Review

Importance of glass transition and water activityto spray drying and stability of dairy powders

Yrjö H. Roos

Department of Food Science, Food Technology and Nutrition,University College, Cork, Ireland

Abstract – Spray-drying is a rapid dehydration method allowing production of high quality dairypowders. In dehydration and subsequent powder handling and storage, however, both chemical andphysical changes, such as caking, lactose crystallisation, and nonenzymatic browning, may impairpowder characteristics and result in loss of powder quality. Many of these changes are related to thephysical state of lactose, as rapid removal of water in spray drying results in the formation of low-moisture, amorphous, noncrystalline structures of lactose and other milk components. The amor-phous components may exist as solid-like glasses or highly supercooled, viscous liquids. The forma-tion of amorphous, glassy lactose during spray drying allows production of a free-flowing powder.High temperatures or residual water contents at the later stages of the drying process, however, maycause stickiness, caking, browning, and adhesion of the powder particles to the processing equip-ment. The glass transition of amorphous lactose occurs in the vicinity of room temperature at a watercontent of about 6.8 g (g × 100)–1 of lactose corresponding to an equilibrium relative humidity of 37%and 0.37 aw (water activity). At higher water contents, as the glass transition of amorphous lactose iswell below storage temperature, dairy powders become sticky and the amorphous lactose may exhibittime-dependent crystallisation. Crystallisation of amorphous lactose may also release sorbed waterfrom the amorphous material, which enhances other deteriorative changes, such as the nonenzymaticbrowning reaction. Amorphous lactose in dairy powders encapsulates milk fat, which, as a result oflactose crystallisation, is released and becomes susceptible for rapid oxidation. The glass transitionand water activity are, therefore, important factors controlling processability, handling properties andstability of dairy powders.

Glass transition / dairy powder / spray drying / stability / water

Résumé – Importance de la transition vitreuse et de l’activité de l’eau pour le séchage par ato-misation et la stabilité des poudres de lait. Le séchage par atomisation est une méthode de déshy-dratation rapide permettant la production de poudres de lait de première qualité. Cependant, au cours

DOI: 10.1051/lait:2002025 475

Communication at the 1st International Symposium on Spray Drying of Milk Products, Rennes,France, October 16–18, 2001.Correspondence and reprintsTel.: 353 21 4902386; fax: 353 21 4270213; e-mail: yrjo.roos@ucc.ie

Lait 82 (2002) 475–484© INRA, EDP Sciences, 2002

de la déshydratation puis de la manipulation et du stockage ultérieurs de la poudre, des changements àla fois chimiques et physiques tels que l’agglomération, la cristallisation du lactose et le brunissementnon enzymatique, peuvent cependant altérer les caractéristiques de reconstitution et entraîner uneperte de qualité de la poudre. Plusieurs de ces changements ont trait à l’état physique du lactose,puisque le retrait rapide d’eau au cours du séchage par atomisation conduit à la formation de lactose etdes autres composants du lait à faible teneur en eau, amorphe, aux structures non cristallines. Lescomposants amorphes peuvent exister sous forme de verre très solide ou de liquides hautement sous-refroidis et visqueux. La formation de lactose amorphe vitreux au cours du séchage par atomisationpermet la production d’une poudre très fluide. Des températures élevées ou des teneurs en eau rési-duelle aux derniers stades du procédé de séchage, peuvent cependant provoquer le collage, l’agglo-mération, le brunissement et l’adhésion des particules de poudre à l’équipement de séchage. Latransition vitreuse du lactose amorphe a lieu à température proche de la température ambiante, à uneteneur en eau d’environ 6,8 g (100 g)–1 correspondant à un équilibre entre l’humidité relative de 37 %et une aw (activité de l’eau) de 0,37. À des teneurs en eau plus élevées, puisque la température de tran-sition vitreuse du lactose est bien en dessous de la température de stockage, les poudres de lait devien-nent collantes et une cristallisation du lactose amorphe dépendant du temps peut se produire. Lacristallisation du lactose amorphe peut aussi larguer l’eau adsorbée du matériel amorphe, ce qui en-traîne d’autres changements détériorants tels que la réaction de brunissement non enzymatique. Lelactose amorphe dans les poudres de lait encapsule la matière grasse lactique, qui, en raison de la cris-tallisation du lactose, est libérée et peut rapidement s’oxyder. La transition vitreuse et l’activité del’eau sont donc des facteurs importants de contrôle de la fabrication, des propriétés de manutention etde stabilité des poudres de lait.

Transition vitreuse / poudre de lait / séchage par atomisation / stabilité de la poudre / eau

1. INTRODUCTION

The solids of dehydrated dairy productsinclude lactose, lipids, proteins, and miner-als. Lactose, proteins, and minerals are mis-cible with water or water-soluble whilelipids have fairly little interactions with wa-ter [10]. The physical state of the non-fatsolids (SNF) is highly dependent on watercontent [2, 3, 10] and, therefore, of great im-portance in defining drying behaviour andstability of dehydrated dairy foods [4, 9].

Lactose in liquid dairy products is dis-solved in a continuous water phase, whichalso contains dispersed fat and proteins.When desired, lactose may be crystallisedbefore dehydration, but preconcentratedliquids may also be spray dried withoutprecrystallisation of lactose. Although thepreconcentrated liquids may become su-persaturated with lactose, crystallisationmay not occur before spray drying if only ashort time is allowed between evaporationand drying. A rapid removal of water in

subsequent spray drying does not allowlactose crystallisation and while water is re-moved, lactose is transformed to a solid-like, amorphous, glass directly from thedissolved state [11, 12]. Therefore, thephysicochemical properties of supersatu-rated, amorphous lactose are important indefining appropriate spray drying condi-tions and storage stability of many dairypowders [9, 15].

Several physicochemical changes ofdairy powders are directly or indirectly re-lated to the glass transition of amorphouslactose. The glass transition is a state transi-tion of amorphous materials occurring be-tween the solid, glassy and supercooledliquid states [9, 17, 18]. The amorphousmaterials are nonequilibrium systems andmany of the changes, including the glasstransition itself, have time-dependent char-acteristics [9]. In general, molecular mobil-ity in the glassy state is limited to molecularrotations and vibrations while above theglass transition, translational mobility

476 Y.H. Roos

appears [18]. Therefore, many amorphousglasses are fairly stable, but long-termstability is lost above the glass transition.For example, amorphous, glassy lactose isfairly stable, but as molecular mobility be-comes significant above the transition, thesolid-like properties are lost [4, 9, 11].There is also a driving force towards thethermodynamically equilibrium crystallinestate and the glass transition also results intime-dependent crystallisation [11–13].Furthermore, stickiness and caking of pow-ders, and rates of diffusion-controlledchemical reactions may be controlled bythe glass transition [9, 17].

The significance of the glass transitionin the spray drying process has receivedfairly little attention although the glass tran-sition related changes of dairy powdershave been well recognised. The present ar-ticle discusses the role of glass transition inspray drying and in the control of stabilityof dairy powders.

2. GLASS FORMATIONIN SPRAY DRYING

Spray drying involves atomisation of theliquid and subsequent evaporation of wateras the material passes through the dryingchamber. Although a number of differentspray driers may be used, correspondingprinciples of glass formation apply accord-ing to Figure 1.

Dehydration of the atomised liquid par-ticles proceeds from the particle surface tothe inner core. A layer of concentrated sol-utes is formed on the particle surface andthere may be a decrease in the particle tem-perature due to evaporative cooling. Theextremely rapid removal of water increasesthe viscosity of the remaining solids and theparticle surface approaches the glassy statebefore colliding with other particles or drierwalls. It has been found that a critical sur-face viscosity resulting in stickiness andcaking is > 107 Pa·s [1] and the generally

Glass transition in spray drying 477

Solids

Water and SolutesDehydration

Plasticiation

Reh

ydra

tion

Solubil

isat o

n

Crystallis

ation

-time

-w

ater

- heat

- water

- heat

-heat

-water

Crystalline Solid

i

s

Figure 1. Formation ofamorphous structures indehydration and the re-lationships betweenequilibrium (solution,crystalline solid) andnonequilibrium (amor-phous solid and liquid)states.

accepted value for the viscosity of glassymaterials is > 1012 Pa·s [18]. Such a highsurface viscosity of drying particle surfacesallows the formation of solid, individualparticles that can be exposed to furthertreatments in subsequent drying stages,e.g., agglomeration on a fluidised bed drieror a belt.

The vitrification of the particle surfacein spray drying is essential in allowing freeflow of the particles through the dryingchamber and avoiding caking of particleswith each other and on the drier surfaces. Atthe end of the drying process, the particletemperature and water content should sup-port the solid, glassy state.

3. PROPERTIES OFAMORPHOUS MILK SOLIDS

The main amorphous components indairy solids are carbohydrates and proteins.These components are both miscible withwater and at least partially miscible witheach other. However, it is likely that mostproteins exist at least partially phase sepa-rated from carbohydrates in dehydrateddairy powders [6].

It is well known that melting tempera-tures of crystalline, water-soluble materialsdecrease with increasing water content[11, 12]. Both amorphous carbohydratesand proteins are also plasticised or softenedby water [17], i.e., their glass transition oc-curs over a water content dependent tem-perature range decreasing with increasingwater content [3–5]. The amorphous carbo-hydrates, however, often dominate the ob-served changes in dairy solids and their tran-sition temperatures correspond to observedchanges in physicochemical properties [2,3]. Unfortunately, fairly little information isavailable on the protein transitions and themiscibility and transitions of carbohydrate-protein mixtures.

The glass transition of lactose and dairysolids has been observed using differential

scanning calorimetry (DSC) [3, 11–13].DSC measures a change in heat capacitythat occurs over the glass transition temper-ature range. DSC is also the most commonmethod in the determination of glass transi-tion temperatures, which are taken from theonset or midpoint temperature of thechange in heat capacity [9]. Other changesassociated with the glass transition arechanges in thermal expansion coefficientand mechanical and dielectric relaxations[9, 18]. The viscosity of the glassy state isoften considered as constant and suffi-ciently high to maintain the solid-like prop-erties. However, the liquid-like propertiesappear over the glass transition and there isa dramatic decrease in viscosity at andabove the transition. Many sugars, includ-ing lactose are transformed rapidly from thesolid glassy state to a syrup-like, stickyliquid.

The glass transition of anhydrous lac-tose, as observed using DSC, has an onsettemperature of 101 oC [12], which is one ofthe highest temperatures measured for “an-hydrous” disaccharides [9]. Glass transi-tion temperatures for anhydrous milkcomponents and solids are given in Table I.The glass transitions observed in milksolids correspond closely to those of purelactose [3, 4]. However, if lactose is hydro-lysed, the observed Tg decreases dramati-cally, because of the much lower Tg of thegalactose and glucose components [3]. Thisalso results in significant changes in thespray drying behaviour and storage stabil-ity of lactose-hydrolysed milk solids [9].

Amorphous carbohydrates, includinglactose and its hydrolysis products are sig-nificantly plasticised by water, which is ob-served from a rapidly decreasing Tg withincreasing water content. The effect of wa-ter on the Tg of milk solids may be predictedusing the Gordon-Taylor equation (1) [3, 9],

Tgw T kw T

w kw1 g1 2 g2

1 2

=++

(1)

478 Y.H. Roos

where Tg is glass transition temperature of amixture of solids with a weight fraction ofw1 and anhydrous Tg1 and water with aweight fraction of w2 and glass transition ofTg2 for pure water. The Tg2 is often taken as–135 oC and k is a constant.

The effects of water on the state andphase transition temperatures of amor-phous food materials is often described us-

ing State Diagrams, which show the transi-tion temperatures over a wide temperaturerange and in maximally freeze-concen-trated systems [9, 17]. The state diagram oflactose is shown in Figure 2. The diagramof lactose is useful in explaining the effectof water on lactose properties in dehydra-tion and dehydrated dairy products. The in-formation of water plasticisation can also

Glass transition in spray drying 479

Table I. Anhydrous glass transition temperatures, Tg, of milk components and solids. Data fromJouppila and Roos [3] and Roos [9].

Material Glass transition temperature (oC)

Galactose 30

Glucose 31

Lactose 101

Skim milk 92

Skim milk with hydrolysed lactose 49

0.0 0.2 0.4 0.6 0.8 1.0-150

-100

-50

0

50

100Solubility(equilibrium mixtureof - and -lactose)α β

Supercooledliquid

Glass

Ice and vitrified solute-unfrozenwater phase

Equilibrium freezing zone

Temperature range for maximumice formation

gT

gT

m

gT'

T'

gC'

Weight Fraction of Lactose

Te

mp

éra

ture

(°C

)

Figure 2. State diagram of amorphous lactose showing lactose solubility, the glass transition temper-ature, Tg, glass transition of maximally freeze-concentrated lactose solutions, Tg’ and the corre-sponding solids content, Cg’, and onset of ice melting in the maximally freeze-concentratedsolutions, Tm’.

be shown with water sorption properties(Fig. 3), which allows evaluation of the ex-tent of water plasticisation of dairy powdersin various storage conditions.

4. GLASS TRANSITIONIN SPRAY DRYING

Several studies have shown that sticki-ness of dehydrated powders occurs as aresult of particle surface plasticisation andconcurrent decrease in viscosity allowingthe formation of liquid bridges betweenpowder particles [7]. It may be assumedthat similar mechanisms control particleproperties in the spray drying process.However, the process involves removal ofthe solvent and plasticiser, which has tooccur with a rate competing with particlevelocity and formation of a dry surface toallow free flow of individual particlesthroughout the dehydration process.

The glass transition temperature of skimmilk solids showing the stickiness and cak-ing zone at about 10 oC or higher above theTg measured by DSC is shown in Figure 4.Although the water content within dryingparticles and between individual particlesmay vary significantly, the particle temper-ature may be assumed to decrease duringinitial dehydration. Lactose exists mostlikely as highly supersaturated syrup in thesolids phase. The particle surface may ap-proach the glassy state when the inner parti-cle core remains within the stickiness andcaking zone providing the free flowingproperties at the end of the drying. At thelater stages, the surface properties may bealtered to allow agglomeration in afluidised bed drier or a separate agglomera-tion step. After dehydration the moisturedistribution within the particles becomesmore even and the water content is reducedto a sufficiently low level to maintain thesolid, glassy state at typical storage condi-tions of dairy powders.

480 Y.H. Roos

20151050-40

-20

0

20

40

60

80

100

0

0.2

0.4

0.6

0.8

1.0

WATER CONTENT [g (g x 100) of Solids]

TE

MP

ER

AT

UR

E(°

C)

WA

TE

RA

CT

IVIT

Y(a

t2

4°C

)

Lactose

Skim Milk Solids

-120151050

-40

-20

0

20

40

60

80

100

0

0.2

0.4

0.6

0.8

1.0

WATER CONTENT [g (g x 100) of Solids]

TE

MP

ER

AT

UR

E(°

C)

WA

TE

RA

CT

IVIT

Y(a

t2

4°C

)

Lactose

Skim Milk Solids

-1

Figure 3. Glass transition temperature and water sorption properties of amorphous lactose and skimmilk solids. The glass transition is depressed to 24 oC at a critical water activity of 0.37 (correspond-ing to critical storage relative humidity of 37%). The corresponding critical water content for lactoseis 6.8 and skim milk solids 7.6 g (g × 100)–1 of solids. Data from Jouppila and Roos [3].

The surface properties of the drying par-ticles are related to surface viscosity [1].The viscosity as a result of water removalincreases rapidly as the glass transition isapproached. The viscosity changes ofamorphous materials are often describedusing the Williams-Landel-Ferry (WLF)equation (2),

log log logaC (T T )

C (T T )T

s s

1 s

2 s

= = =− −

+ −τ

τη

η

(2)

which relates relaxation times above glasstransition to a reference temperature, Ts[19]. If the Tg is taken as a reference tem-

perature, the relaxation time, τ , and viscos-ity, η, are related to their values at the glasstransition (τ g and ηg) and plasticisation,and are defined by the temperature differ-ence, T – Tg, as shown in Figure 5.

Although, the WLF relationship is oftenused to describe changes in relaxationtimes above the glass transition, it does notfollow changes over the transition tempera-ture range. The changes in relaxation timesat and around the transition are better de-scribed by the Fermi relationship [8].

The viscosity changes resulting from theglass transition can also be used to controlagglomeration of fine particles and in themanufacturing of instant powders [9]. Insuch processes, it is essential to allow

Glass transition in spray drying 481

Gla

ssTra

nsi

tion

(°C

)

Water Content (%, w/w)

Particle Temperature

GlassyLactose(Glassy Skim Milk)

LactoseSyrup

Stickiness andCaking Zone

Gla

ssTra

nsi

tion

(°C

)G

lass

Tra

nsi

tion

(°C

)

GlassyLactose(Glassy Skim Milk)

-50

0

50

100

0 5 10 15 20 25

Figure 4. Glass transition ofskim milk solids with a hypo-thetical particle temperature dur-ing water removal in spraydrying, and formation of theglassy solid particles at the endof drying.

1.00WEIGHT FRACTION OF SOLIDS

TE

MP

ER

AT

UR

E

T – Tg

60

40

20

0

(°C)

4.2x10

2.4x10

1.3x10

1.0x10

–10

–8

–5

0

Relative

Relaxation Time

Water plasticisation

Th

erm

alp

lastici

atio

ns

Figure 5. Relative relaxation timesabove the glass transition, as predicted bythe WLF relationship.

controlled stickiness on particle surfacesand adhesion of particles to clusters. Theformation of clusters is followed by re-moval of water and cooling to solidify thesurfaces into the glassy state. Therefore, thepowders will have large particle sizes andremain free flowing and stable at appropri-ate storage conditions.

5. GLASS TRANSITIONAND POWDER STABILITY

Several properties of powders withamorphous lactose or even some amountsof amorphous lactose can be related to itsglass transition [2–5, 9, 13]. These includesurface stickiness and caking [11], time-de-pendent lactose crystallisation [13] and re-lease of encapsulated lipids [16], andincreasing rates of nonenzymatic browning[15] and lipid oxidation [16]. The changesin mechanical properties and diffusion areresponsible for stickiness, caking and lac-tose crystallisation, but the changes in thereaction rates are more complicated and af-fected by other factors, including pH,heterogeneities in water distribution, andmiscibility of proteins and carbohydrates.

5.1. Stickiness, cakingand crystallisation

Stickiness and caking are common prob-lems in handling of powders containingamorphous carbohydrates. Stickiness and

caking appear as the viscosity of the amor-phous components decreases and powderparticles adhere [7]. Further plasticisationis often followed by collapse of structure asa result of increasing flow and lactosecrystallisation, which may occur instantlyat a high level of thermal and waterplasticisation [9]. The development ofstickiness, caking and crystallisation as aresult of plasticisation is shown for skimmilk in Figure 6. The crystallisation of lac-tose is highly time-dependent following thetypical crystallisation rate behaviour ofamorphous solids. The time-dependent lac-tose crystallisation in dairy powders is of-ten observed in water sorption studies[3, 9]. These have shown that above a

482 Y.H. Roos

0 5 10 15 20 25

0

50

100

150Instant Crystallisation of Lactose

Caking Zone

Stickiness Zone

Glassy State

2

7

12

10

10

10

Viscosity (Pa s)

Water Content (%, w/w)

Gla

ssTr

ansi

tion

(°C

)

Figure 6. Glass transition relatedchanges in mechanical properties andcrystallisation in skim milk with amor-phous lactose.

Lactosecrystallisation rateincreaseswith increasingwater activity

0

10

20

30

40

50

Water Activity

Lactosecrystallisation rateincreaseswith increasingwater activity

0.0 0.2 0.4 0.6 0.8 1.0

Wat

er C

onte

nt [g

(g

x 10

0) o

f Sol

ids]

-1

Figure 7. Sorption isotherm of skim milk solids.Crystallisation of amorphous lactose results intime-dependent loss of sorbed water.

critical storage relative humidity, there is aloss of sorbed water (Fig. 7). The loss ofsorbed water in dairy powders correspondsto the difference in water sorption by amor-phous and crystalline lactose. However, itshould be noticed that the loss of sorbedwater is time-dependent and the crystallineform of lactose produced is dependent onthe crystallisation conditions [3–5]. Mostcrystals formed are anhydrous, but at thehigher storage humidities increasingamounts of α-lactose monohydrate isformed. Crystallisation of amorphous lac-tose in sealed packages and in bulk storageresults also in an increase in water activityand acceleration of most deteriorativechanges, such as browning reactions andoxidation [14–16].

5.2. Molecular mobilityand stability maps

The glass transition in dairy powderswith amorphous components is related tomolecular mobility. The amorphous solidsin the solid glass are frozen in a high viscos-ity state and they exhibit only molecularvibrations and side chain rotations. As thematerial undergoes the glass transition,the molecular mobility increases and

translational motion of the molecules ap-pears. This has been related to increasingrates of bimolecular reactions and enzymeactivity in low moisture food systems [9, 17].

A hypothetical stability map for dehy-drated skim milk with amorphous lactose isshown in Figure 8. At low water contents,free fat is accessible to oxygen and mayundergo rapid oxidation. Increasing wateractivity may provide protection of freelipids as a result of water sorption on solidsurfaces. Plasticisation by an increasingwater content results in the decrease of theglass transition. At the critical water activ-ity, the glass transition is decreased to stor-age temperature and further increases inwater activity result in a decrease in vis-cosity of particles, stickiness, caking, andrapid increases in rates of lactosecrystallisation and diffusion controlledreactions [4, 9]. Lactose crystallisation isresponsible for release of encapsulatedlipids and subsequent rapid oxidation indairy powders [16]. The nonenzymaticbrowning reaction has also been observedto proceed at increasing rates above glasstransition and it is substantially acceler-ated by an increase in water activityfollowing crystallisation of amorphouslactose in dairy powders [14].

Glass transition in spray drying 483

1.00.80.60.40.20

WATER ACTIVITY

Oxidation

Diffusion-limited reactions

•Non-enzymatic browning

Enzyme activity

Loss of lysine

Structural transformations

•Stickiness

•Caking

•Collapse

•Lactose crystallisation

Critical a w

RE

LA

TIV

ER

AT

E

1.00.80.60.40.20

WATER ACTIVITY

Oxidation

Diffusion-limited reactions

•Non-enzymatic browning

Enzyme activity

Loss of lysine

Structural transformations

•Stickiness

•Caking

•Collapse

•Lactose crystallisation

Gro

wth

of

Mould

sY

easts

Bacte

ria

Critical a wCritical a w

RE

LA

TIV

ER

AT

E

Figure 8. Stability map for dairypowders containing amorphouslactose. The critical water activ-ity corresponds to the glass tran-sition depression of amorphouslactose to 24 oC, which may en-hance deteriorative changes andloss of quality.

6. CONCLUSIONS

Phase and state transitions, includingglass transition, have an important role in aproper control of the spray drying processof dairy materials and the quality of theproducts obtained. Furthermore, the transi-tions are important in understanding thestability of dehydrated dairy products. Theglass transition of amorphous lactose af-fects the development of the particle prop-erties during spray drying and subsequentagglomeration processes. Further knowl-edge of particle temperature and water con-tent in dehydration is required tounderstand and reduce undesired stickinessand caking of particles in different stages ofthe drying process and powder handling.Moreover, such understanding is essentialin controlling stability of spray dried dairypowders and to avoid deterioration result-ing from lactose crystallisation or chemicaland enzymatic reactions.

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484 Y.H. Roos