LWT 40 (2007) 1593

nm

en

ersi

Qu

sity

m 6

Spray drying is the most common method of dehydrat-

viscosity decreases and this happens when the producttemperature is around or more than its glass transitiontemperature. These phenomenon trigger a chain of events

powder results in poor product quality, lower yield during

Slade, 1986).Due to lactose intolerance in a signicant number of

world populations, lactose reduction in milk and milk

ARTICLE IN PRESSproducts has become a useful strategy in enhancing theconsumption of dairy products. Although milk withreduced or almost zero lactose content is available in the

0023-6438/$30.00 r 2006 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved.

doi:10.1016/j.lwt.2006.11.003

Corresponding author. Tel.: +612 33469 642; fax: +61 7 3365 1177.E-mail address: a.shrestha@uq.edu.au (A.K. Shrestha).ing milk and milk products. It involves rapid removal ofmoisture leading to the formation of amorphous lactosewhich forms a continuous matrix in which proteins, fatglobules, and air cells disperse (Aguilar & Ziegler, 1994).Amorphous lactose and other sugars (if present) in milkpowders can undergo a glassy-to-rubbery transition whenheld at a temperature higher than their glass transitiontemperature (Tg). Amorphous sugar particles are highlyhygroscopic and will absorb water at higher humidityresulting in plasticization that lowers the Tg of the particlessignicantly. The molecular mobility increases when

drying, operation problems and difculties in handling andstorage.The major ingredient in milk powder is lactose which

has a relatively high Tg of 97116 1C (Haque & Roos,2004a; Hill, Craig, & Feely, 1998; Jouppila & Roos, 1994a;Roos & Karel, 1990). The presence of other componentssuch as moisture, protein(s), fat, mineral and lactic acidcan largely affect the physico-chemical behaviours includ-ing water absorption, glass transition temperature andcrystallization of the milk powders (Berlin, Anderson, &Pallansch, 1973; Jouppila & Roos, 1994a, 1994b; Levine &were determined. Spray drying of skim milk with hydrolysed lactose resulted in very low cyclone recovery of 25% and a large amount of

powder remained stuck inside the spray dryer. The equilibrium moisture content of SMPHL was lower than that of lactose for each

range of water activity when humidied for 21 days at 23 1C using saturated salt solutions. Unlike lactose, SMPHL did not lose waterwhen the water activity exceeded 0.432 and no crystallization was noticed at water activity X0.753. The sorption isotherm data forSMPHL tted well with the BET and GAB models with monolayer moisture contents of 7.55 and 8.27 g/100 g, respectively. The glass

transition temperature of anhydrous SMPHL was 49 1C. The critical water activity and moisture content for SMPHL were 0.15 and2.4 g/100 g dry solid, respectively. The low critical values indicated hydrolysis of lactose necessities maintenance of very low moisture of

powder for its long-term stability.

r 2006 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved.

Keywords: Skim milk powder with hydrolysed lactose; Glass transition temperature; Lactose; Stability

1. Introduction leading to stickiness, caking and crystallization (Jouppila &Roos, 1994b; Roos & Karel, 1992). Stickiness of dairyThe moisture sorption behaviour and glass transition temperature of spray dried skim milk powder with hydrolysed lactose (SMPHL)Water sorption and glass transitiohydrolysed ski

Ashok K. Shresthaa,, Tony Howesb, BaCentre for Nutrition and Food Sciences, The Univ

bSchool of Engineering, The University ofcSchool of Land and Food Sciences, The Univer

Received 29 May 2006; received in revised for

Abstract1600

properties of spray dried lactosemilk powder

u P. Adhikaria,b, Bhesh R. Bhandaric

ty of Queensland, Brisbane, QLD 4072, Australia

eensland, Brisbane, QLD 4072, Australia

of Queensland, Brisbane, QLD 4072, Australia

November 2006; accepted 7 November 2006

www.elsevier.com/locate/lwt

equipped with Intracooler II, Perkin-Elmer 7, CT, USA)was used to determine the glass transition temperature ofall spray-dried powders. The purge gas used was drynitrogen (20mL/min). Although onset and endset Tgvalues for samples were calculated in the DSC thermo-gram, only the Tg value determined as half DCp methodat half the extrapolated change in specic heat (DCp)between the glassy state and the rubbery state (orpeak value) is reported in this study. Indium and zinc(Perkin-Elmer standards) were used for temperatureand heat ow calibration. An empty aluminium panwas used as a reference. About 510mg samples werescanned in hermetically sealed 50 mL DSC aluminiumpans (Perkin-Elmers). All analyses were done in triplicate.The rate of thermal scanning was carried out in the

ARTICLE IN PRESSWTmarket, the availability of reduced or hydrolysed lactosemilk powders are not widespread.The glass transition temperature of a carbohydrate is

inversely proportional to its molecular weight (Fox &Flory, 1950; Levine & Slade, 1990; Roos, 1993). Hydrolysisof lactose produces low molecular weight monosacchar-ides, glucose and galactose. The Tg values of glucose andgalactose are 31 and 32 1C, respectively (Roos, 1993; Roos& Karel, 1991a). Previous studies have also reported muchlower Tg in skim milk powder with hydrolysed lactose(Fernandez, Schebor, & Chirife, 2003; Jouppila & Roos,1994b). It is clear that if there is a slight increase inmoisture content, the Tg of milk powders containinghydrolysed lactose will go below room temperature andthese products are likely to be stickier than the correspond-ing powders. This would certainly result in signicantchanges in the spray drying behaviour and storagestability of milk powders containing hydrolysed lactose(Roos, 1993). The sorption behaviour and glass transitiontemperatures of milk powders with hydrolysed lactoseprepared by freeze drying have been studied by Jouppilaand Roos (1994b) and Fernandez et al. (2003). However,no data exists for similar studies using spray-dried milkpowder with hydrolysed lactose.The objectives of this study were to determine the spray

drying behaviour of skim milk with hydrolysed lactose andmeasure the water sorption behaviour and glass transitiontemperature of the resulting powders to predict criticalwater content and storage conditions.

2. Materials and methods

2.1. Materials

Lactose free (hydrolysed) skim milk (brand nameLiddells), manufactured by Liddell Group Pvt. Ltd.Boronica, Vic., Australia, was purchased from the localsupermarket and stored in a refrigerator until spray-dried.The composition of the milk as labelled in the packaging isgiven in Table 1. Edible grade lactose, a-lactose mono-hydrate, from Murray Goulburn Cooperative Co. Ltd.,Melbourne, Australia, was purchased from a localdistributor.

2.2. Sample preparation

An RV10 Rising Film Vacuum Evaporator (SaurinTechnology, Melbourne, Australia), with a capacity of10 kg evaporation per hour was used for the initialconcentration of lactose-free skim milk. The temperatureof water used to heat milk was 70 1C and condensertemperature was 60 1C. A total of 6.2 kg lactose free skimmilk was evaporated to 1.6 kg of concentrated liquid. Thetotal soluble solid of the concentrated milk was measuredas 301brix.

A.K. Shrestha et al. / L1594The concentrated milk solution (301brix) was warmed toabout 50 1C and spray-dried. The spray dryer (SaurinTechnology, Melbourne, Australia) was a twin uid nozzletype with 3 l/h water evaporation capacity. The inlet andoutlet temperatures of the dryer were set at 130 and 65 1C,respectively. The amounts of the powder collected incyclone collector (cyclone recovery) and also by manualsweeping the walls of spray dryer (sweep recovery) werecalculated. Total recovery was calculated by addingcyclone and sweep recovery. Powder recovery was chosenas a measure of spray drying performance as it is easilymeasured with reproducible results (Bhandari, Datta,Crooks, Rigbi, & Howes, 1997). Skim milk powderwith hydrolysed lactose (SMPHL) collected from theoutlet cyclone was immediately vacuum packed in aCryovacsplastic bag and stored in a dry chamber.A 25 g/100 g lactose solution was also spray-dried at180 1C inlet and 80 1C outlet temperatures.The moisture content of the freshly spray-dried powders

was determined using AOAC method 927.05 (AOAC,1990). Water activity of the powders was measured with anAquaLab 3 Water Activity Metre (Decagon Devices, Inc.,Pullman, USA) at 25 1C.

2.3. Glass transition temperature (Tg)

Differential scanning calorimetry (DSC) (Pyris 1

Table 1

Composition of skim milk with hydrolysed lactosea

Nutrients Lactose free skim milk

Energy 114 kJ (27 cal)

Protein 3.4 g

Fat 0.1 g

Carbohydrate 4.8 g

Sugars 4.8 g

Galactose 2.4 g

Lactose o0.1 g/100 gSodium 35mg

Calcium 122mg

aBased on information given on the label.

40 (2007) 15931600following order: (1) Isothermal at 20 1C for 1min;(2) heat scanning from 20 1C to a temperature just over

ARTICLE IN PRESSWTthe expected Tg at 10 1C/min; (3) cooling rapidly to 20 1Cat 50 1C/min; and (4) heat scanning from 20 to 200 1C.The second scanning of each sample was used to reduce theenthalpy relaxation of the amorphous powder whichappears in the rst scan, thereby enhancing the accuracyof Tg measurement on the DSC thermogram. The transferof samples from the container to the DSC pan was done ina sealed Dry Box containing silica gel with regular N2ushing, to avoid unwanted moisture absorption bythe sample.

2.4. Sorption isotherm studies

The spray-dried lactose was dried overnight at 70 1C in avacuum oven followed by further drying in vacuumdesiccators over P2O5 for a week. Considering the lowerTg value of hydrolysed lactose products, spray-driedSMPHL was directly put into a P2O5 containing desiccator.To make sure the powders were fully dried, these werefurther analysed for water activity and residual moisturecontent. About 2 g of amorphous powders, in triplicate,were transferred into the pre-weighed plastic cups with ascrew cap and placed in evacuated desiccators over P2O5and different saturated salt solutions of LiCl, CH3COOH,MgCl2, K2CO3, Mg(NO3)2, KI and NaCl with respectiverelative humidities of 11.4%, 23.1%, 33.2%, 44.1%,52.9%, 68.9% and 75.3% at about 23 1C, giving aw of0.01%RH (Labuza, Kaanane, & Chen, 1985). Thesamples were stored for 21 days at 2324 1C. Afterequilibrium was reached, the samples were tightly closedwith the screw cap, weighed and stored in a dry glasschamber containing silica gel until further analysis for Tg.The moisture content of each sample was measured.The BrunauerEmmettTeller (BET) and Guggenheim

Andersonde Boer (GAB) equations which have previouslybeen used to model water sorption data for dehydratedmilk powders (Haque & Roos, 2004b; Jouppila & Roos,1994a, 1994b; Van der Berg & Bruin, 1981) were used forprediction of water content. The BET isotherm model(Brunauer, Emmett, and Teller, 1938) is given by

aw

1 awm C 1

moC

aw

1

moC. (1)

The BET isotherm was plotted aw/(1aw)m against aw.In the equation, (C1)/moC and 1/moC form the slopeand intercept, respectively, of the straight-line equation(Eq. (2)).In Eq. (3), mo is the BET monolayer value, m and aw are

the equilibrium moisture content and water activity of theproduct, respectively, and C is a constant related to excessenthalpy of sorption. Calculating the values for mo and Cby the linear regression of the experimental data, themoisture content (m) of the sample can be predicted byusing the following equation:

A.K. Shrestha et al. / Lm moCaw1 aw 1 C 1aw . (2)The GAB isotherm model and its second-order poly-nomial equation as given by Van der Berg and Bruin (1981)and Haque and Roos (2004b) are shown as follows:

m moCKaw1 Kaw 1 Kaw CKaw . (3)

Second-order polynomial quadratic equation

aw

m aaw2 baw g, (4)

where

a Kmo

1

C 1

; b 1

mo1 2

C

; l 1

moKC.

The solutions for the above equations were

K b

b2 4ag

q2g

; C bgK

2,

and

mo 1

aKC.

The tting of BET and GAB models was checked bycalculating relative percentage root mean square (%RMSvalue) as given by Haque and Roos (2004b)

%RMS

SNi

mempmp

h i2r

N 100, (5)

where me is the experimental moisture content and mp is thepredicted moisture content, and N is the total number ofexperimental points.

2.5. Prediction of Tg values using the GordonTaylor

equation

A plot of Tg of the samples versus equilibrium moisturecontent of all sorbed samples was constructed. The effect ofmoisture content on Tg values of the products was alsodiscussed. The GordonTaylor equation (Gordon &Taylor, 1952) was used to model data on Tg values forvarious milk ingredients:

Tg w1Tg1 kw2Tg2

w1 kw2,

where Tg is the predicted Tg of a binary system, w1 and w2are weight fractions of solids and water, respectively, andTg1 and Tg2 are the glass transition temperatures of drysolid and water, respectively, and k is an empirical constantfor a system, i.e., 0.69 for lactose (1):water (2) system(Saltmarch & Labuza, 1980). The Tg value of water wastaken at 135 1C (Johari, Hallbrucker, & Mayer, 1987).The k value for amorphous powder was determined byaveraging the k values from experimental Tg and corre-sponding water contents. This k value was used to predict

40 (2007) 15931600 1595the Tg of the given amorphous dairy powder (Haque &Roos, 2004a).

3. Results and discussions

3.1. Spray drying behaviour

Considering the low Tg value of hydrolysed lactose milkand possible stickiness, a rather mild spray dryingcondition of 130 1C inlet and 65 1C outlet temperaturewas used. Only about half of the solids from the milk spraydried could be recovered from spray drying of the lactosefree skim milk (50% total recovery). A signicant portion,half of the total recovered powder was collected in thecyclone collected (25% cyclone recovery), the rest ofthe solids was salvaged by light manual sweeping of thepowders stuck inside wall of spray dryer, particularlyon the conical section of the dryer (25% sweep recovery).It was noted that a large amount of powder (50%)was rmly stuck (caked) inside the wall of the dryer

SMPHL and lactose at room temperature (2324 1C) at

ARTICLE IN PRESSA.K. Shrestha et al. / LWT1596equilibrium after 21 days is given in Table 2 and trend isalso shown in Fig. 1. Comparison of sorption data forSMPHL with those reported by Jouppila and Roos (1994a)for freeze-dried skim milk with hydrolysed lactose humi-died for 1 day showed very close results at all lower wateractivities. It is interesting to note that both the samples

Table 2

Glass transition temperature (Tg) of SMPHL and lactose mixtures

humidied at different activitiesa,b,c

Water activity SMPHL Lactose

Water (%) Tg (1C) Water (%) Tg (1C)

0 0.0 47.773.8 0.0 97.875.50.113 1.770.2 26.273.2 2.270.0 82.271.80.225 3.970.0 9.372.1 4.670.1 57.872.00.328 5.670.1 3.571.3 6.770.0 46.772.60.432 8.170.0 NAd 9.870.2 23.970.50.529 11.170.3 NA 4.070.1 10.772.40.689 20.270.3 NA NA NA0.753 26.170.7 NA NA NA

aMean values7standard deviation of triplicate samples.bMoisture is presented as g H2O/100 g dry solid.which could not be swept. The 74% recovery of lactosein cyclone collector further demonstrated the stickynature of the hydrolysed lactose product. Under theexisting spray drying conditions, it is unlikely to obtainSMPHL powder, as the products are difcult to recoverfrom the dryer.

3.2. Water sorption

Moisture plays important role in glass transition andcrystallization behaviour of amorphous powders thatdetermines its owability, stickiness and storage stability.The experimental moisture adsorption isotherm ofcFor glass transition temperature only mid-point values are reported.dNot analysed.humidied under similar conditions for 1 and 21 days havesimilar moisture content. This indicates that the SMPHLabsorb moisture quickly and are close to equilibriumwithin 24 h and further humidication did not increase thewater content of the powders. The SMPHL absorbedcomparatively less water than lactose at each range ofwater activity, up to awX0.432. This shows spray driedSMPHL has different water absorption capacity than thoseof spray dried lactose. The sorption isotherm curve wassteeper than lactose (Fig. 1). Typically lactose started tolose the sorbed water at awX0.432 indicating signs ofcrystallization. The SMPHL, however, did not lose thesorbed water at any stage of humidication in the givenrange of water activities. It indicates that crystallization inSMPHL did not occur even for aw as high as 0.753.Jouppila and Roos (1994a) also reported no crystallizationfor SMPHL when stored at awX0.764 but SMP crystal-lized at awX0.662 (24 h storage). However, we found thatthe spray-dried SMP starts to crystallize at awX0.432when stored for a week (Shrestha, Adhikari, Howes, &Bhandari, 2006). The result showed that the presence ofhydrolysed lactose markedly affects the water sorptionbehaviour of SMP.The BET and GAB equation were applied to model the

water sorption of SMPHL and lactose. Fig. 2 shows typicalsigmoidal adsorption isotherm curves of milk powders.For SMPHL, moisture data was available for wateractivity range up to 0.753. The predicted water content(g H2O/100 g dry solid) of SMPHL was very close to theexperimental values for both BET and GAB models(Fig. 2). The experimental values for skim milk withhydrolysed lactose from Jouppila and Roos (1994a) alsoshowed very good tting with the BET and GAB models,except at very high aw.

3.3. Sorption isotherm

The sorption isotherms describe the relationship betweenwater activity (aw) and the equilibrium moisture content ofa given food at a constant temperature. The sorptionisotherms can provide data about the shelf life stability of agiven food commodity. One of such parameters is themonolayer moisture content (Xm) which helps to denephysical and chemical stability of foods, since it has a directinuence on lipid oxidation, enzyme activity, non-enzy-matic browning, avour preservation and product struc-ture (Labuza, Tannebaum, & Karel, 1970).The BET and GAB monolayer moisture contents for

SMPHL were 7.72 and 8.27 g/100 g dry solid, respectively(Table 3). The BET monolayer value for SMPHL waslower than 9.34 g/100 g dry solid and GAB value wasslightly higher than 7.72 g/100 g dry solid as reported byJouppila and Roos (1994a). The difference in monolayermoisture could be due to the use of freeze dried samples, ashorter humidication time of 24 h and the water activity

40 (2007) 15931600range of 0.1140.444 for BET and 0.1140.538 for GABused by Jouppila and Roos (1994a). The BET and GAB

ARTICLE IN PRESS

Fig. 2. Water sorption isotherm of SMPHL (A) and lactose (B) humidied for 3 weeks at various water activities at 23 1C using experimental data (),BET predicted (), GAB predicted (m) and from Jouppila and Roos (1994a) (K).

Fig. 1. Moisture contents and glass transition temperatures of SMPHL and lactose humidied for 3 weeks at various water activities at 23 1C: Tg ofSMPHL (), Tg for lactose (K), moisture for SMPHL (~) and moisture for lactose (m).

Table 3

Constants for the sorption isotherm models, GAB and BET, including monolayer moisture contenta

Samples a b g Xmc K c R2 %RMSd

GAB model

SMPHL 0.055 0.008 0.066 8.27 0.97 1.87 0.98 1.14Lactose 0.102 0.041 0.047 6.96 1.10 2.81 0.997 0.24Lactoseb 0.149 0.667 0.045 4.77 1.589 2.93 0.984 0.29Samples b c K Xm R

2 %RMS

BET model

SMPHL 0.064 0.065 1.99 7.72 0.865 1.98

Lactose 0.074 0.048 2.533 8.160 0.997 0.25

Lactoseb 0.076 0.050 2.492 8.026 0.982 0.88

The meaning of above notations is explained in Materials and methods section.aWater activity range of 0.113oaw40.432 selected for lactose and SMPHL.bData from Haque and Roos (2004a) to compare the results.cXm Monolayer moisture content, gH2O/100 g dry solid.dRMS root mean square value.

A.K. Shrestha et al. / LWT 40 (2007) 15931600 1597

lactose

ARTICLE IN PRESSWTThe Tg values of spray-dried SMPHL and lactose atcorresponding moisture contents are given in Table 2 (andFig. 1). The Tg value of SMPHL in dry state was 48 1Cwhich is less than the value for dry lactose, 97.8 1C and dryskim milk powder or SMP at 93 1C (Shrestha et al., 2006).Similarly, Tg values of the SMPHL were less than lactosewhen measured at other water activities (and moisturecontents). Jouppila and Roos (1994b) reported a Tg valueof 49 1C for anhydrous hydrolyse lactose skim milk powderwhich is very close to the present result. Fernandez et al.(2003) measured the Tg values of whole milk powder withpartially hydrolysed lactose (0.6 g H2O/100 g, and lactose,glucose, galactose ratios of 8:15:15) and also a mixture oflactose, glucose and galactose (22:39:39). They reported aTg value of about 58 1C for hydrolysed whole milk powder(moisture about 0.6 g/100 g). The Tg values of pure glucoseand galactose are 30 and 31 1C, respectively (Roos, 1993;Roos & Karel, 1991a). The low Tg value of the SMPHLcompared to SMP was due to the presence of glucose andgalactose in SMP. The higher Tg value (431 1C) ofSMPHL suggests the presence of residual unhydrolysedlactose, more than the label claim of o0.1% in milk orabout 1% on a dry basis. Jouppila and Roos (1994b)and Fernandez et al. (2003) also reported a higher Tgvalue for freeze dried SMPHL and suggested that residualmonolayer values for lactose were 8.16 and 6.96 g/100 g drysolid, respectively. The available data on monolayer valuesfor lactose vary widely: 6.29 for BET and 4.91 for GAB(Jouppila & Roos, 1994a); 6.26 for BET (Jouppila & Roos,1997); 6.93 (at 12 1C) and 4.9 (at 2038 1C) for GAB(Bronlund & Paterson, 2004); and 8.03 for BET and 4.77for GAB (Haque & Roos, 2004a). It shows the monolayervalue for amorphous lactose varies depending on themethod of sample preparation; humidication techniques:storage time and temperature; water activity range used,etc. The monolayer moisture content values (Xm) indicatedthe hydrolysed lactose milk powder is fairly stable atmoisture content of about 8 g/100 g dry solids, and abovethis critical moisture content, the stability of the powderswill be low.Table 3 shows the BET and GAB parameters including

%RMS and R2 values for lactose and SMPHL. Data fromHaque and Roos (2004a) is also included to compare thelactose results. The %RMS values of SMPHL for GABand BET models were calculated as 1.14 and 1.98,respectively, whereas for lactose both models gave almostsimilar values of about 0.25. The regression coefcients ofboth models were closer to unity, except for BET model inSMPHL which is low at 0.865. This indicates that GABmodel is better tted for these powders that BET model.

3.4. Glass transition temperature (Tg) of SMPHL and

A.K. Shrestha et al. / L1598unhydrolysed lactose might have increased the Tg ofthe mixture.The Tg values of SMPHL decreased signicantlywith increase in moisture content. Table 2 shows thateven a slight absorption of water by dehydrated SMPHL(1.7 g/100 g dry basis) almost halved the Tg value. Even atmoderately low humidity (23.8%RH), the Tg value ofSMPHL was below 0 1C (Table 2). Interestingly the powderremained more or less free owing under the ambientcondition which is most likely to be contributed by the milkproteins. SMP contains signicant amounts of proteins(3437 g/100 g) that can affect the water sorption andcrystallization behaviour lactose in the protein lactosematrix. It has been well established that incorporation ofhigh molecular weight compounds that have high Tg haveability to raise the Tg value of the material containing lowTg compounds (Bhandari et al., 1997; Roos & Karel,1991b). This study further veried our previous ndingthat proteins form incompatible mixture with the sugarcompounds and do not inuence the Tg value (Shrestha,Adhikari, Howes, & Bhandari, 2005), which, in presentcase, is solely contributed by residual and hydrolysedlactose compounds. The role of (milk) proteins in retarda-tion of crystallization and subsequent stickiness and cakingof lactose in milk powder is not known and warrantsfurther research.The suitability of DSC as a technique to measure Tg of

protein-rich milk powders is questionable. In previousstudy we observed that the endothermic peak associatedwith glass transition of high protein (low lactose) becomesbroader with increase in protein concentration and nopeaks can be seen if protein level is too high (Shresthaet al., 2005). The Spray Drying Research Group at TheUniversity of Queensland, Brisbane, Australia, has recentlydeveloped a technique based on thermal mechanicalcompression test to measure glassrubber transitiontemperature, Tgr (Boonyai, Howes, & Bhandari, 2005).It consistently gave higher transition temperature when thelevel of protein in amorphous solid increased (Shresthaet al., 2006). Therefore, to analyse the mechanicalbehaviour of the powder (such as stickiness and ow-ability), this technique can be more useful than the DSCtechnique.

3.5. Critical water content and storage conditions

The critical water content/water activity is the valuewhen the glass transition temperature of the product equalsto the room temperature (which is assumed to be around23 1C in this work). All amorphous products are meta-stable, and therefore are liable to caking and crystallizationover time during storage (Bhandari & Howes, 1999).Stability of the amorphous products is largely dictated bythe Tg which in turn depends on storage conditions such aswater activity or humidity and temperature (Roos & Karel,1990; Slade & Levine, 1991). This is more important in lowmolecular products and protein hydrolysates which are

40 (2007) 15931600highly hygroscopic (Aguilera, Delvalle, & Karel, 1995).The sorption data for SMPHL and lactose was obtained by

4. Conclusions

ARTICLE IN PRESS

and

mo

WTtting moisture data into BET equation (model). The Tgvalues at the given aw and water content were predicted byGordonTaylor equation. As expected the Tg valuesdecreased with increase in storage aw or moisture contentdue to plasticizing effect of water on the amorphouscomponents (Fig. 3). The critical aw for SMPHL was 0.15at a storage temperature of 23 1C for 3 weeks. The watercontent of the SMPHL under these conditions was 2.4 g/100 g dry solid. In comparison, pure lactose stored undersimilar conditions had critical aw of 0.39 and water content8 g H2O/100 g dry solid. The critical storage conditions forSMPHL or SMPHL and lactose mixtures were very closeto the values reported by Jouppila and Roos (1994b).

Fig. 3. Relationship between the water activities at 23 1C, water content (m)water content and Tg values of SMPHL and lactose were predicted by BET

activity for both the powders are shown by the arrows.

A.K. Shrestha et al. / LThe lower critical aw and water content clearly indicatedthe vulnerability of SMPHL under processing, handlingand storage conditions. The product would be difcult todry as the conditions of outlet air humidity and tempera-ture of the spray dryer would be closer or higher than thecritical conditions. Since the Tg of SMPHL is so low, themolecular mobility of the solid will be high even at roomtemperature which might have led to deteriorative changessuch as structural collapse, stickiness and caking. Experi-mentally spray drying of lactose-free skim milk showed alarge amount of the powder got sticky and could not berecovered. The outlet temperature of dryer was 60 1C andthe powder had aw about 0.1, which is higher than Tgvalue (49 1C) of anhydrous SMPHL.Glucose and galactose, the major components of

hydrolysed lactose, have low glass transition temperaturesand at higher storage temperature may react with proteinpresent in the powder causing non-enzymatic browning.One of the interesting aspects of SMPHL was the watercontent kept on increasing with increasing storage awunlike lactose that started to release water at awX0.432 andstart to absorb water again awX0.529, due to crystal-lization. There was no visible crystallization in SMPHLSpray drying of SMP with hydrolysed lactose is difcultas most of the powder stuck inside the dryer even at verylow inlet/outlet air temperatures. This is due to the lowglass transition temperature of glucose and galactose, twomajor components of hydrolysed lactose. SMPHL ab-like lactose. No crystallization peaks were seen whenSMPHL was scanned through DSC. SMP has a signicantamount of protein in its matrix, about 34 g/100 g. Itappears that protein delays or masks the crystallizationof glucose, galactose and residual lactose.

glass transition temperature, Tg () of SMPHL (A) pure lactose (B). The

dels and GordonTaylor equation, respectively. The critical Tg and water40 (2007) 15931600 1599sorbed less water than lactose when stored under similar%relative humidity and did not show any sign of crystal-lization of sugars even at higher water activity. Thesorption isotherm data for SMPHL tted well into BETand GAB models with monolayer moisture contents of7.55 and 8.27 g/100 g, respectively. The stability of SMPHLwas much lower than lactose as shown by low criticalvalues for aw and water content in state diagram. Thesedata can be used to assess the proper spray dryingcondition, stickiness and storage behaviour of SMPHL.

Acknowledgement

This research was nancially supported by DairyIngredients Group of Australia (DIGA), Melbourne,Australia (Project No. UQ3412).

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Water sorption and glass transition properties of spray dried lactose hydrolysed skim milk powderIntroductionMaterials and methodsMaterialsSample preparationGlass transition temperature (Tg)Sorption isotherm studiesPrediction of Tg values using the Gordon-Taylor equation

Results and discussionsSpray drying behaviourWater sorptionSorption isothermGlass transition temperature (Tg) of SMPHL and lactoseCritical water content and storage conditions

ConclusionsAcknowledgementReferences