface fat) have been investigated usingESCA [4,9–11]. It was shown that the liquid oils and fullycrystalline fats with high melting points were well encapsu-lated showing low surface fat coverage, while fats with in-termediate melting points were poorly encapsulated with thehighest surface fat coverage. The authors explained that thehighest surface fat coverage in the latter is attributed to de-creased emulsion stability of partially crystalline emulsions.A reso effi-ct y don m-i t fatsu erp dingt wders

them e ofi om-p . Tot atedi face( e in-t ande thena ior.F ders y ofp

2

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ers( ere

Table 1Bulk composition of the industrial spray-dried dairy powders used

Product Composition (wt.%)

Lactose Protein Fat Moisture Ash

CP 12.3 11.5 71.5 2.7 2.0WMP 36.6 27.9 26.6 3.0 5.9

obtained from a local dairy company. The powders werecommercial products which had been freshly manufac-tured and packed for consumer use. The capacities of dry-ers where the powders were made are at least 4 t powderper h. The composition of the powders used is shown inTable 1. The hexane (95%) and iso-propanol (>99%) usedin this study were purchased from Asia Pacific SpecialtyChemicals Ltd. (Auckland, New Zealand). Boron trifluoride-methanol was purchased from Sigma Chemical Company(St. Louis, MO, USA). Deionised water was used through-out.

2.2. Methods

2.2.1. Extraction of milk fat fractionsThe extraction process of different milk fat fractions in

spray-dried dairy powders is schematically shown inFig. 1.The details of each extraction procedure are as follows. Theprocedures were repeated until a sufficient amount of fat wasobtained for analyses. In all cases we are referring to the fatthat is extractable by organic solvents.

2.2.1.1. Extraction of surface free-fat.The free-fat on thesurface of a powder particle is quickly dissolved by very briefexposure to the organic solvents. The free-fat from the innerp r toe n off itho actedbO on afi andw asd e so-l rateu . Thee -fat/gf

2 fta l ofh . Thep filtra-t ent,Uh ature.T wedt con-

similar trend was observed when emulsions with mixtuf different fats were spray-dried, the pure fats are moreiently encapsulated than any mixtures[12]. It is not possibleo distinguish between solid fat and oil by ESCA since theot differ in the C/O ratio. Due to this methodological li

tation, the previous studies assumed that the differenniformly distributed in various fat fractions in the powdarticle and no information could not be provided regar

he melting characteristics of the fat present on the pourface.

The objective of the present study was to investigateelting characteristics of the fat present on the surfac

ndustrial spray-dried fat-containing dairy powders in carison with other milk fat fractions present in the powder

his end, the milk fat present in the powders was differentinto three types; free-fat originated from the powder surdefined as surface free-fat), free-fat originated from therior of the powder particle (defined as inner free-fat)ncapsulated fat. Those various milk fat fractions werenalyzed for fatty acid composition and melting behavurthermore, the melting point of fat present on the powurface was directly estimated by testing the wettabilitowders using various water temperatures.

. Materials and methods

.1. Materials

Two industrial spray-dried fat-containing dairy powdcream powder (CP) and whole milk powder (WMP)) w

art of the particle dissolves much more slowly. In ordextract only surface free-fat and minimize the extractioree-fat from the interior of powders, only a brief wash wrganic solvent was carried out. Surface free-fat was extry a modification of the method described by Buchheim[13].ne gram of the fresh powder was accurately weighed

lter paper (No. 4, Whatman, Maidstone, Kent, UK),ashed with 4× 5 ml of hexane. The powder residue wried under vacuum at room temperature, and the filtrat

ution containing the extracted fat was allowed to evapontil the extracted fat residue achieved a constant weightxtracted fat value was then recorded as g surface freeresh powder.

.2.1.2. Extraction of inner free-fat.The powder residue lefter the extraction of surface free-fat was added to 40 mexane/g powder, and shaken frequently by hand for 48 howder residue and the solvent were first separated by

ion through filter paper (No. 4, Whatman, Maidstone, KK). The powder residue was further washed with 2× 2 mlexane and then dried under vacuum at room temperhe filtrate solution containing the extracted fat was allo

o evaporate until the extracted fat residue achieved a

E.H. -J. Kim et al. / Colloids and Surfaces B: Biointerfaces 42 (2005) 1–8 3

Fig. 1. Schematic diagram of extraction of different milk fat fractions from industrial spray-dried dairy powders., Extraction of surface free-fat, , extractionof inner free-fat, , extraction of encapsulated fat,, extraction of total fat.

stant weight. The extracted fat value was then recorded as ginner free-fat/g fresh powder.

2.2.1.3. Extraction of encapsulated fat.The powder residueleft after the extraction of inner free-fat was added to 4 mlof warm water (50◦C)/g powder and vortexed for 2 min todissolve the powder matrix and release the encapsulated fat.The resulting solution was extracted with 45 ml hexane/iso-propanol (3:1, v/v)/g powder. The suspension was shakenfor 15 min, and centrifuged at 1000×g for another 15 min.The clear organic phase was collected and the aqueous phasewas re-extracted with the solvent mixture. The organic phasecontaining the extracted fat was allowed to evaporate untilthe extracted fat residue achieved a constant weight. The ex-tracted fat value was then recorded as g encapsulated fat/gfresh powder.

2.2.1.4. Extraction of total fat.One gram of the fresh pow-der was added to 4 ml of warm water (50◦C) and vortexedfor 2 min. The resulting solution was extracted with 45 mlhexane/iso-propanol (3:1, v/v). The suspension was shakenfor 15 min, and centrifuged at 1000×g for another 15 min.The clear organic phase was collected and the aqueous phasewas re-extracted with the solvent mixture. The organic phasecontaining the extracted fat was allowed to evaporate un-t . Thee freshp

2de-

t ond-i plew wasa wast with2 es-sA ded

to stop the reaction, followed by a further 1 ml of hexane.A 2 �l of the organic phase was injected into a fused sil-ica capillary column (DB-WAX; 30 m× 0.25 mm ID; filmthickness 0.25�m; J&W Scientific Co., Folsom, CA, USA)which was placed into a Shimadzu 17A gas chromatographequipped with a flame ionization detector connected to aQP5000 quadrupole mass spectrometer (Shimadzu Corp.,Kyoto, Japan). The oven temperature was programmed tostart at 50◦C (2 min) and increase to 240◦C at 2◦C/min.The injector temperature and the detector temperature weremaintained at 230◦C. The carrier gas was helium and itsflow rate was 1.5 ml/min. The fatty acid methyl esters wereidentified by comparison of retention times with those of astandard mixture from Sigma (St. Louis, MO, USA). Usu-ally, 35 fatty acids could be detected. For clarity, only themain fatty acids and groups of fatty acids (e.g. C18:1 as thesum of all positions of the double bond) were presented.There can be some differences between analyses, mainlywith the most volatile compound. This can have an influ-ence at the proportion of fatty acids. To avoid this problem,the most volatile fatty acid C4:0 was excluded from the re-sults.

2.2.3. Differential scanning calorimetry (DSC)A differential scanning calorimeter (DSC-SP, Rheometric

S ard( hef bles,w mptys ther-m 70f ys-ta at− eo uresw RSIO , NJ,U

il the extracted fat residue achieved a constant weightxtracted fat value was then recorded as g total fat/gowder.

.2.2. Analysis of fatty acid compositionThe fatty acid composition of the milk fat fractions was

ermined after conversion of fatty acids into the correspng methyl esters. An amount of 0.1 g of the milk fat samas accurately weighed into a vial, and 10 ml of hexanedded to dissolve the sample. A 1 ml of this solution

ransferred to a screw-cap reaction vessel and mixedml of boron trifluoride-methanol. The contents of the vel were then shaken well and reacted at 70◦C for 60 min.fter cooling to room temperature, 1 ml of water was ad

cientific, NJ, USA) was calibrated with an Indium standmp 156.66◦C,�Hf 28.41 J/g,). About 10 mg portions of tat samples were accurately weighed in aluminium crucihich were sealed and placed into the calorimeter. An eealed crucible was used for reference. DSC meltingograms were determined by heating the samples at◦C

or 10 min to completely melt the fat and eliminate all cral nuclei, after which the samples were cooled to−65◦Ct a cooling rate of 10◦C/min. The samples were kept65◦C for 10 min before heating to 60◦C at a heating ratf 5◦C/min. Solid fat content (SFC) at various temperatas calculated from the melting thermograms with therchestrator partial area program (Rheometric ScientificSA).

4 E.H. -J. Kim et al. / Colloids and Surfaces B: Biointerfaces 42 (2005) 1–8

2.2.4. Wetting (wettability) testThe wettability of powders was determined by static wet-

ting test as described by Freudig et al.[14] with some modi-fication. A constant amount of powder (1 g) was transferredinto a glass cylinder placed on a glass slide covering a waterreservoir (diameter = 50 mm). The glass slide was then with-drawn to bring the powder into contact with 25◦C water. Thewetting time, which is necessary for the submersion of thelast powder particle, was measured. The measurements wereperformed using water at various temperatures (10–90◦C).

3. Results and discussion

3.1. Masses of milk fat fractions

The amounts of various milk fat fractions sequentially ex-tracted from industrial spray-dried dairy powders are shownin Table 2. In cream powder, only about 30% of total fatpresent were encapsulated and a substantial part of total fatwas present as free-fat. Particularly, about 40% of total fat waslocated the near surface region in the form of a thick layer.In whole milk powder, almost 90% of total fat present wereencapsulated and only a small part of total fat was present asfree-fat.

(sur-f qualt ffer-e ctions( s lowb holem tedf re.

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ex-t pre-so ding

TA airyp

P

l fat

CW

Table 3Individual fatty acid composition of the milk fat fractions extracted fromindustrial spray-dried cream powder (CP)

Fatty acids Composition (%)

Surfacefree-fat

Innerfree-fat

Encapsulatedfat

Totalfat

6:0 Caproic 2.28 2.03 1.83 1.878:0 Caprylic 1.40 1.28 1.00 1.1710:0 Capric 3.54 3.33 2.54 3.1412:0 Lauric 4.45 4.23 3.44 4.0514:0 Myristic 13.99 13.53 11.83 13.1015:0 Pentadecylic 1.77 1.79 1.60 1.7716:0 Palmitic 32.11 31.67 30.39 31.3416:1 Palmitoleic 2.02 2.10 1.97 2.1018:0 Stearic 12.72 12.99 13.53 13.3318:1 Oleic 22.33 23.41 24.69 23.7118:2 Linoleic 1.91 2.04 5.10 2.6318:3 Linolenic 1.49 1.61 2.09 1.79

to similarities in melting points) inFig. 2. Firstly, the re-sults show that there is a compositional difference betweenthe free-fat and the encapsulated fat. The free-fat fractions(surface free-fat and inner free-fat) had slightly higher con-centrations of C6–C18 saturated fatty acids and slightly lowerconcentrations of C16–C18 unsaturated fatty acids comparedto the total fat, whereas the encapsulated fat showed the re-verse tendency with more marked concentration differences.These observations on the free-fat fractions concur with thetrend obtained by Lindquist and Brunner[15], although theirresults were based on spray-dried whole milk powder andonly minor dissimilarities were observed. Secondly, the re-sults show that there is a compositional difference even be-tween the free-fat fractions. The surface free-fat had slightlyhigher concentrations of C6–C18 saturated fatty acids andslightly lower concentrations of C16–C18 unsaturated fattyacids than the inner free-fat.

F milkf satd.:s

It is expected the sum of the masses of fat fractionsace free-fat, inner free-fat and encapsulated fat) to be eo the mass of total fat. However, there was a slight dince in the masses. The sum of the masses of fat frasurface free-fat, inner free-fat and encapsulated fat) way approx. 0.01 g/g powder, in both cream powder and wilk powder. This difference might be due to the extrac

at remained behind on the filter paper and the glasswa

.2. Fatty acid composition of milk fat fractions

Milk fat is a complex mixture of triglycerides, composf a large number of different fatty acids. This great variet

atty acids leads to a heterogeneous composition of trirides and a very broad melting range, which varies betpproximately−40 to +40◦C [7]. The fatty acid compos

ion is, therefore, an important and a most basic paramor determining melting characteristics of milk fat.

The fatty acid compositions of the milk fat fractionsracted from industrial spray-dried cream powder areented by individual fatty acids inTable 3and, for simplicityf presentation, by selected fatty acid groupings (accor

able 2mount of the milk fat fractions extracted from industrial spray-dried dowders

roduct Amount of fat extracted (g fat/g fresh powder)

Surface free-fat Inner free-fat Encapsulated fat Tota

P 0.266 0.211 0.228 0.715MP 0.007 0.015 0.235 0.266

ig. 2. Fatty acid composition (selected fatty acid groupings) of theat fractions extracted from industrial spray-dried cream powder (CP).aturated fatty acids, unsatd.: unsaturated fatty acids.

E.H. -J. Kim et al. / Colloids and Surfaces B: Biointerfaces 42 (2005) 1–8 5

Table 4Individual fatty acid composition of the milk fat fractions extracted fromindustrial spray-dried whole milk powder (WMP)

Fatty acids Composition (%)

Surfacefree-fat

Innerfree-fat

Encapsulatedfat

Totalfat

6:0 Caproic 2.31 2.08 2.03 2.088:0 Caprylic 1.98 1.87 1.82 1.8610:0 Capric 5.42 5.24 5.03 4.9012:0 Lauric 6.47 6.51 6.25 6.2214:0 Myristic 15.81 16.15 15.55 15.3815:0 Pentadecylic 1.46 1.48 1.47 1.4416:0 Palmitic 30.76 30.80 30.17 30.4016:1 Palmitoleic 1.58 1.61 1.67 1.6918:0 Stearic 12.10 12.17 12.58 12.5718:1 Oleic 19.57 19.58 20.55 20.6518:2 Linoleic 1.35 1.29 1.55 1.5218:3 Linolenic 1.19 1.20 1.33 1.28

As already mentioned above, the saturated and unsatu-rated fatty acid content of milk fat affects its melting behavior.In general, increases in short-chain fatty acids (C4–C8) andlong-chain unsaturated fatty acids (C16:1 and C18:1–C18:3)with concurrent decreases in long-chain saturated fatty acids(C16:0 and C18:0) result in milk fat with lowered melt-ing points and a greater proportion of low-melting triglyc-eride species. Conversely, increases in long-chain saturatedfatty acids with concurrent decreases in short-chain fattyacids and long-chain unsaturated fatty acids result in milkfat with higher melting points and a greater proportion ofhigh-melting triglyceride species[16–21]. Taking this intoaccount, the results here indicate that high-melting triglyc-eride species are more concentrated in the free-fat whereasmore low-melting triglyceride species in the encapsulated fat.Furthermore, the high-melting triglyceride species present infree-fat are accumulated at the surface of powder.

Table 4andFig. 3show the fatty acid composition of themilk fat fractions extracted from industrial spray-dried wholemilk powder by individual fatty acids and by selected fattyacid groupings, respectively. The results show similar trendsto those observed in the cream powder, i.e. higher concen-trations of C6–C18 saturated fatty acids and lower concen-trations of C16–C18 unsaturated fatty acids in the free-fatfractions (surface free-fat and inner free-fat) compared to thee cesb e ob-s t arev ap-s eadym lsoc

3

tedf SCp ver

Fig. 3. Fatty acid composition (selected fatty acid groupings) of the milk fatfractions extracted from industrial spray-dried whole milk powder (WMP).satd.: saturated fatty acids, unsatd.: unsaturated fatty acids.

its melting range (melting profile) and the actual percentageof solid fat at various temperature (SFC profile)[18].

Figs. 4 and 5show the melting profiles of the milk fat frac-tions extracted from industrial spray-dried cream powder andwhole milk powder, respectively. In both figures, the meltingthermograms of the surface free-fat, the inner free-fat andthe encapsulated fat all were very similar to that of the corre-sponding total fat. All exhibited a wide melting range from−40 to +40◦C and four distinct peaks, i.e. a broad shoulderrepresenting high-melting triglyceride species, one peak rep-resenting middle-melting triglycerides species and two peaksrepresenting low-melting triglyceride species. These obser-vations indicate that the surface free-fat, the inner free-fatand the encapsulated fat all are complex mixtures of vari-

F fromi

ncapsulated fat. However, little compositional differenetween the surface free-fat and the inner free-fat wererved. Here, data for the encapsulated fat and total fairtually identical. This would be expected in that the enculated fat accounts for about 90% of the total fat, as alrentioned in Section3.1. This close agreement of data a

onfirms the precision of the analytical method.

.3. Melting profile of milk fat fractions

The melting behavior of the milk fat fractions extracrom the two powders was studied by means of DSC. Drovides a generalized view of the melting of milk fat o

ig. 4. DSC melting thermograms of the milk fat fractions extractedndustrial spray-dried cream powder (CP).

6 E.H. -J. Kim et al. / Colloids and Surfaces B: Biointerfaces 42 (2005) 1–8

Fig. 5. DSC melting thermograms of the milk fat fractions extracted fromindustrial spray-dried whole milk powder (WMP).

ous triglycerides with melting points ranging from−40 to+40◦C. The same as the total fat. There were general sim-ilarities observed, but the size of particular peaks differedbetween fat fractions. The consequence of these peak sizedifferences is discussed in Section3.4.

3.4. Solid fat content profile of milk fat fractions

SFC profile of the milk fat fractions extracted from in-dustrial spray-dried cream powder is shown inFig. 6. SFCobtained by DSC is a relative measure, and we assume 100%crystallinity at−40◦C. As already observed in the meltingthermograms, all milk fat fractions showed gradual meltingthroughout their melting range in a very similar manner. How-ever, the free-fat fractions (surface free-fat and inner free-fat)had higher a solid fat content at all temperatures than the en-capsulated fat. The solid fat content of total fat fell betweenthe encapsulated and surface free-fat fractions. It was nearlyidentical to the profile of the inner free-fat fraction. The totalfat profile was also very similar to the surface free-fat profile.This is not surprising since the free-fat fractions account for

F omi

Fig. 7. Solid fat content profile of the milk fat fractions extracted fromindustrial spray-dried whole milk powder (WMP).

about 70% of the total fat in cream powder (Table 2). Thesurface free-fat had greatest solid fat content at all temper-atures. For example, at 20◦C, the surface free-fat contained35% solids, the inner free-fat 30%, the encapsulated fat 28%.These results indicate that there is some fractionation withthe slight accumulation of high-melting triglyceride speciesat the surface of the powders. The results here are in excel-lent agreement with the results obtained in Section3.2 andthe solid fat content profile clearly revealed the differencesfound in their fatty acid composition.

Fig. 7shows the SFC profile of the milk fat fractions ex-tracted from industrial spray-dried whole milk powder. Theresults showed the similar trend as observed in the spray-driedcream powder, i.e. greatest solid fat content in the surfacefree-fat and lowest solid fat content in encapsulated fat. Forexample, at 20◦C, the surface free-fat contained 31% solids,the inner free-fat 28%, the encapsulated fat 26%. Buchheim[22] reported that the surface of whole milk powder was lo-cally covered with thin fat layers and the surface fat showedthe crystalline state (few monomolecular layers of approx.50A). This previous microscopic observation was found tobe in good agreement with the results obtained here.

3.5. Melting point of surface free-fat

fattya rentm er-e causet onlyt , thef e ofp in thes fore,e

s of‘ sti-m iousw t fat-c andc per-

ig. 6. Solid fat content profile of the milk fat fractions extracted frndustrial spray-dried cream powder (CP).

In the observations above, significant differences incid composition and melting behavior between the diffeilk fat fractions were not found. However, the small diffnces, consistently detected, seem to be important. Be

he extraction procedure of surface free-fat extracts nothe ‘real’ surface free-fat but also ‘near’ surface free-fatat extracted is a ‘blend’ of free-fat localized near surfacowder and the small differences are seen as a net effectubstantial fraction. Much larger differences are, therexpected to occur in ‘real’ surface fat.

In order to directly measure the melting characteristicreal’ surface fat, the melting point of surface fat can be e

ated by measuring the wettability of powders using varater temperatures. In general, it has been known thaontaining powders are not easily wetted in cold wateran achieve a satisfactory wettability when the water tem

E.H. -J. Kim et al. / Colloids and Surfaces B: Biointerfaces 42 (2005) 1–8 7

Fig. 8. Wettability of industrial spray-dried fat-containing dairy powders atvarious water temperatures.

ature is higher than the melting point of the fat[23,24]. Toimprove the wettability of these powders in cold water, a nat-ural surfactant soya lecithin is usually coated on the powdersurface. However, an earlier study[25] has shown that thewettability of fat-containing powders, which are not easilywetted in cold water, can be significantly improved by coat-ing at least a part of the powder surface with mono-and/ordi-glyceride of a medium chain fatty acid. This suggests that,in the absence of a surfactant, the wettability of powders isdetermined by the melting point of the fat present on theoutermost powder surface, not the melting point of the fat.Fig. 8 shows the wettability of industrial spray-dried creampowder and whole milk powder at various water temperatures(10–90◦C). The values are an average of three measurements.As shown, industrial spray-dried cream powder and wholemilk powders could not be completely wetted in cold water(10 to∼37◦C) within reasonable periods (> 15 min). How-ever, the wetting time of the powders was sharply decreasedat 42◦C for cream powder and 38◦C for whole milk powder.This indicates that the fats present on the surface of creampowder and whole milk powder melt at 42 and 38◦C, re-spectively. Though the composition analysis shows a smalltendency that the cream powder had harder fractions on sur-face than that of the whole milk powder, the wettability testsshow marked differences.

4

cedb ltingc e. Int thes holem fatf d fat)p ana-l Thes arec ome

fractionation with the accumulation of high-melting triglyc-eride species in the free-fat and even more at the surface of thepowders. The results were further confirmed by wettabilitytests.

Acknowledgements

The first two authors thank Fonterra Research Centre fora PhD scholarship which facilitates a research program in thearea.

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psu-hnol.

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[ face

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[ ried

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[ of

[ ion95.

[ ac-

. Conclusions

Many surface-related powder properties will be influeny the quantity of fat on the powder surfaces, but the meharacteristics of surface fat is also of great importanchis study, the melting characteristics of fat present onurface of industrial spray-dried cream powder and wilk powder were investigated. To this end, various milk

ractions (surface free-fat, inner free-fat and encapsulateresent in the powders were sequentially extracted and

yzed their fatty acid composition and melting behavior.tudy showed that, in both powders, all milk fat fractionsomplex mixtures of various triglycerides, but there is s

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