The Effect of Surface Composition on the Functional Properties of Milk Powder

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The effect of surface composition on the functionalproperties of milk powders

J.J. Nijdam, T.A.G. Langrish *

Chemical Engineering Department, University of Sydney, Sydney, NSW 2006, Australia

Received 18 December 2004; accepted 2 August 2005Available online 27 September 2005

Abstract

The migration of lactose, protein and fat within milk droplets and particles in a spray dryer is investigated with a view to eventuallymodelling this process using computational fluid dynamics. Both protein and fat accumulate preferentially at the surface of the milk par-ticles as they dry, at the expense of lactose. This has repercussions for the rate of particle agglomeration and wall deposition within thespray dryer, and the functional properties of the dried powder, because the fat and lactose surface concentrations affect the stickiness ofthe milk particles. The surface fat coverage, and hence the particle stickiness, is particularly sensitive to small changes in fat contentbetween 0% and 5%, which is likely to be important for the control of powder properties and the operation of spray drying equipmentin skim milk production. In addition, a higher drying temperature favours the appearance of lactose over protein at the surface of themilk particle. We postulate that higher temperatures hasten the formation of a surface skin, which hinders the migration of surface-activeprotein towards the surface. Finally, we have confirmed observations made by various other researchers on the morphological evolutionof a milk droplet as it dries, which involves the formation of a skin and a vacuole, and the inflation and subsequent shrinkage of theparticle.2005 Elsevier Ltd. All rights reserved.

Keywords: Spray drying; Caking; Stickiness; Particle morphology; Drying temperature; X-ray photoelectron spectrometry; Agglomeration

1. Introduction

Milk is spray dried for easier storage, handling andtransport. The spray drying process involves atomisingthe milk within a flow of hot air, where water is progres-sively evaporated from the droplets until dried milk parti-cles are produced. The functional properties of the

resultant milk powder, such as particle size distribution,bulk density, flowability and solubility, determine its stor-age, handling and transport capabilities. There has beenconsiderable work done in the past decade by various re-search groups around the world to numerically model thespray drying process using computational fluid dynamics(CFD), with the ultimate aim of predicting these powder

properties in order to facilitate the design of improvedspray dryers and new milk products. Thus far, successfulvalidation work has been carried out on various CFD mod-els, which has highlighted their ability to accurately predict(1) the transient and swirling airflow patterns found inmany spray dryers (Guo, Langrish, & Fletcher, 2003;Langrish, Williams, & Fletcher, 2004), (2) the turbulent

dispersion of droplets and particles within a spray andhence their trajectories through the dryer (Berlemont,Desjonqueres, & Gouesbet, 1990; Nijdam, Guo, Fletcher,& Langrish, 2004a; Ruger, Hohmann, Sommerfeld, &Kohnen, 2000), and (3) the coalescence and evaporationof droplets (Nijdam et al., 2004a; Nijdam, Guo, Fletcher,& Langrish, 2004b; Ruger et al., 2000).

How changes in the morphology of a droplet aremodelled once sufficient moisture is evaporated that a skindevelops on its surface is less clear. Indeed, the transforma-tion of a milk droplet into a particle is rather complex. This

0260-8774/$ - see front matter 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jfoodeng.2005.08.020

* Corresponding author. Tel.: +61 2 9351 4568; fax: +61 2 9351 2854.E-mail address:[email protected] (T.A.G. Langrish).

www.elsevier.com/locate/jfoodeng

Journal of Food Engineering 77 (2006) 919925
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process involves the concentration of various milk compo-nents, such as lactose, fat and protein, at the surface of thedroplet as moisture evaporates, and the subsequent devel-opment of a skin. It concerns the formation of a vacuole(a vapour bubble) within the droplet, once the surface skinappears, that repeatedly inflates and deflates (Hassan &

Mumford, 1993; Hecht & King, 2000a; Walton, 2000),which affects the porosity of the particle. Another impor-tant phenomenon is the migration of milk componentsthrough the aqueous phase to the skin and on towardsthe surface, because the surface moisture, lactose and fatconcentrations affect the stickiness and agglomeration ofparticles, which influence the particle size distribution, bulkdensity and flowability of the resultant milk powder. More-over, the presence of fat on the particle surfaces renders themilk powder hydrophobic, such that its solubility in wateris reduced, in addition to making it readily susceptible tooxidation and subsequent rancidity (Pisecky, 1997).

Models have been developed to predict the evolution of

a vacuole (inflation and deflation cycle) within the dropletas it dries (Hecht & King, 2000b; Sano & Keey, 1982),although these models are not practical to simulate manyparticles simultaneously within a spray dryer in additionto modelling the trajectories and drying of these particles.A promising method for simulating the drying of a multi-tude of moist particles involves the concept of the charac-teristic drying curve, which is an empirical, lumpedparameter approach in which the complexities of internalmoisture transport are avoided by relating the drying rateto the average moisture content of the particle (Harvie,Langrish, & Fletcher, 2002). However, the clear disadvan-

tage of this technique is that the surface moisture contentand temperature cannot be predicted, which are importantvariables for determining whether the glass transition tem-perature of surface lactose has been exceeded, and there-fore whether the particle is sticky (Jouppila & Roos,1994; Lloyd, Chen, & Hargreaves, 1996). This is likelyto be a key consideration when modelling particle agglom-eration and wall deposition within spray dryers. Stra-atsma, van Houwelingen, Steenbergen, and De Jong(1999)have overcome the shortcomings of the characteris-tic drying curve method by developing a mathematicalmodel to simulate the internal diffusion of moisture withinparticles as they dry while simultaneously calculating thetrajectories of these particles through the spray dryer.Similar equations can also be used to predict the diffusionof any other milk components within the droplet/particleas it dries, although this is likely to be computationallyexpensive.

While the issue of skin and vacuole evolution and drop-let/particle drying are touched upon in this paper, we focusprimarily on the migration of milk components (lactose, fatand protein) to the surface of the milk particle during thespray drying process. An understanding of these transportmechanisms is necessary in order to predict the surfacecomposition, and hence agglomeration of particles, and

ultimately the properties of the dried powder. The effect

of surface fat concentration on the functional propertiesof the milk powder is particularly studied in this paper,since very little work has been published in the literatureon this subject (Ozkan, Walisinghe, & Chen, 2002). X-rayphotoelectron spectroscopy (XPS) is used to measure thesurface composition of spray dried milk powders with dif-

ferent fat contents according to the method developed byFaldt, Bergenstahl, and Carlsson (1993). In order to pro-vide a measure of the agglomeration potential of particleswithin the spray dryer, we investigate the effect of surfacecomposition on the caking ability of the powder using astandard sieving test. The particle size distribution of themilk powders is measured using the laser diffraction tech-nique, and the structure of individual particles is studiedby scanning electron microscopy.

2. Experimental

2.1. Sample preparation

Skim and full-cream milk solutions were made to a con-centration of 41.2% (weight basis) by combining skim andfull-cream milk powder (bought from a local supplier) withdistilled water at a constant temperature of 50 C. Milksolutions with various fat contents were then prepared bymixing different ratios of these skim and full-cream milkconcentrates. The compositions of the milk solutions usedare given inTable 1, although we are relying on the accu-racy of the nutritional information provided on the pack-aging to determine these compositions.

The milk concentrates were spray dried in a BUCHI

Mini Spray Dryer B-290 (Switzerland), which operatesco-currently with a two-fluid self-cleaning spray nozzlehaving a cap orifice of diameter 1.5 mm. The BUCHI dry-ing chamber is cylindrical and vertically orientated with alength and diameter of approximately 500 mm and150 mm, respectively. During the experiments, the liquidfeed to the nozzle was 8 ml/min and was kept constant at31 C using a water bath. The flow of air from the nozzlewas 440 L/h, while the flow of drying air was 38 m3/h.Two inlet air temperatures, 120 C and 200 C, were testedin order to show the effect of drying intensity on the surfacecomposition. These inlet air temperatures corresponded tooutlet air temperatures of approximately 80 C and 125 C,respectively. After drying, the powder samples were trans-ferred to sealed bottles and refrigerated.

Table 1Composition of milk solutions

Milk solution Fat (%solids) Protein (%solids) Lactose (%solids)

1 (skim) 1.1 39.3 59.62 1.8 39.0 59.23 3.4 38.4 58.34 6.7 37.0 56.35 14.0 33.9 52.1

6 (full-cream) 29.8 27.3 43.0

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2.2. Electron spectroscopy

In order to perform the surface composition analysis, itwas assumed that the solids in the milk were composed ofonly three components: lactose (carbohydrate), protein andfat. These components typically amount to 95% of the total

solids in milk, with the remainder consisting of variousminerals, and therefore a significant portion of the milkcomposition was not taken into account during the analy-sis. In addition, it was assumed that the protein consistedentirely of casein, which normally constitutes 86% of thetotal proteins in milk, with the remainder consisting ofwhey proteins, such as lactalbumin (Pisecky, 1997). Thus,the extracted surface compositions are not precise, andthe implications of this inaccuracy are discussed below.

The relative atomic concentrations of carbon, oxygenand nitrogen at the surface of the milk powder sampleswere analysed using X-ray photoelectron spectroscopy(XPS). These elemental ratios were converted into surface

coverage ratios of lactose, protein and fat by assuming alinear relationship between these elemental surface compo-sitions and the elemental compositions of each of the puremilk components (lactose, protein and fat) making up thesample, according to the method described by Faldt et al.(1993) and subsequently adopted by Kim, Chen, andPearce (2003)for milk powders. We have measured the fol-lowing elemental compositions in the pure components:lactosecarbon (56.7%) and oxygen (43.3%); caseincarbon(70.5%), oxygen (17.2%), nitrogen (12.3%); fatcarbon(90.8%), oxygen (9.2%). We have also measured the ele-mental composition of whey protein concentrate powder,

which consists of milk proteins that remain once caseinhas been removed, as follows: carbon (76.2%), oxygen(15.6%) and nitrogen (8.2%). The whey protein tested inthis work was not pure, containing additional lactose(7.5%), fat (3.5%) and various other components (4.5%),which would tend to augment the carbon concentrationand reduce the nitrogen concentration. By making anallowance for this impurity, we suggest that the whey pro-tein and casein compositions are not dissimilar, so thatthey can be lumped together in the analysis as protein.Thus, only minerals are not accounted for properly in thiswork. However, these minerals contribute only 5% of theoverall composition of the milk, and this is not sufficientto affect the conclusions drawn here. Note that we obtainedthe lactose (99% pure), anhydrous milk fat (99% pure)and whey protein concentrate powder from FonterraCo-operative Group Limited (Temuka, New Zealand),and the casein (from bovine milk) from SigmaAldrich(St. Louis, MO, United States).

The samples of milk powder were manually pressed intopellets for the surface analysis. X-ray photoelectron spectrawere determined using a monochromatic Al Ka source onan ESCALAB220i-XL instrument (VG Scientific, UK).The X-ray source was operated at 120 W, and a passenergy and step-size of 100 eV and 0.5 eV were used,

respectively. The take-off angle of the photoelectrons was

perpendicular to the sample, and the area analysed was acircular region of 0.5 mm in diameter. The pressure in thevacuum chamber during the analysis was less than1 109 mbar. The peak areas for the important photo-electron emission signals (carbon, oxygen, nitrogen, sul-phur, phosphate, etc.) were measured and converted into

relative elemental concentrations using Eclipse software.The elemental concentrations of carbon, oxygen and nitro-gen amounted to at least 99% of all the elements registered,and thus the remaining elements were ignored in the anal-ysis. This indicates that the minerals (calcium, sodium andphosphate) in the milk powder were not present at suffi-ciently high concentrations at the surface of the particlesto significantly affect the surface composition analysis.

2.3. Degree of caking

The degree of caking was measured using the methoddescribed by Pisecky (1997). A 1.5 g sample was first

oven-dried for 1 h at 102 C. After cooling in a desiccator,the sample was weighed, quickly transferred to a stainlesssteel sieve, and shaken for 5 min in a shaking apparatus.The powder that passed through the sieve was weighed,and the caking index was determined by the equation:

CMT MF

MT 100 1

whereMTis the total mass of powder andMFis the mass offines that passed through the sieve. Sieves with differentapertures were previously tested in order to determine whichsieve gave the greatest caking sensitivity for the particle size

distribution produced by the BUCHI spray dryer. A sievewith an aperture of 106 lm was used, although the resultsfor the 212 lm tests are also reported. The sieving tests werecarried out at an ambient air temperature of 23.5 C.

2.4. Particle size distribution

The particle size distribution of the milk powders wasmeasured using a laser light diffraction instrument, Master-Sizer S (Malvern Instruments, Malvern, UK) according tothe method described by Pisecky (1997), in which a smallsample of the powder is suspended in isopropanol in a cuv-ette under magnetic agitation during the size measurement.

The particle size distribution was monitored every 2 min dur-ing each measurement until successive readings became con-stant. This allowed the agglomerates, which were most likelyformed in the powder rather than within the drying chamberwhere the particle number concentration was too low forparticle collisions to occur, to break up due to the shearingaction of the magnetic agitator. The droplet size is expressedasD(v, 0.5), the volume-weighted median diameter.

2.5. Scanning electron microscopy

Milk powder samples were attached to double-sided

adhesive carbon tabs mounted on SEM stubs, coated with

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gold/palladium, and examined with a Phillips SEM505(Philips Export BV, The Netherlands) operated between3 and 4 kV.

3. Results and discussion

The surface fat coverage is significantly higher than theaverage fat content, as shown inFig. 1, which suggests thatfat is present at higher concentrations at the surface of eachmilk particle than in the interior. For example, when theaverage fat content of the milk powder is 5%, the surfacefat coverage is in the vicinity of 35%. Thus, it appears thatfat is transported towards and accumulates at the surfaceof the milk particle during drying, which results in a non-uniform distribution of fat throughout the solid matrix,as previously demonstrated by Kim et al. (2003). Higherdrying temperatures appear to marginally favour the accu-mulation of fat at the surface, as shown by the slightlyhigher surface fat coverage of the milk powders dried at

200 C compared with 120 C. Differential scanning calo-rimetry (DSC) measurements by Jouppila and Roos(1994)indicate that the melting point of fat in milk powderis between 10 and 30 C. Given that the outlet temperatureof the BUCHI spray dryer was at least 80 C in these tests,the fat was in a mobile fluid form throughout the spraydrying process, and was therefore readily transported tothe surface of the droplets/particles.

Fig. 1shows that a small change in the average fat con-tent at low fat concentrations results in a large change inthe surface fat coverage. When the average fat content isincreased from 0% to 5%, the surface fat coverage increases

from 0% to 35%. However, the surface fat coverage is lessaffected by increases in the average fat content at higher fatconcentrations, only rising by a further 25% as the averagefat content is increased from 5% to 30%. It will be demon-strated below that this rapid change in surface fat coverageat low average fat contents strongly affects the surfaceproperties of the milk particles, which is likely to haveimportant repercussions for the production of skim milk,

where small changes in the fat content of skim milk concen-trate fed to the spray dryer can affect the agglomeration po-tential of the milk particles and the functional properties ofthe dried powder. Faldt and Bergenstahl (1996)have ob-served similar trends in surface fat coverage with averagefat content for emulsions of whey protein, lactose and soy-

bean oil.Figs. 2 and 3 shows that protein has a higher surfaceconcentration than lactose at the lower drying temperatureof 120 C, even though there is more lactose than protein inthe powder by a factor of one-half, as shown in Table 1.This is in accordance with data presented byFaldt and Ber-genstahl (1996), who explain that there is an accumulationof surface-active protein at the airwater interfaces of thedroplets, so that protein appears in relatively high concen-tration on the dry powder surface. At the higher dryingtemperature of 200 C, this trend is reversed and more lac-tose appears at the surface of the powder than protein,although the ratio of lactose to protein on the surface is

still generally lower than the average value in the powder,

0

10

20

30

40

50

60

70

0 5 10 15 20 25 30

Average Fat Content (%weight)

SurfaceFatCoverage

(%)

120 C

200 C

Fig. 1. The surface fat coverage of milk powders with different average fat

contents spray dried at 120 C and 200 C.

0

10

20

30

40

50

60

0 5 10 15 20 25 30


SurfaceProteinCoverage(%)

120 C200 C

Fig. 2. The surface protein coverage of milk powders with differentaverage fat contents spray dried at 120 C and 200 C.

0

10

20

30

40

50

60

0 5 10 15 20 25 30


SurfaceLactoseCoverag

e(%)

120 C

200 C

Fig. 3. The surface lactose coverage of milk powders with different

average fat contents spray dried at 120 C and 200 C.



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which further demonstrates the affinity of surface-activeprotein with the airwater interfaces of the droplets. Wepostulate that, at lower drying temperatures, protein hasmore time to migrate to the surface of the droplet beforesufficient moisture is evaporated that a skin forms, whichsubsequently hinders further transport of protein to the

surface, especially when much of the solvent (in this casewater) is removed from the skin. At higher drying temper-atures, less protein can migrate towards the surface, be-cause moisture is evaporated more quickly and thereforethe surface solidifies and immobilises the protein sooner.This is in contrast with fat, which is in a mobile fluid formthroughout the drying process, since the drying tempera-ture is above the melting point of fat.

Kim et al. (2003) have a different explanation for theapparent concentration of fat and protein at the surfaceof a milk particle. They have stated that moisture contentgradients effectively concentrate the solutes (fat, proteinand lactose) at the surface, where the moisture content is

lower, which causes Fickian diffusion of these solutes to-wards the core of the milk particle. According to theseworkers, lower molecular weight solutes, such as lactose,diffuse inwards more rapidly than higher molecular weightsolutes, such as protein and fat, which consequently con-centrate at the surface. However, this transport mechanismcannot be rationalised with the observed increase in surfacelactose concentration at higher drying temperatures(Fig. 3), which should enhance the Fickian diffusion of lac-tose towards the core, according to the StokesEinsteinequation for diffusion of solutes in liquids (Kim et al.,2003), rather than retard it as observed experimentally.

We have proposed that surface affinity causes protein toaccumulate at the surface of a milk droplet, and that skinformation, which is more rapid at higher drying tempera-tures, hinders this process. Note that protein accumulationat the surface of a milk particle is important, because itaffects the lactose surface coverage, which is known tostrongly influence the caking of milk powders duringhumid storage when the glass transition temperature isexceeded (Jouppila & Roos, 1994).

There appears to be a strong correlation between thecaking ability of the powder and the surface fat coverage,as shown by the similarity of the shapes of the caking index(Fig. 4) and surface fat coverage plots (Fig. 1). When theaverage fat content and, consequently, the surface fat cov-erage of the milk powder is high, the powder is very stickyat ambient temperature, so that it has a caking indexapproaching 100%, which implies that very little milk pow-der passes through the sieve. The caking index remainshigh even when the average fat content of the powder is re-duced from 30% to 5%, whether a 106 lm or 212 lm sieveis used. However, below 5% average fat content, there is asharp reduction in the surface fat coverage, as shown inFig. 1, so that the stickiness of the powder, and hence itscaking ability, correspondingly diminishes quickly. Theindividual milk particles are significantly smaller than the

aperture of the sieves, as shown in Fig. 5, which indicates

that the milk powders should easily pass through the sievestested here, provided they do not stick and agglomerate.Ozkan et al. (2002) have explained that any fat presenton the surface of individual particles forms weak bridgesbetween these particles, which help to bind them togetherto form larger agglomerates. In the experiments presentedhere, it was observed that the individual milk particles inthe powders with high fat content balled together to formsuch agglomerates during the sieving process, some ofwhich were over 1 mm in diameter, which were too largeto pass through the sieve apertures. It is clear from Fig. 4that the caking index changes very little above 30% averagefat content, because a large proportion of the surface ofeach particle is coated in fat, irrespective of the averagefat content of the particle, as shown in Fig. 1. Note thatthe caking ability of the powder is independent of the dry-ing temperature over the range of temperatures tested,which is not surprising given the similarity in the surfacefat coverage curves for each of these temperatures, asshown inFig. 1.

The particle size measurements shown inFig. 5suggest

that the size of the milk particles decreases as (1) the drying

0

20

40

60

80

100

0 5 10 15 20 25 30

Average Fat Content (%)

CakingIndex(%)

120 C (106 um sieve)



Fig. 4. The effect of the average fat content on the degree of caking formilk powders spray dried at 120 C and 200 C.

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30

Average Fat Content (%)

Volumeweightedmediandiamete

r,

D(v,0.5

),(um)

120C

200 C

Fig. 5. The volume weighted median diameter D(v, 0.5) of milk powderswith different average fat contents spray dried at 120 C and 200 C.



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temperature decreases, and (2) the average fat content ofthe powder increases. These trends are consistent withtrends in the observed sizes of the milk particles shown inthe scanning electron micrographs ofFigs. 6 and 7. Numer-ous observations of shattered particles in these scanningelectron micrographs indicate that the particles are hollow,

which implies that a vacuole (a vapour bubble) forms with-in a particle soon after a skin develops on the surface, thatinflates once the particle temperature exceeds the localambient boiling point and the vapour pressure within thevacuole rises above the local ambient pressure. When thedrying temperature is sufficiently high, moisture is evapo-rated very quickly and the skin becomes dry and hard, sothat the hollow particle cannot deflate when vapour con-denses within the vacuole as the particle moves into coolerregions of the dryer. However, when the drying tempera-ture is lower, the skin remains moist and supple for longerso that the hollow particle can deflate and shrivel as itcools. Thus, milk particles dried at 200 C are spherical

and smooth (Fig. 6), while milk particles dried at 120 Care smaller and have a shrivelled appearance (Fig. 7). Theseconcepts have been discussed previously by Hassan andMumford (1993), Walton (2000) and Hecht and King(2000a).

Finally, the milk particles may decrease in size as the fatcontent increases, as shown inFig. 5, due to a change in thesurface tension of the milk concentrate fed to the nozzle,which would alter the atomisation process. The possibleinfluence of fat on the mechanical properties of the skin,which may affect the inflation of the particles, is not ex-cluded either. The scanning electron micrographs (Figs. 6and 7) clearly show that there are larger numbers of smallparticles in the powders with higher average fat content,which confirms the trend shown in Fig. 5, and that highersurface fat coverage helps to bind milk particles together.The powders with very little fat content do not bind to-gether in this fashion, as shown in Figs. 6 and 7, which isconsistent with the results of the caking tests (Fig. 4), indi-cating that individual particles with very little surface fatpass readily through a sieve, provided the apertures areadequately sized.

4. Conclusions

Both fat and lactose have a strong influence on the stick-iness of milk particles. The stickiness of these particles isparticularly sensitive to small changes in the fat content be-

tween 0% and 5%, which is due to a rapid change in surface

Fig. 6. Scanning electron micrographs of milk powders spray dried at200 C: (a) skim milk powder (low fat content) and (b) full-cream milk

powder (high fat content).

Fig. 7. Scanning electron micrographs of milk powders spray dried at120 C: (a) skim milk powder (low fat content) and (b) full-cream milkpowder (high fat content).



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fat coverage from 0% to 35%. Moreover, it appears thatprotein accumulates at the surface of a milk droplet/parti-cle at the expense of lactose during drying (possibly due tothe surface-active nature of the protein), which will also af-fect particle stickiness when the glass transition tempera-ture of lactose is exceeded. Thus, the experimental results

highlight the importance of predicting the transport of eachof these milk components within a droplet/particle as mois-ture is evaporated in order to properly model particulateagglomeration and wall deposition in a spray dryer, andto estimate the properties of the spray-dried milk powder.We have confirmed observations made by various otherresearchers on the morphological evolution of a milk drop-let as it dries, which involves the formation of a skin and avacuole, and the inflation and subsequent shrinkage of theparticle.

Acknowledgements

We wish to thank Mr. K. Kota (Chemical EngineeringDepartment, University of Sydney) and Dr. Bill Bin Gong(School of Chemistry, University of New South Wales) fortheir technical assistance, Dr. James Winchester (FonterraCo-operative Group Limited, Temuka, New Zealand) forhis academic support, and Mr. John Gabites (FonterraCo-operative Group Limited, Temuka, New Zealand) forproviding the lactose, whey protein concentrate and anhy-drous milk fat. This work has been supported by an Aus-tralian Research Council Discovery Grant.

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The Effect of Surface Composition on the Functional Properties of Milk Powder