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Colloids and Surfaces B: Biointerfaces 75 (2010) 377384

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Colloids and Surfaces B: Biointerfaces

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How surface composition of high milk proteins pspray-d

C. Gaiani . Jaca LIBio, Nancy U . 172, 5b INRA, Agrocam ce

a r t i c l

Article history:Received 22 MReceived in re15 SeptemberAccepted 15 SAvailable onlin

Keywords:ProteinsDairy powdersDryingSurface compositionXPS

on inssentsurfaent ospra

hotoey, boentre, li

whereas lactose is found in the core. This surface enrichment is also highly affected by the spray-dryingtemperature. More lipids, more proteins and less lactose are systematically observed at the surface ofpowders spray-dried at lower outlet air temperatures. The nature of proteins is also found essential;surface enrichment in lipids being much stronger for whey proteins containing powders than for caseincontaining powders. Additionally, we found a direct correlation between the lipids surface concentration

1. Introdu

Spray-drand more ptent, this pof microbiaof lipids oxemulsion sttransport cdrying of mleading to thlipids and mover the padients withproteins poextracted dtion in amin

Abbreviatiolate; PCA, prinphotoelectron

CorresponE-mail add

0927-7765/$ doi:10.1016/j.and the wetting ability for the 25 powders studied. 2009 Elsevier B.V. All rights reserved.

ction

ying is a commonly used technique in the food industryrecisely in the dairy sector. By decreasing water con-rocess provides several advantages as a minimizationl deterioration of the spray-dried products, a reductionidation [1] and, ideally, a preservation of the originalructure [2]. It ensures also a reduction of storage andost and an easier handling of the material [3]. Spray-ilk concentrates involves a quick removal of water,e formationof adrymatrix containingproteins, lactose,inerals. New processing technologies have emerged

st 10 years allowing the development of novel ingre-unique functional properties. Among these, high milkwders are an example [4]. Native whey isolate (NWI)irectly from milk (i.e. not whey), presents a concentra-o acids exceeding that of conventional whey proteins.

ns: Lac, lactose; NMC, native micellar casein; NWI, native whey iso-cipal component analysis; Tout, outlet air temperature; XPS, X-rayspectroscopy.ding author. Tel.: +33 3 83 59 58 77; fax: +33 3 83 59 57 72.ress: [email protected] (C. Gaiani).

The production process of NWI is carried out at a low temperaturewith membrane technologies (microltration and ultraltration),and as such preserves the native structure of the protein. Nativemicellar casein (NMC), which is obtained by tangential membranemicroltration of skimmed milk, is also an attractive material dueto its high protein content and can be used as a relevant modelof milk micelles. Therefore, high milk proteins powders are usefulingredients for food products such as nonfat yogurt, ice cream andcheese.

Several important technological properties of these powdersdepend on particle-water interactions (wettability, rehydration,. . .) [5,6], particleparticle interactions (owability, stickiness, . . .)[7] and particleair interactions (oxidation, . . .) [1]. These inter-actions are in turn inuenced by morphological and physicalproperties [8], aswell as by the chemical bulk and surface composi-tion [9,10] of powders particles. Better understandingof the surfacecomposition, especially of lipids, is therefore needed to predict andappreciate some functional properties of dairy powders. Key func-tional properties of milk powders being largely affected by surfacecomposition, it is fundamental to understand how the major milkcomponents are distributed within the particle. Among the differ-ent techniquesused for characterizing thesurfaceand/orquantiedthe lipids, scanning electron microscopy (SEM) and solvent extrac-tionhave traditionally beenused.However the distinction between

see front matter 2009 Elsevier B.V. All rights reserved.colsurfb.2009.09.016rying temperaturea,, M. Moranda, C. Sancheza, E. Arab Tehranya, Mniversit, Laboratoire dIngnierie des Biomolcules, 2 avenue de la Fort de Haye, B.Ppus Ouest, UMR 1253, Science et technologie du lait et de luf, F 35042 Rennes, Fran

e i n f o

ay 2009vised form2009eptember 2009e 20 September 2009

a b s t r a c t

High milk proteins powders are commthese powders is expected to play an eTherefore, an eventual control of thecould be very useful in the improvemFor this purpose, the inuence of vepowders was investigated by X-ray pnative micellar casein and native whe

The results show a surface enrichmders. Whatever the drying temperatu/ locate /co lsur fb

owders is inuenced by

quota, P. Schuckb, R. Jeantetb, J. Schera

4505 Vandoeuvre Les Nancy Cedex, France

gredients in many food products. The surface composition ofial role during their storage, handling and/or nal application.ce composition by modifying the spray-drying temperaturef powder quality and the development of new applications.y-drying temperatures upon the surface composition of thelectron spectroscopy. The major milk proteins were studied:th more or less enriched in lactose.in lipids for all the powders and in proteins for many pow-pids and proteins are preferentially located near the surface

378 C. Gaiani et al. / Colloids and Surfaces B: Biointerfaces 75 (2010) 377384

the various components and their quanticationwas impossible bySEM and this technique was principally used to visualize the sur-face topography of powder particles [10,11]. The amount of surfacelipids could be quantied by solvent extraction, but it was demon-strated thatsurface andDespite itsunderstand

Within(XPS) has bcompositiotent [9,14,1proteins, lacharacterizsuch as wealso used t[1416]. Mstrated thewhereas lacthe core [9,study concwith the spwas limitedatures and danalyzed (s

The aimvariable spr150 C, upoders. For thX-ray photwetting pro

2. Materia

2.1. Materi

Native m(Rennes, Frmicroltratthrough wa90) was kiobtained byof microltwas obtainders are comand packed

Casein aanhydrousout XPS calfrom NMC a

2.2. Labora

The powa solids cowere left unrehydrationavoidmicrowere then ptions. The b(NMC), 0/1and 80/20/0using a Minat the follow150 C. Tout

air temperature and the liquid feed rate. Consequently, Tout wasxed after exploratory trials between 70 and 150 C (80 C beingthe outlet air temperature used by our industrial partners).

ysico

Chemwatryings deaticmeterred awer

DegraturitatepernKjeldjeldaes).

Wettwet

g asThed foettin

Physiparting (5mW300Fr feefor prage

es). Tparti

S an

EquipXP

pecthromsamd th

ed a3

0eV.05 eode wing tsuset 1.6im coerefoetersts wictra2.2.kgroctraextractable lipids obtained originates from the powderunfortunately also from the bulk of the particle [12,13].crucial importance, dairy powder surface compositioning is still very limited.the last 10 years, X-ray photoelectron spectroscopyeen successfully applied for investigating the surfacen of dairy powders especially for low fat powders con-5]. From the C, O and N percentages, surface contents inctose and lipids were deduced. XPS was already used toe powder surface in relation with functional propertiestting [9,10,15] and owability [7]. The technique waso study powder surface modications during storageany comparisons between bulk and surface demon-over-representation of lipids and proteins at the surfacetose and minerals were found mostly encapsulated in10,14]. On the best knowledge of the authors, only oneerns powder surface investigation by XPS in relationray-drying temperature [17]. Nevertheless, this studyto the examination of only two spray-drying temper-ifferent powders than those studied in this work were

kim and whole milk powders).of the present work was to consider the inuence ofay-drying outlet air temperatures, ranging from 70 ton the surface composition of high milk proteins pow-is purpose, surface composition was investigated by

oelectron spectroscopy (XPS) and related to powdersperties.

ls and methods

als

icellar casein powder was a king gift from UMR STLOance). NMC was obtained from tangential membraneion (0.1m) of skimmed milk followed by puricationter dialtration. Native whey isolate powder (Prolactandly provided by Lactalis (Laval, France). NWI wasmembrane tangential ultraltration and dialtration

rate collected during NMC production. Lactose powdered by Armor Protines (Loudac, France). All the pow-

mercial products that has been freshly manufactured.nd whey proteins from bovine milk (Sigma, France) andlactose (Fluka) were used as reference powders to carryculations on pure grade powders. Lipids were obtainednd NWI powders after a Folch extraction.

tory-scale production of powders

ders (NMC, NWI and lactose) were rst rehydrated tontent of 15% (w/w) the day before spray-drying andder stirring overnight at 20 C in order to obtain a totalof micellar casein. Sodium azide was also added to

bial development. Differentmixeddispersions (500ml)repared by combining NMC, NWI and/or lactose solu-lends (NMC/NWI/Lac) used in this work were: 100/0/000/0 (NWI), 70/0/30 (NMC+Lac), 0/70/30 (NWI+ Lac)(NMC+NWI). Finally, each dispersion was spray-driedi Spray Drier Buchi B290 (Buchi SARL, Rungis, France)ing outlet air temperatures (Tout): 70, 80, 110, 130 andcannot be controlled directly, but depends on the inlet

2.3. Ph

2.3.1.The

after dgen waenzymwere dmeasuiments

2.3.2.Den

precipthe suto thefromKanalys

2.3.3.The

wettincation.requirethe w

2.3.4.The

scatterwith awithapowdetationare aveanalysof the

2.4. XP

2.4.1.The

Ultra smonocto thetape anrecord700mand 16with 0lens m

Durtemwacurrenlens trand thparamponen

Spe(Visionthebacfor spechemical characterization of powders

ical analysiser content of powders was determined by weight loss1 g of powder at 105 C for 5h [18]. The total nitro-

termined by Kjeldahl [18]. Lactose was determined byethod using an Enzytec Lactose/D-Galactose kit. Lipids

mined according to the Folch method [19]. Ashes werefter incineration at 550 C during 5h [18]. All the exper-e done in triplicate.

ee of whey protein denaturationated whey proteins were precipitated at pH 4.6. Thewas removed by centrifugation (30min 1000 g) andatant was analyzed for the protein content accordingahl method. The degree of denaturation was deducedhl analysesbeforeandafterprecipitation (meanof three

ing propertiesting properties of powders were determined by staticdescribed by the FIL method [20] with some modi-powder (1 g) was poured into 10ml water. The time

r all the particles to be submersed is recorded and calledg time. Measurements were done in triplicate.

cal propertiesicle size distributions were determined by static lightMastersizer S, Malvern Instruments Ltd., Malvern, UK)

HeNe laser operating at a wavelength of 632.8nmlens. The sizedistributionsweredeterminedusingadryder attachment and the standard optical model presen-articles dispersed in air was used. The results obtaineddiameters calculated from Mie theory (mean of threehe criterion selected was the d(50) that means that 50%cles have a diameter lower than this criterion.

alyses

mentS analyses were carried out with a Kratos Axisrometer (Kratos Analytical, Manchester, UK) using aatic Al K source. The powder samples were spreadple holder using a double side conductive adhesiveen degassed overnight prior analyses. All spectra weret 90 take-off angle, the analyzed area being about00m. Survey spectra were recorded with 1.0 eV stepanalyzer pass energy and the high resolution regions

V step and 20eV pass energy. In both cases the hybridas employed.

he data acquisition the Kratos charge neutralizer sys-donall specimenswith the following settings:lament

A, charge balance 2.4V, lament bias 1.0V andmagneticil 0.375A. The CC (CH) carbon was set to 284.60 eVre used as an internal energy reference. With thesewe could obtain C1s signal with sharp, symmetric com-th a FWHM of 1.2 eV.were analyzed using the Vision software from Kratos2). A Shirley base line was used for the subtraction ofundandGaussian/Lorentzian (70/30%)peakswereusedl decomposition. Quantication was performed using

C. Gaiani et al. / Colloids and Surfaces B: Biointerfaces 75 (2010) 377384 379

Table 1Relative elemental composition of the reference samples measured by XPS (exclud-ing inorganic ions).

Initial powder Relative atomic concentration (mol%)

LactoseMean valuTheoreticaErrorc

CaseinMean valuTheoreticaErrorc

Whey proteiMean valuTheoreticaErrorc

Lipids (mainMean valuTheoreticaErrorc

a Mean valub Calculatedc Difference

value)/(mean

the photoemgiven in the

2.4.2. AppliThe foun

powders haative atomisurface layematrix formpounds maCa2p, S2p, anall of the poin the quan

IC = P ICP

IO = P IOP

IN = P INP

100 = P +In protei

of C in proteof C +mol olactose. In t= surface cgen, andnitfrom the arthe mole fraIOLi were thand INP , INLatose and lippowders (pused to carr

By solvintein, lactosthe directthe relativemass conce[14,17,21] ttrations dueto the relati

2.5. FTIR

2.5.1. EquipmentFTIR in transmission mode was performed on NWI powders in

o eva(70,metset wng rad sas ofg 25usheh whrefer penentr, Ka

Dataab

y-Gom1.tra. Sre. Tas uectr

ults

wde

ChemchemCpoeralsowdeundy conell wperimLac as alsoncesC O N

ea 61.62.5 38.42.8 lb 52.2 47.8

15.2 24.5

ea 68.60.3 17.60.8 13.80.1lb 65.0 19.0 16.0

5.2 7.9 15.9

nsea 68.00.5 18.00.9 14.10.2lb 65.0 19.0 16.0

4.4 5.5 13.5

ly phospholipids)ea 80.30.6 18.40.7 1.30.2lb 82.0 16.0 2.0

2.1 13.0 53.8

e of three independent measurements S.D.on the basis of the chemical formula.s between values (%) = [(mean value theoretically calculatedvalue)]100.

ission cross-sections and the transmission coefcientsVision package.

cation of XPS to the surface content of dairy powdersdation of the technique and its application to dairys been described elsewhere [9,10,14]. Briey, the rel-c concentration of carbon, oxygen and nitrogen in ther (10nm) of the powder was quantied and used in aula related to the surface content of the different com-king up the sample (i.e. lactose, proteins and lipids).d P2p havebeenalsodetected at levels lower than1% forwders. Consequently, these elements were neglected

tication procedure:

+ La ICLa + Li ICLi (1)+ La IOLa + Li IOLi (2)+ La INLa + Li INLi (3)

La + Li (4)

order tdryingspectrocal cellScannience anSamplegrindinthen crthroug

Thepowdecompo(Bruke

2.5.2.Raw

Savitsk1580 cto specsoftwaway) wFTIR sp

3. Res

3.1. Po

3.1.1.The

ForNMof minNWI palso fogloballably wthe exNMC+ticles idifferens, according to the chemical stoechiometry, P= (molins +mol of O in proteins +mol of N in proteins)/(molf O+mol of N); the same equations hold for lipids andhe following (Eqs. (1)(4)), this ratio will be dened asontent. IC, IO, IN were themole fractions of carbon, oxy-rogen in the sample surface. Thesevalueswereobtainedeas of the C1s, O1s and N1s XPS peaks. ICP , ICLa , ICLi werections of carbon in protein, lactose and lipids; IOP , IOLa ,e mole fractions of oxygen in protein, lactose and lipids, INLi were the mole fractions of nitrogen in protein, lac-ids. Experimental values obtained from the referenceure casein, whey proteins, lactose and lipids) have beeny out these calculations (Table 1).g the matrix, (P), (La), (Li), respectively the pro-

e and lipids surface contents were determined. Forcomparison with the bulk composition of powders,surface content calculated must be converted into

ntration. However, as already noted by some authorshe proportions of the sums of C+O+N molar concen-to lactose, protein and lipid are very close (within 1%)

ve mass concentrations of these compounds.

8m what

3.1.2. WheyWith in

powders (iddenaturatioFor NWI, th70 and 150denaturationents analythe FTIR speforNWI powof the absoamide I banfor 99% andquently, onpresented iopposed atily identieTout increas1618 and 1sheet structluatewheydenaturation andaggregationduring spray-80, 110, 130 and 150 C). A Tensor 27 mid-FTIR Brukerer (Bruker, Karlsruhe, Germany) equippedwith an opti-ith a KBr beamsplitter and a DTGS detector was used.

te was 20kHz and 256 scans were used for both refer-mple between 4000 and 850 cm1 at 2 cm1 resolution.powders were mixed with KBr (potassium bromide) bymg samples and 475mg KBr. The powder mixture wasd in a mechanical die press to form a translucent pelletich the beam of the spectrometer can pass.rence was rst recorded at 20 C on air and then thellet was analyzed. All treatments excepted principalanalysis (PCA) were carried out using OPUS software

rlsruhe, Germany).

treatmentssorbance spectra were smoothed using a 13 pointslay smoothing function and cut between 1720 andElastic baseline correctionusing200pointswas appliedpectra were then centered and normalized using OPUShe Unscrambler v7.6 software (CAMO ASA, Oslo, Nor-sed to calculate the PCA on centered and normalizeda.

rs characterization

ical and size characterizationsical composition of the powders is reported in Table 2.

wders, lactose and lipids traces are present. The amountis high and found in the ash fraction. In comparison,rs present fewer minerals. Lactose and lipids traces arein these powders. The moisture of all the powders isstant at 4%. The chemical composition agreed reason-ith the different mixes realized (Section 2.2). Indeed,ental ratio protein/lactose is found close to 70/30 fornd NWI+ Lac powders. The average size of powder par-determined after spray-drying (Table 2). No signicantare found between the powders, the d50 being aroundever the spray-drying temperature.

proteins denaturation and aggregationcreasing Tout, the only signicant difference betweenentical bulk composition) is the percentage of proteinn. This percentage clearly increases with Tout (Table 2).e denaturation increases from 2.8% and 19.5% betweenC. Lactose addition to NWI (NWI+ Lac) leads to lessn (from 1.7% to 8.7%). In addition, principal compo-sis (PCA) was used to study the correlation betweenctra (amine I) and the different outlet air temperaturesders. PCA are vectors described by linear combinations

rbancies measured in the infrared spectrum related tod. Principal components 1 (PC 1) and 2 (PC 2) account1% respectively of spectral changes observed. Conse-

ly the rst component is of interest. Its loading plotn Fig. 1 shows that bands at 1618 and 1705 cm1 arethe region near 1654 cm1. These three regions are eas-d as the most important absorption bands affected byed. A simultaneous increase in the adsorption band at705 cm1 reveals the formation of intermolecular -ures and intramolecular -strand or turns as the outlet


Table 2Characterization of the powders spray-dried (mean of triplicate analysis S.D.).

Outlet air temperature (C) Powder Chemical composition (%) Denaturation (%) d50 (m) Wetting time (s)

Moisture Ash Lipids Proteins Lactose

70

NMC 4.7 0.2 7.5 0.0 0.30.1 87.3 1.2NWI 4.9 0.0 2.3 0.2 0.40.2 91.9 0.8NMC+Lac 3.9 0.1 6.4 0.4 TRACES 63.1 0.8NWI+ Lac 4.7 0.3 1.9 0.5 0.50.1 64.4 1.3NMC+NWI 3.8 0.1 7.1 0.1 TRACES 88.8 1.5

80

NMC 5.0 0.2 7.7 0.6 0.80.0 86.4 1.1NWI 4.8 0.4 2.1 0.2 0.30.4 92.2 0.7NMC+Lac 3.8 0.3 6.9 0.2 0.70.1 62.0 0.5NWI+ Lac 3.8 0.0 1.6 0.5 TRACES 65.7 0.3NMC+NWI 5.9 0.2 7.4 0.3 TRACES 86.1 1.1

110

NMC 4.9 0.0 7.6 0.1 0.50.2 86.7 1.4NWI 4.8 0.0 2.2 0.0 TRACES 92.2 1.5NMC+Lac 5.0 0.7 5.6 0.0 TRACES 62.7 1.8NWI+ Lac 4.1 0.2 1.3 0.0 TRACES 66.1 1.9NMC+NWI 3.4 0.5 6.9 0.2 TRACES 89.1 0.7

130

NMC 3.3 0.3 8.2 0.4 0.50.1 87.6 0.8NWI 3.5 0.8 2.3 0.0 0.60.4 92.3 0.9NMC+Lac 3.8 0.2 7.1 0.1 TRACES 63.2 1.9NWI+ Lac 3.3 0.1 1.4 0.1 0.50.1 65.6 1.7NMC+NWI 5.7 0.2 7.0 0.5 TRACES 86.8 1.5

150

NMC 5.0 0.5 7.9 0.2 0.30.0 86.5 1.4NWI 4.9 0.4 2.4 0.1 0.40.1 91.6 1.5

0.50.60.7

Lac: lactose; N

air temperaistic of -his the spect

3.1.3. ElemThe elem

sured on plipids) and uThe elemensonably weand from litratio was nvalues.

Fig. 1. Loadinspectra of NWaverage of 3 ex

wde

Bulk/diffethe sableprayentedAs exC poNMC+Lac 4.8 0.3 6.3 0.0 TRACES 63.3 NWI+Lac 4.3 0.1 1.3 0.1 TRACES 67.8 NMC+NWI 3.9 0.3 7.1 0.3 0.60.1 88.7

MC: native micellar casein; NWI: native whey isolate.

ture rises. Concurrently, a decrease in bands character-elix/unordered structures is noticed (1654 cm1). Thisral signature of proteins aggregation [22,23].

ental composition of the reference powdersental composition of the reference powders was mea-ure components (whey proteins, casein, lactose andsed to calculate powder surface composition (Table 1).tal composition of lipids, whey and casein agreed rea-

3.2. Po

3.2.1.The

are inused (Terally scommature.of NMll with the values calculated from the chemical formulaerature. However, for lactose, the atomic concentrationot in full agreement with the expected stoichiometric

g plot of rst PC obtained after principal component analysis of FTIRI powders spray-dried at 70, 80, 110, 130 and 150 C. Each value is theperiments.

present. Whsurface appthe core of tbulk) and li5%). For NWvery high (msurface enrpresent atand NWI (8(96%), lipidslactose is lasurface usincentration (enrichmentformulationsurface enrpowders th

3.2.2. EffectThe surf

slightly moair temperalipids decreand NMC+tion decrearatio of lac0.2 0.0 7.5 0.1 932 530.5 0.1 2.8 0.9 8.3 0.2 1498 35

26.6 0.5 7.9 0.0 543 3928.5 0.4 1.7 0.4 8.5 0.1 1085 390.3 0.0 7.9 0.0 1001 430.1 0.0 6.9 0.1 631 310.6 0.0 4.4 1.1 8.6 0.1 1290 34

26.6 0.1 7.2 0.0 431 3628.9 0.0 2.5 0.1 9.1 0.1 991 270.6 0.0 7.4 0.2 882 210.1 0.1 8.6 0.1 642 210.8 0.0 9.2 0.2 8.8 0.0 1120 37

26.7 0.3 6.0 0.1 205 2728.5 0.1 7.4 0.4 7.1 0.1 666 150.6 0.0 7.4 0.2 811 270.4 0.1 8.6 0.2 639 121.3 0.3 12.2 0.1 8.6 0.2 1179 27

27.9 0.4 6.4 0.1 202 1229.3 0.2 6.5 0.4 8.7 0.2 557 150.5 0.0 8.3 0.2 758 160.3 0.0 8.0 0.2 623 160.7 0.1 19.5 0.3 8.7 0.2 1131 8

25.6 0.3 8.1 0.1 201 1526.6 0.4 8.7 0.6 8.1 0.1 499 120.5 0.2 8.3 0.0 628 13

rs surface composition

surface differencesrences observedbetweenbulk and surface compositioname direction whatever the spray-drying temperature3). Consequently, as industrial dairy powders are gen--dried at 80 C, comparison between bulk and surface isin details only for powders spray-dried at this temper-pected, the principal constituent found at the surface

wder is protein (99.4%). Lactose traces (0.6%) are also

en 30% of lactose is added to the bulk (NMC+Lac), theears very different. Lactose is supposed to be located inhe powder (only 16% at the surface instead of 30% in thepids are found over-represented at the surface (aroundI powders, the percentage of lipids at the surface isore than 30%). When lactose is added (NWI+ Lac), the

ichment in lipids is important (21%) and lactose is lessthe surface (only 12.7%). For powders containing NMC0/20), proteins are the main component at the surfaceare also foundover-represented at the surface (4%) and

cking. A differentiation between casein and whey at theg the XPS technique was impossible; their atomic con-in C, O and N) being too close. To summarize, a surfacein lipids and in proteins is observed whatever powderand the spray-drying temperature. Furthermore, the

ichment in lipids is much stronger for NWI containingan for NMC containing powders.

of drying temperatureace composition of casein containing powders is onlydied by the drying temperature. When the outletture increased (from 70 to 150 C), the percentage ofased from 5.3% to 0% and from 8.9% to 0% for NMCLac, respectively (Fig. 2). While the lipids concentra-ses slightly for casein containing powders, the surfacetose to protein increases slightly from 0.18 to 0.25 for


Table 3Powder composition on the dry matter basis and surface composition (assumed that the powders are composed of the three main compounds, i.e. protein, lactose and lipids)(mean of duplicate analysis).

Outlet air temperature (C) Powder Bulk composition (mass%) Surface composition (proportion ofC+O+N molar concentration %a)

Protein Lactose Lipids Protein Lactose Lipids

70

NMC 99.4 0.2 0.4 93.9 0.8 5.3NWI 99.0 0.6 0.4 66.1 0.1 33.8NMC+Lac 70.3 29.7 76.8 14.3 8.9NWI+ Lac 69.0 30.5 0.5 62.9 9.5 27.6NMC+NWI 99.7 0.3 89.6 0.3 10.1

80

NMC 99.0 0.1 0.9 99.4 0.6 0.0NWI 99.0 0.6 0.4 69.8 0.0 30.2NMC+Lac 69.4 29.8 0.8 79.3 16.0 4.7NWI+ Lac 69.5 30.5 66.3 12.7 21.0NMC+NWI 99.4 0.6 95.6 0.0 4.4

110

NMC 99.3 0.1 0.6 97.8 2.2 0.0NWI 99.1 0.9 88.6 0.0 11.4NMC+Lac 70.1 29.9 83.5 16.5 0.0NWI+ Lac 69.9 30.1 81.1 18.8 0.1NMC+NWI 99.3 0.7 94.3 0.2 5.5

130

NMC 99.0 0.5 0.5 98.5 1.5 0.0NWI 98.0 1.4 0.6 88.1 0.0 11.9NMC+Lac 69.4 30.6 80.3 19.7 0.0NWI+ Lac 68.8 30.7 0.5 79.2 20.8 0.0

92.9 0.0 7.1

150

Lac: lactose, Na Close to th

a bulk ratioders, Tout mIndeed, lipifrom 27.6%(Fig. 2).Whthe outlet tincreases frders containby Tout. For10.1% to 0%

In concluof proteinsthe surface

Fig. 2. Changeair temperaturnative micella

ettin

wetpo

. HowNMC+NWI 99.4 0.6

NMC 99.3 0.4NWI 98.8 0.7NMC+Lac 71.2 28.8NWI+ Lac 71.8 28.2NMC+NWI 98.8 0.6

MC: native micellar casein, NWI: native whey isolate.e mass concentration of the compound (%).

of 0.43 (Fig. 3). For whey proteins containing pow-odied strongly the percentage of lipids at the surface.ds surface content decreased from 33.8% to 10.6% andto 3.8% for NWI and NWI+ Lac powders, respectively

ile the lipids surface concentrationdecreases strongly as

3.3. W

TheAll the1500 semperature rises, the surface of ratio lactose to proteinom 0.15 to 0.25 (Fig. 3), for a bulk ratio of 0.43. Pow-ing a mix of casein and whey (80/20) are also affectedthese powders, lipids surface content decreased from(Fig. 2).sion, a lowerdrying temperature favors theappearance

over lactose at the surface of the particles and enhancesenrichment in lipids.

in lipids surface composition (%) of the powders with varying outlete (from 70 to 150 C) (mean of duplicate analysis). Lac: lactose; NMC:r casein; NWI: native whey isolate.

powders. Ashortest wetose beforeparticularlypowder sprting time ois signicansame for NW

Fig. 3. Surfacewith varying oLac: lactose; N0.3 96.5 3.5 0.00.5 85.4 4.0 10.6 80.3 19.7 0.0 77.0 19.2 3.80.6 99.2 0.8 0.0

g properties

ting properties of the powders are reported in Table 2.wders studied are completely wetted in less thanever, important differences are observed between the

s expected, powders containing lactose presented thetting times. By its hygroscopic nature, addition of lac-spray-drying could greatly improve water transferif the lactose is locatedat the surface. For example,NMCay-dried at 80 C is wetted in 630 s whereas the wet-f NMC+Lac powder spray-dried in the same conditiontly shortened around 230 s. The tendency is exactly theI powder (1290 s) in comparison with NWI+ Lac pow-

lactose/surface protein ratio obtained for powders containing lactoseutlet air temperature (from70 to 150 C) (mean of duplicate analysis).MC: native micellar casein; NWI: native whey isolate.


Fig. 4. Wettab(mean of threeindependent aisolate.

der (691 s).of the protewetting timtion betweeFig. 4. Forbetween thcorrelationcontain NM0.99 for NMpowders thand 0.89 fobecomewesurface lipid

4. Discussi

4.1. Powde

4.1.1. Evalupowders (Ta

The eleproteins reculated from[9,10,14,21experimentsamples [2from NMC[15,25], lipiphospholipvalues for lifor experimpreparationto the theorwere not inues and the24.5% for Odue to carbosphere duriproduct maexplained bysis. Threeto the adhepacking. Ththe surface

disk is not perfectly covered, some regions could be exposed to theX-ray photons. For example, comparative determinations for purelactose show that the relative atomic concentration of the sampleprepared by dusting could deviate from the theoretical concentra-

rticu

Surfasurfagreblener inrabley lipitablicouldrs. Rdy.e buns int [26), diff

samlipidswith

caseiey gild at tha

latioties

all foTouttionurfasets

e comowncopitiesuslyg prorati

esenility of the powders spray-dried at various outlet air temperaturesanalyses) in relation with lipids surface composition (mean of two

nalyses). Lac: lactose; NMC: nativemicellar casein; NWI: nativewhey

Important variations are also observed with the natureinwetted. NMCpowders present systematically shorteres thanNWIpowders. A correlation toevaluate the rela-n wetting properties and lipids surface is presented in

the 25 powders studied, a direct correlation is founde wetting time and the surface concentration. Highercoefcients are obtained for subset of samples whichC and have the same formulation: r2 =0.99; 0.98 andC, NMC+Lac and NMC+NWI, respectively. For wheye correlation coefcients are lower, respectively 0.85r NWI and NWI+ Lac. To summarize, powder particlesttedmore rapidlyathighspray-dried temperatures (lesss).

on

rs surface composition

ation of the matrix formula obtained from referenceble 1)mental composition in proteins (casein and wheyferences) agreed reasonably well with the values cal-

the chemical formula and obtained in other studies]. The differences observed between theoretical andal values may be due to some impurities in reference4]. Residual lipids used as references were extracted

tion pa[21].

4.1.2.The

was inby XPSatomizcompaered b[10] esencespowdeour stuaveragicatiocontencaseinFor thelowersistentfoundthat thwe couconten

4.2. Reproper

Forat lowcorrelalipids sfor subSurfacare knhygrosproperpreviowettinsurfaceThe prand NWI powders. In agreement with others studiesds in thesehighproteinspowders are found tobemainlyids. Comparison between theoretical and experimentalpids agreed well. The O percentage was slightly higherental values. Lipids could be slightly oxidized during, resulting in an increase in oxygen content comparedetical value. For lactose, the atomic concentration ratiostotal agreement with the expected stoichiometric val-difference between values was high (15.2% for C and

). The C peak intensity was more important and may ben contaminant and impurities collected from the atmo-ng the sample transfer into the XPS chamber or duringnufacturing [24]. The differences observed may be alsoy the sample preparation that occurs before XPS anal-methods are commonly used to x the powder samplesive tape [21]: dusting (used in this study), pressing ande sample mounting technique may have an effect oncomposition obtained. With the dust technique, if the

wetting prowetting timature. It theimportant tof whey pro

Neverthsurfacemayerties of thpowder rehtion, and stkept constaconnectedspecic surdifferent foporosity orhigh tempeever, particshown) welarly for the C1s as observed with the lactose reference

ce compositionace composition found for casein containing powdersement with previous study [9]. These authors analyzedds of NMC and NMC+Lac spray-dried in an industrialstead of a laboratory one in this study. The results were. The surface of whey powders was found largely cov-ds (30% at 80 C). For the same powder, another studyshed a lipid surface content higher (53%). The differ-be explained through the bulk composition of the

esidual lipids were found around 6% instead of 0.4% inHowever, it is well known that small variations in thelk lipid concentration could results in very large mod-

the lipids surface content, particularly at low lipids,27]. Depending on proteins nature (whey proteins orerent results were found on lipids surface enrichment.e blends (prot/lac: 100/0 and 70/30), casein presentssurface enrichment than whey proteins. This was con-the results of earlier investigations [1,28,29] which

ns more surface-active than whey proteins in the senseve a lower surface tension at air-water interface. Thus,ssume that casein powder would give a higher surfacen whey protein powder.

n between spray-drying temperature and wetting

rmulations, lipids surface content was more importantand could explain the poorer wetting times found. Awas established between the wetting properties andce content. This correlation was linear (0.85< r2


4.3. Inuence of spray-drying temperature upon surfacecomposition

4.3.1. LipidsThe mo

spray-dryinsignicantlyair temperawith anotheperatures (Tthe lipid surand low feecontent (30consideredmodicatioperature dias all the suto the best kture on surfproteins po

4.3.2. LactoHigher d

over proteilactose/protaining powwhey proteteins denateffect on thimportant cthat increasand aggregto secondaing, unfoldthe heat trefavorably foteins hydrothe interfacincrease hytion) and toboth inuen

4.3.3. SurfaDependi

tioned abouone (scenartiononair/lthis scenariface andappfat dropletsond hypoth[35,36]. Duface is accocenter of thular weightOver time,enriched inlower diffusbe enriched

It is likelbe possibletion of surfathen duringthe surfacehypothesesto migrate.

to our results, surface-active components are found at the surface(proteins and phospholipids) at low Tout. As the spray-drying tem-perature increased, the surface may solidify and immobilizes thecomponents quickly. Consequently, surface-activemolecules could

ve enarticair tsprad an

clus

as ssurfs pohis slledl of peveabo

ure,ntrol

wled

thanXPSs thascie

nces

. Keogbilityk ingrillqvses, Charsa

ay-dryearch. Mist(2002aiani,erize mcts, Joaianiaviouature06) 14.J. Kimndust(3) (20Bumauencery Jouaianiitioncts on(1) (20.J. Kimddairpertie. MistrostruBumadeter52.. Vignry pow07) 18ldt, Ssity Daiani,shipsing stmistrst remarkable change in surface composition withg temperature is observed in lipids surface content. Itdecreased for all the powders studied when the outlet

ture increased. These observations were in agreementr studywhich investigated only two spray-drying tem-out = 85 and 105 C) [17]. In addition, greater changes inface composition were reported for skim milk powdersd solids content (10%), compared to high feed solids%). In the present study, the feed solid content can beas low (15%) and may explain the important surfacens observed. For whole milk powder, the drying tem-d not appear to greatly affect the surface compositionrface was covered by lipids [17]. In the literature andnowledge of the authors, inuence of drying tempera-ace composition was not yet investigated on high milkwders.

se to protein ratiorying temperature favors the appearance of lactose

n at the surface of the particle (Fig. 3). Nevertheless,tein ratio was only slightly modied for casein con-ders whereas the ratio was considerably increased forins powders. This difference may be explained by pro-uration. Heating casein micelles below 140 C has littleeir stability whereas the same heat treatment induceshanges in the structure of whey proteins. It was founding Tout induced whey proteins denaturation (Table 2)ation (Fig. 1). The sensitivity of the amide I vibrationry structure make it possible to study proteins fold-ing, aggregation, etc. [23]. It has been proposed thatated proteins are less surface-active and competes lessr the interface than the untreated protein [30]. Pro-

phobicity andproteinmolecularmass are both affectinge. Increasing the outlet air temperature could result todrophobicity of whey proteins (as a result of denatura-increase its molecular mass (as a result of aggregation)cing interface formation.

ce formation mechanismsng on the authors, two scenarios are generally men-t the surface formation of the drying droplet. The rstio 1) was based on surface-active components adsorp-iquid interface of the spraydroplet [14,34]. According too, protein adsorbs preferentially to the air/liquid inter-ears on thepowder surface after spray-dryingwhereas,are largely found inside the particles [14,29]. The sec-esis (scenario 2) is based on solid/solutes segregationring spray-drying, diffusion of water towards the sur-mpanied by an opposite diffusion of solids toward thee particle. As a consequence, solutes with a high molec-will accumulate more slowly than smaller molecules.the outer surface of the droplet would then becomethe larger molecules. As lipids and proteins present aivity than lactose and inorganic ions, the surface wouldin these components [10,36].

y that a combinationbetween these two scenarios couldin this work. During the rst step (atomization), migra-ce-active components toward the surface could occur,the last step (spray-drying) a diffusion of solutes fromto the interior of the spray droplets could follow. Theseassumed that there is enough time for the componentsThis is probably not the case in this study. According

not hanal poutletduringreache

5. Con

It wor lessproteinature. Tcontrocontro

Howknownthe futand co

Ackno

Wevidingauthorand its

Refere

[1] M.KStamil

[2] A. Mpha

[3] A. GsprRes

[4] V.V82

[5] C. Gacteffe

[6] C. Gbehper(20

[7] E.Hof i46

[8] T.J.inDai

[9] C. Gposeffe49

[10] E.Hdriepro

[11] V.Vmic

[12] T.J.the42

[13] M.Ldai(20

[14] P. Fver

[15] C. GtiondurCheough time to migrate at the surface; the surface of thele being closer to the bulk composition. By increasingemperature over 110 C, the diffusion of componentsy-drying seems to be interrupted before the system hasequilibrium.

ion

hown that surface composition modication (moreace lipids and/or lactose) was possible on high milkwders bymodifying the spray-drying outlet air temper-tudy couldhelp in the future to formulatepowderswithsurface composition which in turn will allow a betterowder functionalities.r, one point needs more investigation. Very little isut dynamics of the formation of the drying droplets. Inknowledge on this point will be required to understandmechanisms behind surface formation.

gements

k J. Lambert, LCPME Engineer CNRS (Nancy), for pro-analysis and J.J. Ehrhardt for scientic assistance. ThenkalsoARILAITRECHERCHE (Paris) fornancial supportntic committee is gratefully acknowledged.

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urnal of Dairy Science 88 (2005) 27002706., J. Scher, P. Schuck, J. Hardy, S. Desobry, S. Banon, The dissolutionr of native phosphocaseinate as a function of concentration and tem-using a rheological approach, International Dairy Journal 16 (12)271434., D.C. Xiao, D. Pearce, Effect of surface composition on the owabilityrial spray-dried dairy powders, Colloids and Surfaces B: Biointerfaces05) 182187., Free fat in spray-dried whole milk. 3. Particle size. Its estimation,of processing parameters and its relation to free fat, Netherland Milkrnal 25 (1971) 5372., J.J. Ehrhardt, J. Scher, J. Hardy, S. Desobry, S. Banon, Surface com-of dairy powders observed by X-ray photoelectron spectroscopy andtheir rehydration properties, Colloids and Surfaces B: Biointerfaces06) 7178., X.D. Chen, D. Pearce, Surface characterization of four industrial sprayy powders in relation to chemical composition, structure andwettings, Colloids and Surfaces B: Biointerfaces 26 (3) (2002) 197212.ry, H.N. Hassan, D.J. Robinson, Effect of lactose and protein on thecture of dried milk, Food Structure 11 (1992) 7382., Free fat in spray-dried whole milk. 2. An evaluation of methods formination of free-fat content, Netherland Milk Dairy Journal 25 (1971)

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How surface composition of high milk proteins powders is influenced by spray-drying temperatureIntroductionMaterials and methodsMaterialsLaboratory-scale production of powdersPhysicochemical characterization of powdersChemical analysisDegree of whey protein denaturationWetting propertiesPhysical properties

XPS analysesEquipmentApplication of XPS to the surface content of dairy powders

FTIREquipmentData treatments

ResultsPowders characterizationChemical and size characterizationsWhey proteins denaturation and aggregationElemental composition of the reference powders

Powders surface compositionBulk/surface differencesEffect of drying temperature

Wetting properties

DiscussionPowders surface compositionEvaluation of the matrix formula obtained from reference powders (Table 1)Surface composition

Relation between spray-drying temperature and wetting propertiesInfluence of spray-drying temperature upon surface compositionLipidsLactose to protein ratioSurface formation mechanisms

ConclusionAcknowledgementsReferences

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