glass transition and food technology - a critical appraisal.pdf

2444 JOURNAL OF FOOD SCIENCE—Vol. 67, Nr. 7, 2002 © 2002 Institute of Food Technologists

Concise Reviews in Food Science

Glass Transition and Food Technology:A Critical AppraisalM. LE MESTE, D. CHAMPION, G. ROUDAUT, G. BLOND, AND D. SIMATOS

ABSTRACT: Most low water content or frozen food products are partly or fully amorphous. This review willdiscuss the extent to which it is possible to understand and predict their behavior during processing and storage,on the basis of glass transition temperature values (Tg) and phenomena related to glass transition. Two mainconclusions are provisionally proposed. Firstly, glass transition cannot be considered as an absolute thresholdfor molecular mobility. Transport of water and other small molecules takes place even in the glassy state at asignificant rate, resulting in effective exchange of water in multi-domains foods or sensitivity to oxidation ofencapsulated materials. Texture properties (crispness) also appear to be greatly affected by sub-Tg relaxationsand aging below Tg. Secondly, glass transition is only one among the various factors controlling the kinetics ofevolution of products during storage and processing. For processes such as collapse, caking, crystallization,and operations like drying, extrusion, flaking, Tg data and WLF kinetics have good predictive value as regardsthe effects of temperature and water content. On the contrary, chemical/biochemical reactions are frequentlyobserved at temperature below Tg, albeit at a reduced rate, and WLF kinetics may be obscured by other factors.

Keywords: glass transition, glassy state, amorphous regions, relaxation, WLF, water activity

Introduction

THE HUGE VARIETY OF POSSIBLE APPLICATIONS OF GLASS TRANSITION

in food science and technology was highlighted in the 1980sby Levine and Slade. Stimulated by an extensive series of papersand presentations from these authors (Levine and Slade 1986;Slade and Levine 1988, 1991) and by theoretical and experimen-tal progress in materials science, a constantly increasing numberof studies in the food area refer to glass transition. It seems time-ly to review current literature, in order to discuss the extent towhich it is possible to understand and predict the behavior offoods during processing and storage on the basis of glass transi-tion-related phenomena. Most food products with reduced mois-ture content are partly or totally amorphous. Depending on thestorage temperature and their composition (mainly water con-tent), they are glassy and may be expected to be rigid (eventual-ly crispy) and stable, or contain a rubbery or liquid phase andthen be soft and prone to physical and chemical changes. Pro-cesses such as baking, air- and freeze-drying, extrusion, and flak-ing may also operate through the glass transition range. Sincethe theoretical basis of glass transition that are of interest to foodscience were already reviewed (Angell and others 1994; Perez1994; Simatos and others 1995b) a few points will only briefly bementioned here. We will focus on experimental data dealing withfood related materials.

Glass transition and molecular mobility

DefinitionGlass transition (or glass-liquid transition GLT) is the name

given to phenomena observed when a glass is changed into a su-percooled melt during heating, or to the reverse transformationsduring cooling. Both are non-crystalline states; but while theglass is a rigid solid, the supercooled melt, which is observed be-tween the GLT and the melting point, can be a viscoelastic “rub-ber” in the case of a polymeric material, or a mainly viscous liq-

uid, for low molecular weight materials. The GLT is a kinetic andrelaxation process associated with the so-called � relaxation ofthe material. At temperatures above the GLT the material, if sub-mitted to a perturbation, can recover after a characteristic relax-ation time (�): the supercooled melt is in a metastable state. Theliquid-like structure of the melt is “frozen” in the glass, which isan out-of-equilibrium state (Figure 1). The GLT region is the tem-perature range where the relaxation time of the material is simi-lar to the experimental time scale.

Mobility above GLTA characteristic feature of mechanical properties of the super-

cooled melt is the strong temperature dependence in the tem-perature range above the glass transition temperature (Tg): theapparent activation energy (Ea) commonly attains 200 to 400kJ.mole-1; it decreases when temperature increases. The mostpopular expressions to describe this behavior are the Vogel-Tam-mann-Fulcher (VTF) [1] and the Williams-Landel-Ferry (WLF) [2]expressions:

�T = �0 exp(B/ (T-T0)) (1)

log(�T /�Tg ) = -C1g (T-Tg)/ (C2g + (T-Tg)) (2)

where �T and �Tg are viscosities at T and Tg respectively; �0, B, T0,C1g, and C2g are phenomenological coefficients. Both expressionscan be inter-converted. Expressions similar to Eq 1 and 2 can bewritten with the � obtained for example with mechanical spec-troscopy (Figure 2).

C1g and C2g can fluctuate (Ferry 1980) around the “universal”values given by Williams and others (1955) (17.4 and 51.6 respec-tively) as a function of the considered material. The variations ofC2g and of B correspond to the classification proposed by Angell(Angell and others 1991, 1994) of strong/fragile materials accord-ing to the variation of their dynamic properties through the glass

JFS: Concise Reviews and Hypotheses in Food Science.

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ienceGlass transition in food . . .

transition. The fragility parameter m was introduced to differen-tiate fragile systems (m between 100 and 200) which are highlysensitive to temperature changes above Tg, from strong ones (mbetween 16 and 100) which are less disturbed when passingthrough the glass transition. By definition, m is the slope of thescaled Arrhenius plot of the viscosity when the temperature ap-proaches Tg from above, Ea being the apparent activation ener-gy (kJ.mole-1):

m = Ea/ (RTg ln10) (3)

This parameter m can be calculated with the VTF and WLF coeffi-cients:

m = C1g + C1g2 T0 ln10/ B or m = (C1g/ C2g) Tg (4)

or it may be deduced from DSC or mechanical spectroscopy data.A discussion of the application of various methods to estimate mfor food materials can be found in Simatos and others (1995b).

Low molecular weight sugars can be classified as rather fragilematerials. They seem to be located in a narrow domain of the fra-gility diagram (Figure 3). For proteins, the scarce experimentaldata seem to indicate a strong behavior: m � 40.5 for poly-L-as-paragine (15 to 25% water) (Angell and others 1994) and similarvalues for elastin and gluten (Simatos and others 1995b). Pullu-lan-starch blends were found to show strong behavior (m � 42 to51) increasing with water content (Biliaderis and others 1999).Fragility was reported to increase in the order: pullulan<dextranand phytoglycogen<amylopectin, and to decrease for amylopec-tin with increasing water content (Borde and others 2002).

Mobility below GLTIn the glass, long range cooperative motions are restricted.

Motions (vibrations of atoms, reorientation of small groups of at-oms) are mainly local, not involving the surrounding atoms ormolecules. The temperature dependence of dynamic propertiesis generally considered to obey Arrhenius law, with an apparentactivation energy that is lower than at T>Tg but still rather high(Perez 1994).

Several sub-Tg relaxations can be observed in biopolymersand low molecular weight sugars (� and � relaxations). Their ori-gin is still being discussed. As observed in polysaccharides theycould correspond to rotation of lateral groups (� relaxation at lowtemperature) or to local conformation changes of the main chain(� relaxation closer to T�) (Montes and others 1998). The Ea val-ues range between 40 to 70 kJ.mole-1 for � relaxation in maltoseand glucose.

When a glassy material is stored between T� and Tg, a micro-structural evolution may take place, which corresponds to thesystem approaching the metastable equilibrium, with some ex-tra loss in enthalpy and volume (Figure 1). This “physical aging,”also named “annealing” when carried out to develop desiredproperties, can be regarded as a continuation of the � relaxation.From observations in the field of materials science, it is knownthat the more compact molecular organization and the strength-ening of interactions result in changes in many physical proper-ties: increased rigidity and brittleness, decreased dimensions,and transport properties. Physical aging is expected to be of im-portance for the stability of low moisture products and is current-ly receiving a lot of attention. Experimental study of the processallowed the determination of the relaxation parameters (see be-low). The rate of enthalpy relaxation was decreased, as expected,upon addition of a substance with a higher Tg, for instance dext-

ran in sucrose (Blond 1994; Shamblin and Zografi 1998). In con-trast, increasing the weight fraction of fructose in glucose-fruc-tose mixtures also resulted in a decrease of the aging rate, al-though Tg was depressed (Wungtanagorn and Schmidt 2001a).

Relaxation timeRelaxation times obtained by means of various techniques

can be represented as a function of temperature in a mobilitymap (Figure 4). Besides the characteristic relaxation time �, 2 oth-er empirical parameters are used to describe the relaxation pro-cesses associated with GLT.

Non-exponentiality. The process cannot be described by asingle relaxation function. To represent the distribution of relax-ation times, a somewhat arbitrary but mathematically conve-nient expression is most frequently used, a so-called stretchedexponential (KWW expression):

�(t) = exp [– (t / �)� (5)

where � is the property studied as a function of time (t). The pa-rameter � is close to 1 for strong liquids (nearly exponential relax-ation). For fragile liquids it changes from near 1 at high tempera-ture to a value close to 0.3 to 0.5 near Tg. � is generally lower(broader distribution of relaxation times) for polymers than forlow molecular weight materials, and for mixtures.

Non-linearity. The characteristic relaxation time � is changingwith time t and with the material structure, which depends on itsprevious history. This dependence is commonly represented bythe following expression:

� = �0 exp [x �h/ RT + (1-x) �h / RTf ] (6)

Figure 1—Evolution of enthalpy (or specific volume) (a) andof heat capacity (or of thermal expansion coefficient) (b)versus temperature in the vicinity of glass transition, dur-ing cooling (1), rewarming (2) or after physical aging dur-ing time t (3). Tgo and Tgm are the 2 points (onset, middle)that are commonly used to define Tg. TfA and TfB are thefictive temperatures characteristic of the glass structurein states A and B.

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2446 JOURNAL OF FOOD SCIENCE—Vol. 67, Nr. 7, 2002


Glass transition in food . . .

The fictive temperature Tf, for a material being at the actual tem-perature T, is considered to represent its structural state (Figure1). The parameter x (0 < x < 1) defines the relative contributionsof temperature and structure to �(if x is close to 1, � mainly de-pends on the aging temperature) �h is the activation energy atTg for the supercooled material. The parameters �, �, x can be es-timated from enthalpy relaxation experiments using DSC aftervarying annealing times (Hodge 1994). This was done for mal-tose (Lammert and others 1999, Noel and others 1999) and glu-cose-fructose mixtures (Wungtanagorn and Schmidt 2001b). In-creasing the fructose weight fraction in the blend induced anincrease in x (linearity increase) and a decrease in � (broader dis-tribution of relaxation times). The value of � and the evolution ofTf with time can give an indication about the relative structuralstability of materials (Shamblin and others 1999). For a more ac-curate assessment of the theoretical relevance and technologicalusefulness of parameters � and x in the food area, there is a needfor more experimental data with different material types.

Glass transition temperature(s)Most popular methods to determine the temperature range

of GLT currently are differential scanning calorimetry (DSC) andmechanical spectroscopy (or dynamic mechanical thermal analy-sis = DMTA). Both types of techniques may provide significantlydifferent values (Kalichevsky and others 1992a,b; Biliaderis andothers 1999). The discrepancies may be understood by keepingin mind 2 series of factors: i. differences induced by the analysisof experimental data, for instance definition of Tg in the DSCcurve (onset, midpoint, endpoint, see Figure1), choice of featurein DMTA (drop in E’, maximum of E” or of tan �); ii. differences as-cribable to the coupling of different structural units (with partic-ular relaxation times) with the imposed perturbations, whichmay differ as regards the nature of the stress or the experimentaltime. It is therefore not possible to consider a unique glass tran-sition temperature. Namely, following the convention in use inmaterials science, the use of Tg designation should be restrictedto the DSC value (10 K.min-1), the temperature derived fromDMTA being named T� (with indication of the measurement fre-quency). The important discrepancies between published val-ues of Tg may, in addition, originate from uncertainties in the wa-ter content and/or chemical degradation (Vanhal and Blond1999).

For practical purposes, the method of measurement shouldbe chosen according to the application. If sensory properties oftexture, or dynamic processes such as collapse or agglomerationwere of concern, the relevant method would most probably bemechanical spectroscopy or change in the Young modulus or vis-cosity with temperature (with the relevant experimental time).Although more difficult to determine with many food products,Tg (DSC) may be relevant, once the correlation with the dynami-cally measured temperature or with the phenomenon of interesthas been demonstrated. Determination of glass transition tem-perature is further complicated with food products because oftheir chemical and microstructural complexity. Glass transitionsoften extend over a large temperature range, due to a broad dis-tribution of relaxation times and/or to unresolved transitions cor-responding to different components. Distinct transitions associ-ated with different phases can be observed as reported foramylopectin-gelatin mixtures (Mousia and others 2000). TheDMTA thermograms exhibited 2 glass transitions, which could beassociated with the 2 phases identified by FTIR microscopy. Themeasured T�’s were consistent with the values predicted on thebasis of water partitioning between the 2 components according

to their respective sorption isotherms. Moreover, this studypoints to the fact that distinct Tg’s (or T�) may exist in multipha-sic systems, not only because of distinct Tg for the componentsthemselves, but also because of the water distribution. Most realfoods are multiphasic systems. More attention should be givento the effect of microstructure on glass transition related proper-ties and to possible phase transitions occurring in componentssuch as lipids. As it appears that common techniques (DSC, me-chanical spectroscopy) often give an indication of a transition ata macroscopic level, a correct understanding of the glass transi-tion related phenomena implies resorting to different tech-niques. Moreover, nuclear magnetic resonance, electron spin res-onance, solute translational diffusion, and so on, may giveinformation on the molecular mobility in various phases and mi-crostructural locations.

The question of the relevant glass transition temperature infrozen products must be raised. Because of the cryo-concentra-tion process, 2 different glass transition temperatures have to beconsidered, depending on the storage temperature of the prod-uct (Figure 5). The glass transition temperature of the maximallyfreeze-concentrated phase is considered to occur in a tempera-ture range around Tg’. If the product is stored at a temperature Ts

below Tg’ it may be expected to be composed of ice and a freeze-concentrated phase in the glassy state and long-term stabilitymay be anticipated. By contrast, if Ts is between Tg’ and Tm, thefreeze-concentrated phase is more diluted, its concentration Cs

being determined on the curve Tm at temperature Ts. The glasstransition temperature of the partially freeze-concentratedphase is now Tgs, determined on the curve Tg for concentrationCs (Champion and others 1997a).

PlasticizationThe temperature of glass transition is strongly dependent on

the molecular weight of the material. The expression:

1/ Tg = 1/ Tg + K/ DP (7)

Figure 2—Relaxation time of sucrose solutions versus (T-Tg) (TgDSC). The curve is the WLF prediction (Eq. 2) with�Tg=1.6x1012 Pa.s, C1g=19.8, C2g=51.6 K obtained from fit-ting to viscosity data. Experimental points: for water con-tents 6-18% DMTA (�=1/2�f where f is the frequency of theloss modulus maximum at T); for water contents 34.7-42.5% shear viscosity (�=�/G

� where � is viscosity and G

�the elasticity modulus at infinite frequency ��4x104 Pa). Datafrom Champion 1998

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where DP is the degree of polymerization, K a constant, Tg thehigh molecular weight limit of Tg, is used to describe the molecu-lar weight dependence of Tg in an homogeneous polymer series.It has been proved to apply to carbohydrates (Orford and others1989). For a compatible mixture a single GLT is observed, thetemperature of which taking an intermediate value that de-pends on the mass fraction of components (Tg curve on Figure5). In food, plasticization (Tg decrease) is mainly due to water,but other small solutes may also act as plasticizers. One amongnumerous applications is the addition of glycerol to raisins tokeep them soft at a low water activity when mixed with cereals.Branching in polysaccharides may work as internal plasticization,inducing a small decrease in Tg when compared to linear chains(Bizot and others 1997). To the contrary, cross-linking is known tostrongly raise the glass transition temperature; for instance, cal-cium tetraborate was found to significantly raise both Tg and T�

of alginate/poly(vinyl alcohol) blends (Miura and others 1999).The literature is replete with figures showing the variation of Tg(or T�) versus water content for food components or real prod-ucts. The semi-theoretical Couchman and Karasz expression:

lnTg = �1n (mi �cpi lnTgi) / �1

n (mi �cpi )

should allow the prediction of Tg from mi the mass fraction, Tgi

the glass transition temperature and �cpi the increment in heatcapacity at Tgi for each component i. However, given the uncer-tainties in the Tgi and �cpi for biopolymers and the somewhatpoor agreement with experimental values, particularly when onecomponent is water, the empirical Gordon-Taylor expression ismost often preferred:

Tg = (m2 Tg2 + k m1 Tg1) / (m2 + k m1)

where m2 et m1 are the mass fractions of solid and water respec-tively, Tg2 and Tg1, their glass transition temperatures and k a fit-ting parameter. A problem with both relations is that the glasstransition temperature of water is still uncertain (Velikov andothers 2001), it is sometimes taken as a fitting parameter in theGordon-Taylor expression.

Mobility of water around Tg. multi-domains foodsExperimental results evidence that the mobility of water re-

mains high in glassy food systems. From NMR measurements ofthe rotational mobility in oligo and polysaccharides, the trendscan be summarized as follows (Ablett and others 1993): the waterhas a higher degree of mobility than the solute, even below Tg;the water mobility at Tg increases with increasing molecularweight of the solute; the higher the molecular weight of the sol-ute, the lower the temperature below Tg at which the mobility ofwater is observed to begin to increase (also see Figure 6a). The ro-tational mobility of water in a maltose glass is estimated to beonly 105 times lower than that of liquid water (Hemminga andothers 1999) whereas the viscosity is about 1015 times greater inthe sugar glass than in pure water. Figure 6 illustrates the differ-ence in mobility for water (no significant enhancement of thetemperature dependence near Tg, mobility increasing with themolecular weight of the solute) and for the matrix (significant en-hancement of the temperature dependence near Tg).

Similar conclusions can be derived from studies about watertranslational diffusion (Dtrans or Dapp). The diffusion coefficientobtained from desorption experiments in dried glucose syrupsand maltose was shown to follow Arrhenius behavior above andbelow Tg, with the same apparent activation energy of 60

kJ.mole-1, thus showing a total decoupling of the water diffusivi-ty from the macroscopic viscosity. The diffusion was faster in ma-trices of the oligomeric chains of the glucose syrups than in mal-tose matrices at equal water content (Tromp and others 1997).Similarly, NMR measurements of the water self-diffusivity didnot show any important evolution at Tg. At this point, Dtrans wasaround 4x10-11 m2.s-1 in pullulan with 19.1% water content (Ablettand others 1993). These results follow the same line as formerstudies (Bruin and Luyben 1980; Furuta and others 1984), whichhad shown the water diffusivity varying continuously with watercontent and exhibiting relatively high values in glassy materials.

An heterogeneous distribution of water is sometimes desiredin food products, for instance to associate soft and crispy / crack-ly textures, as in pizza, ice-cream + cone, mixtures of cereals plusraisins. To maintain this texture differential, it is necessary tolimit the transfer of water from the soft (moist, rubbery) domainto the rigid (dry, glassy) one. The rate of transfer is proportionalto both the diffusivity of water and the difference in water poten-tial (or water activity) between each domain. Water diffusivity re-mains relatively high even in the glass so that the transfer has tobe reduced by means of edible films or barriers. Appropriate for-mulations of the 2 domains may allow reductions in the differ-ence in (apparent) water activity while keeping the difference inwater content and texture. A good example of application of theglass transition concepts to food technology is designing a con-fectionery with a core rich in fructose within a sucrose envelope.With the same aw the glass transition temperature is higher forsucrose than for fructose. The core (supercooled liquid) can thenbe soft within the rigid (glassy) coating.

Mobility of solutesPhysical data. In liquid systems well above Tg, the transla-

tional diffusion (Dtrans) and rotational diffusion (Drot) of mole-cules can be predicted from the so-called Debye-Stokes-Einstein

Figure 3—Fragility diagram: Arrhenius representation ofviscosity where T is scaled to Tg. From: (a: SiO2 , b: glycéroland h: triphenylphosphite) Angell and others 1994, (c: su-crose) Champion 1998, (d: fructose and e: glucose) Ollettand Parker 1990, (f: maltose) Noel and others 1991, (g:trehalose) Magazu and others 1998. Water content (%) asindicated in the inset. The relaxation time scale applies forsucrose 8%.

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relations (DSE):

Dtrans = kT / 6��rc Drot = kT / 8��r3 c (7)

where k = Boltzmann constant, T = temperature, � = viscosity, r =hydrodynamic radius of the diffusant, c = coupling factor be-tween the molecule and the matrix. A relaxation time associatedto the translational mobility (�D) may be defined as the time nec-essary for the lateral motion of a structural unit on a distanceequal to its characteristic dimension (Perez 1992). With smallmolecules the latter may be taken as the molecule diameter (2r):

�D = (2r)2 / Dtrans (8)

The DSE equations are not valid to describe the diffusion of asmall solute dispersed in a polymer network where the macro-scopic viscosity (commonly measured) does not reflect the localenvironment of the diffusing species and is not the factor thatcontrols diffusion. The translational and rotational mobilities ofsmall probes dispersed in concentrated sucrose solutions(57.5%) were not significantly affected, or were only slightly re-duced, upon addition of respectively 1% or 10% polysaccharides,in spite of the large increase in viscosity (Contreras-Lopez andothers 2000).

To check the validity of Eq. 7 in systems where the diffusingprobe was of a size similar to that of the matrix material, Dtrans offluorescein was measured in concentrated sucrose solutions (30-90%) in a temperature range from -15 to 20 °C using the FRAPmethod (fluorescence recovery after photobleaching) (Champi-on and others 1997b). The evolution of the fluorescein Dtrans withtemperature did follow Eq. 7 when the temperature was higherthan 1.2 Tg (Tg / T = 0.86) (Figure 7 and 8 ). The Dtrans valuesmeasured in solutions of various concentrations are all on thesame curve, which demonstrates that Dtrans is effectively con-trolled by viscosity. When the temperature was closer to Tg, thediffusion of fluorescein was faster than predicted from Eq.7.

The decoupling of translational diffusion and viscosity in thesame temperature range, (between Tg and 1.2 Tg) had been 1streported for probes dispersed in various organic liquids or poly-

mers (Blackburn and others 1996; Fujara and others 1992; Halland others 1998; Le Meste 1995). The values of Dtrans for probesdispersed in polymers are greatly dependent on the probe sizeand shape (Hall and others 1998). They may also be affected bythe existence of interactions such as hydrogen bonds betweenthe probe and the polymer (Hall and others 1999). By contrast,rotational diffusion data, obtained from a variety of techniques,always appear to scale with the matrix � relaxation dynamics (ex-cepted when probes dispersed in a polymer are smaller than aminimum size (Hall and others 1998)). The decoupling betweentranslational and rotational diffusions has been explained by thespatial heterogeneity of supercooled materials, comprising do-mains with varying mobility, both types of motions being differ-ently sensitive to the local variations of relaxation time (Black-burn and others 1996; Fujara and others 1992; Hall and others1998). The decoupling temperature being in the range where �and � relaxations merge (Perez and Cavaille 1994), suggests thatthe translational diffusion of the probe could be facilitated by lo-cal motions of the matrix when the temperature approaches Tg(Champion and others 1997b).

The practical importance of this decoupling has to be empha-sized, with regard to the stability of amorphous products: in thevicinity of Tg, the translational diffusivity may be 2 to 5 orders ofmagnitude higher than predicted from the viscosity and theStokes-Einstein equation. The temperature dependence also ismuch weaker (apparent activation energy =59 kJ.mole-1 for theDtrans of fluorescein in sucrose below 1.2 Tg).

Figure 4—Arrhenius representation of the characteristicrelaxation times for maltitol deduced from viscosity, me-chanical and dielectric spectroscopies. Faivre and others1999

Figure 5—State diagram of the sucrose-water system. Tgand Tm indicate the temperatures of glass transition andof “equilibrium” freezing/melting respectively, versus su-crose mass fraction. Tg’=glass transition temperature ofthe maximally freeze-concentrated phase (here defined asthe intersection of curve Tg (DSC Tgonset) and curve Tm(UNIQUAC model). Cs and Tgs=concentration and glass tran-sition temperature of the partially freeze-concentrated liq-uid when the product is stored at a temperature Ts>Tg’.From Blond and others 1997

Tgs

Ts

Cs

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EncapsulationIn the food industry there is a growing interest for encapsula-

tion technologies, which are designed to protect the encapsulat-ed material (or “active”) and to allow only controlled release. Thisis particularly the case for flavoring components, which are proneto loss by evaporation, oxidation or ingredient interactions. Al-though a variety of methods have been proposed to encapsulateflavors, spray-drying and extrusion are still the most commontechniques.

A great number of studies have been devoted to the retentionof aroma during drying, freeze-drying, extrusion, or storage ofdried products. It is well known that in favorable conditions, aro-ma can be retained to a much larger extent than expected fromtheir volatile character (see Flink 1975 for a review). At low watercontent the diffusion coefficient for organic compounds dropsrapidly, even more rapidly than that for water. This “selective dif-fusion” process (Thijssen 1971), with some modifications, is gen-erally accepted as the main process explaining the retention ofaroma being entrapped in glassy materials (carbohydrates orproteins) (King 1988). Release of volatiles is promoted by tem-perature and water content conditions that bring the dry glassymaterial to the rubbery state (To and Flink 1978; Whorton andReineccius 1995). The potential interest of glass transition inview of controlled release from glasses was the basis of a patent(Levine and others 1991).

The study of release kinetics is complicated by the structureof dried/extruded products and changes induced by plasticiza-tion. The release of propanol from freeze-dried sucrose and su-crose-raffinose matrices was shown to increase with (T-Tg) fol-lowing WLF kinetics with the “universal” coefficients, in a rangeof (T-Tg) approximately 10 to 30 °C (Levi and Karel 1995b). Ac-tually the release kinetics appeared to be controlled by crystal-lization (in the sucrose matrix) or by collapse (in the sucrose-raffinose matrix). The release of volatiles was also associatedwith the collapse of spray-dried maltodextrins (Whorton andReineccius 1995). Volatiles loss during rehumidification offreeze-dried foods models was interpreted in terms of a diffu-sion-based model where the web thickness within a sample in-creases with time because of structure collapse (Omatete andKing 1978). The introduction of an effective diffusion coeffi-cient, which should be time-dependent, was suggested to ob-tain a realistic description of processes above Tg (Karel and oth-ers 1993). While the completion of collapse results in a declineof the release rate, crystallization is accompanied by a total lossof the volatiles (Levi and Karel 1995b; Senoussi and others1995). For flavors encapsulated in carbohydrate matrices by anextrusion process, it was concluded that release (at low watercontents) due to diffusion through the matrix was relativelyslow as compared to release due to matrix cracking (Gunningand others 1999). With undercooled maltose-water mixtureswhere a more regular macroscopic structure could be expected,and with steady water content, the release of volatiles could bemodeled as a Fickian diffusive process (Gunning and others2000). Close to the glass transition of the material, the tempera-ture dependence of the apparent diffusion coefficient showeda decoupling from viscosity similar to the one described forprobes in sucrose-water mixtures.

The dependence on water content of the effective diffusioncoefficients of water and ethanol in maltodextrins films could befitted to the model developed originally by Fujita (1961) from thefree volume theory to express the concentration dependence ofthe diffusion coefficient of a solvent in a synthetic polymer (Fu-ruta and others 1984). However, the comparison of the curve fit-

ting parameters to other data would be necessary.

Edible films and barriersEdible films and coatings are expected to be concerned by

glass transition, as both their mechanical and barrier propertiesare strongly affected by temperature, ambient humidity andplasticizer content. There is an abundant literature on glass tran-sition temperature of biopolymer films and on the influence ofwater and other plasticizers. Nevertheless, quantitative data cor-relating their functional properties to glass transition phenome-na are scarce. In an extensive study on polyol-plasticized pullu-lan-starch blends (Biliaderis and others 1999), the permeabilityto O2 and CO2 could be fitted to Arrhenius behavior both belowand above Tg. The Arrhenius plots showed a distinct break in thetemperature range of Tg (DSC) and T� (drop in E’). The apparentactivation energy for permeability (EaP) was larger above glasstransition than below, amounting to 74 to 113 kJ.mole-1; it de-creased for increasing water contents. These values, however, aresmall when compared to the apparent activation energy for the �relaxation, which was between 226 and 296 kJ.mole-1. Evensmaller values of EaP above Tg were reported for O2, CO2, andethylene permeability in gluten films (20 to 56 kJ.mole-1) (Mujicaand others 1997). Similar features were described for the perme-ability or diffusion coefficients of gases in natural and syntheticpolymers. Consistent with the predictions from the free-volumetheory of diffusion, experimental observations showed that as

Figure 6—NMR relaxation time T2 (a) and second momentM2 (b) versus (T-Tg) for freeze-dried maltodextrins (DE 2,21, 40) equilibrated at a water activity of 0.4. T2 representsthe rotational mobility of the more mobile protons (mainlywater). M2 is considered to be inversely related to mobilityof the less mobile protons (matrix protons + possibly somewater protons). Data from Grattard and others 2002

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the weight fraction of small molecules increases (for instance, in-crease in water content in the above examples) the apparent ac-tivation energy of diffusion above Tg decreases and the inflexionat Tg becomes less apparent (Duda 1999; Ramesh and Duda2001). Although the free-volume theory for diffusion allows, atleast qualitatively, to explain the temperature and water contentdependence of permeability, it should be remembered that per-meability (P) is the product of diffusivity (D) and solubility (S) ofthe diffusant in the matrix:

P = D S

The activation energy for P is:

EaP = EaD + �HS

where �HS is the dissolution enthalpy. Depending on the respec-tive hydrophilic/hydrophobic character of the film and the per-meant, EaP may be variously affected.

Oxidation kineticsThe relatively high mobility of water and oxygen in glassy ma-

trices is responsible for the limited shelf-life of encapsulated ma-terials or dried food products. The permeation rate of oxygen intoa freeze-dried (sucrose-maltodextrin-gelatin) matrix was foundto show Arrhenius behavior (EaP = 74 kJ.mole-1) below and aboveTg and to control the oxidation kinetics of the encapsulated oil(Andersen and others 2000).

Evidently, the permeation rate is influenced by the matrixstructure (crystallinity, porosity, tortuosity) and the distributionof oil in the case of an emulsion. Glass transition appears to onlyhave a weak direct impact on the diffusivity of small moleculessuch as gases and water. A number of studies however show thatthe structural modifications induced by glass transition signifi-cantly affect the actual permeation. Crystallization of the matrixwas shown to induce the release of the encapsulated oil, thus en-hancing its oxidation (Shimada and others 1991). Oxidation ofsaffron carotenoids and of the beetroot pigment betanin encap-sulated in polymer matrices was observed in conditions of watercontent and temperature where the matrices were glassy, con-firming the permeation of O2 in the glass. The lower degradationrates were actually observed when the matrix was collapsed (Se-lim and others 2000; Serris and Biliaderis 2001). Depending onthe experimental conditions, partial or total release of the oil mayresult from collapse; while the released oil is oxidized, the frac-tion remaining entrapped appears to be protected (Labrousseand others 1992; Grattard and others 2002).

Glass transition knowledge may be beneficial to encapsula-tion, edible films, and coating technologies by helping to defineprocessing parameters and especially the matrix formulation.More investigations are necessary however, to better understandthe mobility of small solutes in glassy and rubbery matrices andparticularly the influence of water. Moreover, it must be kept inmind that in practical situations release or permeation kineticsmay be affected by events such as collapse or crystallization.

Structure and Texture

Texture of low moisture productsCrispness, a popular texture attribute of various low moisture

foods, is lost when their water content is raised above a thresh-old, which was found between 6 to 9% for crackers, popcorn, andpotato chips (Katz and Labuza 1981), breakfast cereals (Sauva-

geot and Blond 1991), dried white bread and extruded flat bread(Fig. 9). As a first explanation, the crispness loss was directly re-lated to glass transition (Ablett and others 1986, Slade and Le-vine 1993, Roos 1995). Crispness is associated with a low-densitycellular structure that is brittle and generates a high- pitchednoise when fractured. The loss of brittleness was attributed tothe drop in rigidity modulus that is characteristic of the glass-rubber transition in polymers. Due to the plastifying effect of wa-ter, the temperature of glass transition was assumed to be de-creased to the ambient temperature where crispness wasassessed. A glass transition could indeed be recognized in a realfood product such as bread using thermomechanical analysis(TMA) and later DMTA (Le Meste and others 1992, 1996). It wasshown, however, that the water content at which the loss of sen-sory crispness, as well as the brittle to ductile transition observedin mechanical tests, occurred in white or extruded breads, corre-sponded to a glass transition temperature (T�)much higher thanthe testing temperature (Le Meste and others 1996, Roudaut andothers 1998) (Figure 9). Similar results were obtained with corncakes (Li and others 1998) and with starch extrudates (Attenbur-row and others, 1992, Nicholls and others 1995). This most im-portant texture change thus takes place while the material is inthe glassy state. It was suggested that it could be best modeledwithout any relation with glass transition, by empirical expres-sions such as the Fermi equation (Peleg 1996) (Fig 9).

The underlying microstructural events are not elucidated yet.Secondary relaxations are evidenced by DMTA and dielectricspectroscopy below glass transition in dry and extruded breads(Le Meste and others 1996, Roudaut and others 1998, 1999) (Fig10). It is difficult to definitely relate these features with the tex-ture changes, because the latter result from a variation in watercontent at ambient temperature, whereas the former are detect-ed in a temperature / frequency scanning at constant water con-tent. The values of tan �(DMTA),however were shown to increasefrom the water content (9%) at which the acoustic emission dropwas initiated, which allows to conclude to a significant increase inmobility in the glassy state, coinciding with the crispness loss

Figure 7—Translational diffusion coefficient (FRAP) of fluo-rescein in sucrose solutions. The curve is the predictionaccording to the Stokes-Einstein relation (Eq. 7), with vis-cosity predicted from the WLF model (parameters in Fig-ure 2 caption). Water contents indicated in the inset. Cham-pion and others 1997b

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(Figure 11). The loss tangent increase could result from sub-Tgrelaxations becoming possible at 25 °C when the water content israised above 9%, or could be associated with motions just pre-ceding the onset of glass transition (� relaxation) (Roudaut andothers 1998).

Fracture tests evidence another important event in glassy ce-real products: an hardening effect is observed, beginning atabout 5% and maximum between 9 and 11% water content withdried and extruded breads, which is also detected in sensoryanalysis and which is followed by a softening at higher hydration(Fontanet and others 1997, Roudaut and others 1998). Similarobservations have been reported for corn cakes (Li and others1998) and “fat-free apple chips” (Konopacka and others 2002). Itmay be noted that similar effects have also been described forstarch extrudates (Attenburrow and others 1992, Nicholls andothers 1995) and films (Chang and others 2000). The first changein texture could, therefore, be an increase in fracture stress: be-ing less easily fractured, the product is perceived as less crispy(Roudaut and others 1998). This hardening effect was ascribedto antiplasticization (Roudaut and others 1998; Chang and oth-ers 2000). The addition of a diluent to a polymer, although de-creasing Tg, can hinder the polymer chain motions, resulting inan increased rigidity (Vrentas and others 1988). This antiplastify-ing effect of water in starch systems has been attributed to ashort-range reorganization (Fontanet and others 1997) resultingin a density increase by filling the defects in the glass structure(Benczedi 1999, Chang and others 2000). It is worth noting thatthis antiplastifying effect was observed under high deformationconditions. Water at low concentration may therefore act as plas-ticizer under low deformation conditions and as antiplasticizerunder high deformation conditions (Chang and others 2000).

Collapse and cakingThe changes in mechanical properties related to glass-liquid

(or rubber) transition are the governing factors in many food pro-cessing operations. Structure collapse of the product during air-or freeze-drying, or during the storage of dried products, is re-sponsible for the reduction in volume and porosity, which resultsin the loss of desirable appearance and volatile substances andin poor rehydration. It is generally considered that structure col-lapse should be avoided, although it may be credited of somebeneficial aspects such as a reduced sensitivity to oxidation.Powder stickiness and caking are phenomena related to collapse.

This imposes important constraints on drying and storage in thedry state of products with a high relative content of low molecu-lar weight solutes such as sugars, minerals, and protein hydroly-sis products. Here again, a controlled caking process (or agglom-eration) is used to improve the appearance and handling ofpowders and their dispersion in water.

It is well known that the factors controlling structure collapse,stickiness, and caking are temperature and water content andthat these processes are time dependent. The collapse tempera-ture of powders (To and Flink 1978) as well as the sticky point(the temperature at which the force to stir a powder in a tube in-creases sharply) has been shown to decrease when water contentincreases. During freeze-drying, structure collapse occurs when,as a result of an increase in water pressure for instance, the heatinput exceeds the drying needs, inducing a rise in temperatureat the sublimation interface. Structure collapse in the course offreeze-drying was shown to happen when the viscosity of thecryo-concentrated phase had fallen to the range of 107 to 104 Pa.s(Bellows and King 1973). Below this limit, the interstitial networkcould not withstand the collapsing effects of the capillary forces.The same critical viscosity range was shown to determine col-lapse in freeze-dried materials (Tsourouflis and others 1976). Asimilar mechanism was proposed to explain caking phenomena:the formation of interparticle bridges between adjacent particlesand then aggregation take place when the surface viscosityreaches the critical range 108 to 106 or 107 to 105 Pa.s for sucrose-fructose-maltodextrin mixtures or coffee extract respectively (forparticle diameters 3 to 4 µm and 30 to 40 µm respectively andcontact time 1 to 10 s.) (Downton and others 1982; Wallack andKing 1988).

The connection of collapse and caking with glass transition isdemonstrated by the parallel evolution of agglomeration tem-perature and Tg as a function of water content. The sticky point(Ts) and glass transition temperature of the sucrose-fructosemixture model were observed to be similarly affected by increas-ing moisture content, with Ts close to the Tgend values, that isabout 20°C above Tgonset (Roos and Karel 1991a). Similar resultswere reported for the collapse temperature (Tc) of freeze-driedmaltodextrins, but with Tc values about 30 to 70°C above Tgonset

(Roos and Karel 1991b). It must be stressed that the critical vis-cosity levels, and then the (T-Tg) values are dependent on thecharacteristic times of the methods used to monitor the changes.The link between collapse and caking and glass transition is fur-ther supported by many observations showing that collapse andcaking temperatures are raised as Tg is, when the average molec-ular weight of the product increases, for instance with maltodex-trins (collapse) (Tsourouflis and others 1976; To and Flink 1978;Roos and Karel 1991b) or starch addition in powdered soy sauce(caking) (Hamano and Sugimoto 1978). The increase in Tg ofsugar solutions following the addition of maltodextrins was re-ported to be correlated with an increase in drying yield (thanksto the reduction of product sticking on the spray-dryer walls)(Busin and others 1996).

Both collapse and caking have been demonstrated to obeyWLF kinetics. The collapse of a freeze-dried sucrose-raffinosemodel, measured by the decrease in specific volume, was report-ed to change exponentially with time. The relaxation time valuescould be fitted to the WLF equation with the “universal con-stants”, for a (T-Tg) range between 15 and 30 °C (Levi and Karel1995). Caking of a spray-dried fish protein hydrolyzate was alsofound to follow a first order kinetic model. The temperature de-pendence of the relaxation time was characterized by a WLF rela-tionship, with adjustable C1 and C2 coefficients, for (T-Tg) be-

Figure 8—Translational diffusion coefficient of fluoresceinin sucrose solutions (Cf. Fig. 7) to show the decoupling ofDtrans from viscosity for Tg/T>0.86. (T/Dtrans� =constant whenthe DSE law is obeyed: solid line). Champion and others1997b


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tween about 20 and 80 °C (Aguilera and del Valle 1995). Becauseof the rather narrow (T-Tg) range, in the 1st example, the C1, C2

values may not be meaningful. What is most important, however,is the high level of the mean apparent activation energy (>200-400 kJ.mole-1), which points to the large temperature depen-dence of the phenomenon.

Prevention of collapse or caking is 1st based on low tempera-ture and/or low water content. This means, for instance, coolingthe walls of the spray-dryers and the design of towers with di-mensions large enough so that the droplets would not reach thewalls before being in the non-sticky domain. The beneficial ac-tion of anticaking agents, which has been recognized for long, isexplained by several mechanisms, the rise in glass transitiontemperature by increasing the average molecular weight of theamorphous phase being only one of them (Aguilera and others1995).

It may be concluded from the presently available evidencethat structural collapse, stickiness and caking/agglomeration,being primarily dependent on flow rate, are successfully ex-plained using the glass transition concept. From the current liter-ature, it appears that these phenomena are never observed inthe temperature/water content domain below glass transition.Even in cases where some chemical evolution in the glassy statewas reported, collapse could not be observed at T<Tg. Their ki-netics can be described based upon the equations characterizingthe dynamic properties, such as viscosity or relaxation time,above Tg, although coefficients specific to the product should bedetermined. The glass transition temperature measured, for in-stance, by DSC appears to be a good predictive parameter oftheir domain of occurrence. Some shift above this value mayhowever be observed. The anticaking action based on a physicalbarrier between particles, cited above, is one example. It has alsobeen shown that addition of small amounts of high molecularweight substances may prevent the collapse during freeze-dry-ing, without changing the DSC Tg (Le Meste and others 1979).

Drying and extrusionThe transition between the rigid glass and the viscoelastic su-

percooled melt has many other applications in food processing,particularly aiming at deliberate structures. Drying of plantproducts may be carried out with rough initial conditions, allow-ing an external layer to pass suddenly from the fully hydratedstate much above the glass transition to the dry glassy state. Thisrigid external layer helps in maintaining the original productshape and in developing an internal porous structure favorableto the final drying and to a fast rehydration. With some materialshowever, this layer may work as an impermeable crust, which canbe desirable or not depending on the end-use of the product.The formation and thickness of the crust are controlled by therelative rates of drying and collapse. A predictive model could beproposed, by accounting for viscoelastic deformations andchange in Tg during drying (Achanta and Okos 1995). Compres-sion is, on the contrary, used to reduce storage and transport vol-ume of freeze-dried products. Water content and temperatureduring compression can be adjusted according to the glass tran-sition concepts. In the snack and cereals industry, this knowl-edge is used to design the operations of flaking, puffing, extru-sion-cooking, although published studies are scarce. Theexpansion during extrusion-cooking was modeled using the WLFvariation of viscosity above Tg. The influence of extrusion tem-perature and initial water content on the final density could besatisfactorily predicted. Simulation showed for instance thatbubble growth can start to occur when the temperature is about

30 °C above Tg, viscosity being of the order of 107 to 108 Pa.s un-der these conditions (Fan and others 1994).

CrystallizationCrystallization is a very important process regarding the quali-

ty of food products. Depending on the product, the absence orpresence of crystals, as well as their size and shape, are critical fac-tors, for instance to the desired texture properties of confectioner-ies or ice-creams and for the free flowing characteristics and disso-lution of powders. Moreover, crystallization may promote therelease of substances entrapped in the glass, for instance water,which will increase the water content of the remaining amorphousphase. Crystallization comprises 2 steps: nucleation and growth.Nucleation is the formation of a crystalline phase in the melt or ina supersaturated/supercooled solution. According to classical nu-cleation theories, the probability of appearance of nuclei, J, is afunction of a thermodynamic and a kinetic terms

J = A exp [- (�G* + �G’/ RT]

�G* is the critical free energy required for the formation of a nu-cleus of the critical size at a given temperature and concentra-tion. �G’ is the activation energy for transport between the bulksolution and the nucleus. When nucleation occurs at a tempera-ture T close to Tm, �G* is very high, resulting in a very low J value.The nucleation probability increases with the degree of super-cooling (Tm-T). As T decreases however, the term �G’ becomesdominant, the closer T to Tg, the more nucleation is controlled bymolecular mobility. The nucleation rate can therefore be expect-ed to take a maximum value at some temperature between Tgand Tm.

The crystal growth rate also depends on several processesthat are differently influenced by temperature. Schematicallythe crystal growth implies, on the one hand, incorporation ofgrowth units into the crystal lattice, on the other hand, transportof these units from the bulk to the crystal interface, surface diffu-sion to an appropriate site and heat transfer in the reverse way.Whatever the mechanism assumed for the incorporation, the 2processes involved in crystal growth can be expected to be in-versely controlled by temperature, resulting in a bell shaped

Figure 9—Glass transition temperature and crispness ofextruded flat bread as a function of water content. Thesolid line is the fitting of Fermi equation to crispness data:P=P0/[1+exp ((w-wc)/b)] where P and P0 are magnitude ofcrispness, respectively for water content w and in the drystate, wc is the characteristic water content where P=P0/2, b is a constant describing the steepness of the transi-tion.


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curve with a maximum at some temperature between Tg and Tm.To describe the variation of the overall rate of crystallization as afunction of temperature, several semi-empirical expressionswere proposed, particularly for polymers, such as the followingone (Lauritzen and Hoffmann 1973):

K1/n = A’ exp [-U* / R(T-T)] exp (- N / T �T f ) (9)

K is the isothermal crystallization rate constant at temperature Tderived from the Avrami equation: Xt = 1-exp (- K tn) where Xt isthe fraction of the material that has crystallized at time t and nthe Avrami index. A’ is a pre–exponential constant that includesall terms independent of temperature. N is a constant that wasrelated to the surface energy in the case of polymers, �T(=Tm-T)is supercooling, f = 2T / (T+Tm). U* is a constant related to the ac-tivation energy for the transport of crystallizing units. T is a hy-pothetical temperature below which such transport is inhibitedand has been found in practice to be approximately 30°C belowTg. The transport term in [9] has the form of the VTF expression.For large supercooling, when this term is controlling the crystalli-zation rate, the process can therefore be observed to exhibit VTF/ WLF kinetics. Considering a broader temperature range, the in-terplay between transport and adhesion to the crystal surface isexpected to again result in a bell shaped curve between Tg andTm.

The (time-dependent) crystallization temperature (Tcr) inamorphous sugars determined from DSC with a heating rate of5°C/min was reported in a number of studies to be approximate-ly halfway between their Tg and Tm respective values (Roos andKarel 1991a, 1992; Saleki-Gerhardt and Zografi 1994; Gabarraand Hartel 1998). The effect of water was found to be about thesame on Tcr and Tg as indicated by a fairly constant value of(Tcr-Tg) (Roos and Karel 1991a). The time to complete crystalli-zation (tcr) in samples of lactose with water content betweenabout 1% and 8% could be fitted to a WLF type equation:

Log tcr = log tg + C1 (T-Tg) / (C2 + T-Tg)

where the “universal values” were used for C1 and C2; the time tgwas determined by curve fitting (Roos and Karel 1992). The (T-Tg) range in these experiments was 10 °C to 45 °C. A Lauritzen-Hoffmann-like fit was found to apply to crystallization rates mea-sured for dry sucrose and lactose with water content about 3% to7% in temperature ranges between 25 °C or 50 °C to 90 °C or 130°C above Tg (Kedward and others 1998, 2000). This model wasalso applied to crystallization in a 50% aqueous starch gel (Marshand Blanshard 1988) and to the retrogradation of waxy maizestarch extrudates (Farhat and others 2000). It is generally be-lieved that crystallization over practical time scales occurs exclu-sively above Tg; however the drug indomethacin was reported toundergo crystallization over a time scale of days or hours down totemperatures 30 °C below its Tg, provided the material had beentriturated in the glassy state (Yoshioka and others 1994).

The possibility of inhibiting the crystallization in dried prod-ucts by raising Tg through the addition of high molecular weightcompounds was explored in many studies. Mixing the crystalliz-ing sugar with other substances effectively reduces crystalliza-tion rate, although a connection with a change in Tg can not bedemonstrated ( Jouppila and Roos 1994; Gabarra and Hartel1998). In lactose-trehalose mixtures, crystallization was delayedwithout any increase in Tg (Mazzobre and others 2001). It is mostprobable that the additives interfere with crystallization not onlyby decreasing molecular mobility (connected with an increase in

Tg) but also by hampering the incorporation of the growth unitsin the crystal lattice, an effect depending on the molecular struc-ture of the additives. It is well known that in mixtures of sugars,the crystallization rate is depressed, compared to that of purematerials.

In dried products, it is generally considered that crystalliza-tion proceeds through nucleation and growth (Saleki-Gerhardtand Zografi 1994), the amorphous state of the freeze-dried sam-ples being checked by X-ray diffraction (Kedward and others1998). On the contrary, in frozen products all the “freezable” wa-ter has generally been transformed into ice during the freezingstage. The issue is then the coarsening of the ice structure duringfrozen storage, which is considered to occur through severalmechanisms, including accretion, migratory recrystallization (orOstwald ripening), where differences in the stability of crystals,based on size differences, cause small crystals to disappear andlarge crystals to grow, and finally isomass rounding, which is alsobased on differences in stability for surfaces with different cur-vature radii (Hartel 1998). Several studies with fructose solutions(Sutton and others 1996) and with ice creams (Hagiwara andHartel 1996) suggested that ice recrystallization was driven by anon-convective diffusion process, as the time dependence of themean crystal size could be described by the expression:

r = r0 + K t1/3

where r is the mean crystal radius, r0 the initial value, K the crys-tallization rate (Hartel 1998). The temperature dependence of Kwas checked for WLF kinetics, using a constant temperature (Tg’or an arbitrary temperature T0 obtained by fitting). The authorsreported the fit of K to WLF kinetics was reasonably good, al-though recognizing that the range and the number of testedstorage temperatures (-20 °C to -5 °C, that is 14 °C to 29 °C above

Figure 10—Storage modulus (E’), loss modulus (E”) and lossfactor (tan �) for extruded bread with 5% water contentshowing � and � relaxations. Measurement frequencies:5, 20, 40 Hz


106

tan ��

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the DSC Tg’, for ice creams; -20 °C to -10 °C, that is, 25 °C to 35 °Cabove Tg’ for fructose solutions) were too small to allow exact de-termination of the best model for temperature dependence.Moreover a constant reference temperature appears irrelevant tocheck WLF kinetics in this situation where the composition of theamorphous phase and therefore its Tg value are variable in thetested temperature range. Actually, commenting on the data forthe ice crystal growth in frozen beef, as well as other processes ofdeterioration in frozen foods, we stressed the point that the tem-perature dependence of these processes was much too weak tobe correlated with the drastic decrease in viscosity above Tg’, as aresult of the WLF effect associated to ice melting (Simatos andBlond 1991). We suggested that (besides using more appropriateWLF parameters C1 and C2) this discrepancy could be solved us-ing Tg values much lower than Tg’. Indeed, to take into accountthe dilution induced by the melting of ice, the temperature tak-en as reference (Tg in Eq. 2), should be the variable Tgs (Figure5) (Simatos and Blond 1993).

When added at a rather high concentration to solutions of lowmolecular weight solutes, polymers induce an increase in the tem-perature commonly considered as Tg’ on DSC curves. Evidencehas been presented, however, for the onset of molecular mobilityremaining close to the glass transition temperature of the smallsolutes solution (Simatos and others 1995a). Macromolecules (var-ious polysaccharides and gelatin) are commonly added to icecream mixes as “stabilizers” as they are known to exert a beneficialeffect on the texture of the final product. At the very low concen-trations that are used in the ice cream industry, these stabilizersdo not induce any visible effect on the DSC Tg’ values (Levine andSlade 1988; Goff and others 1993; Blond 1994). The main mecha-nism of the favorable effect of these stabilizers on the sensoryproperties of texture may relate to the perception in the mouth, ei-ther through a change in the viscoelastic properties of the unfro-zen phase or any other sensory effect. Stabilizers however havebeen shown to reduce recrystallization rate in ice creams stored attemperatures above Tg’ (Caldwell and others 1992; Hagiwara andHartel 1996). The potential mechanisms suggested to explain thisinhibition include: restriction of mobility around Tg’ (Blond 1994);mechanical limitation of the crystal growth related to rigidity of thefreeze-concentrated phase (Muhr and Blanshard 1986; Blond1988); adsorption of the macromolecules on the crystal surface(Sutton and Wilcox 1998). In stabilized ice creams, there was a gen-eral trend where the recrystallization rate increased with increas-ing (T-Tg’) but the data did not fit well WLF kinetics (with constantTg’) (Hagiwara and Hartel 1996).

Chemical stability

ProteinsIt has been suggested that, to ensure the preservation of

structure and activity of proteins in the course of freeze-drying ortheir long-term storage in the dry state, all that is needed is tomaintain the system below its glass transition temperature(Franks 1990). The higher the Tg of the matrix, the better shouldbe its stabilizing effect. Experimental results published so farhave only partly verified this hypothesis. It has been shown forvarious dried enzyme preparations that storage above Tg accel-erated the loss of activity. The rate of glucose-6-phosphate dehy-drogenase inactivation (in a glucose-sucrose matrix) was report-ed to conform to the WLF relation (with coefficients C1 � 12.5 andC2 � 92 for water content 6% to 7%) (Sun and others 1998). Therate of inactivation of a pectinlyase preparation also greatly in-creased at temperature above Tg (Taragano and Pilosof 2001).

For tyrosinase (Chen and others 1999a) and invertase (Chen andothers 1999b) (in PVP matrices with various water contents), theincrease in the rate of inactivation as a function of (T-Tg) wasmuch less pronounced.

Moreover, inactivation was observed to occur during storagebelow Tg for a number of systems (Schebor and others 1996;Mazzobre and others 1997; Sun and others 1998; Chen and oth-ers 1999a,b; Taragano and Pilosof 2001). Alpha (�) amylase wasmore stable in rubbery matrices of lactose or trehalose than in aglassy PVP matrix (Terebiznik and others 1998). The protectiveefficiency of saccharides, maltodextrins and PVPs does not in-crease with their respective Tg (Rossi and others 1997; Terebiznikand others 1998). Provided that they remain amorphous aboveTg, disaccharides are more protective than expected from theirrelatively low Tg. The stabilizing effect of sugars may be attribut-ed to specific hydrogen–bond interactions between the proteinand the sugar replacing water (Crowe and others 1993). Dehy-dration induced conformational change of poly-L-lysine, from arandom coil conformation to an extended � sheet one, can beprevented either by immobilization (vitrification) resulting fromfast air-drying, or by sufficient interaction with a protectantthrough hydrogen-bonding. Protection was increased in the or-der: dextran-sucrose-glucose, opposite to the order of increasingTg, but the same as for the tightness of hydrogen bonds with thepolypeptide, as revealed by FTIR absorption spectra (Wolkersand others 1998). The high efficacy of trehalose, which had beenrelated to its highest glass transition temperature among thetested sugars (Green and Angell 1989), could be associated, inaddition, with other properties, such as: restricted mobilityabove Tg and ability to resist to phase separation and crystalliza-tion during storage (Sun and Davidson 1998), higher stability ofthe disaccharide bond (Schebor and others 1999b), existence ofsolid state polymorphic forms (Sussich and others 2001).

Chemical reactionsTo determine the impact of Tg related phenomena on the

chemical stability of foods, investigations have been mainly fo-cused on non-enzymatic browning reactions (Karmas and others1992; Roos and Himberg 1994; Lievonen and others 1998; Belland others 1998; Schebor and others 1999a; Craig and others2001). Studies have also dealt with the hydrolysis of sucrose (in

Figure 11—Influence of water content on the loss factormeasured at 25 °C (DMTA 5 Hz) for white bread (�) andextruded bread (�). Roudaut and others 1998.


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low pH media: Schebor and others 1995, 1999b, or by invertase:Chen and others 1999; Kouassi and Roos 2001), aspartame deg-radation (Bell and Hageman 1994), thiamin hydrolytic cleavage(Bell and White 2000), cleavage of peptide bond and unimolecu-lar dissociation of tetrahydropyran (Streefland and others 1998).From these various works, it can be provisionally concluded thatthe temperature of glass transition does not constitute an abso-lute stability threshold and that, above this temperature, the re-action kinetics do not obey WLF kinetics. Almost all of the quotedstudies reported finite reaction rates in glassy products. The re-action rate increased with the difference (T-Tg) (sometimes witha maximum above Tg: Bell and White 2000). When the variationof Tg however, was induced by changes in water content, the ex-pected single relationship between the rate and (T-Tg) was notobserved (Karmas and others 1992; Roos and Himberg 1994; Belland others 1998). The reaction rate evolution with temperaturewas most often described as uniform, of Arrhenius type, evenwithin the glass transition temperature range (Roos and Him-berg 1994; Lievonen and others 1998). A break in the slope of theArrhenius plot in the Tg range was identified by Karmas and oth-ers (1992). The apparent activation energies however remainedlow, even at T>Tg : 50 to 100 kJ.mole-1 (Karmas and others 1992),130 to 140 kJ.mole-1 (Craig and others 2001). These values arequite smaller than the activation energies commonly observedfor dynamical properties in the glass transition range.

In some systems, reactions in the glassy state may have beenrendered possible by a phase separation, resulting in microre-gions with a higher water content (and then a lower Tg) than thewhole matrix. It should be also noted that in most studies, reac-tants were mixed together during the preparation step, limitingthe need for translational diffusion. A decoupling of reactantsmobility from the matrix viscosity may also be an explanation forthe occurrence of finite reaction rates at or below Tg.

Some observations suggest that the temperature itself ratherthan the difference (T-Tg) controls the non-enzymatic browningkinetics (Roos and Himberg 1994; Schebor and others 1999a). Aswas recalled by Karel and Saguy (1991) physical chemistry booksstate that, for reactions that are influenced by diffusion of reac-tants, the reaction rate constant kapp may be described by the re-lation:

kapp = kact / (1 + kact / �D)

where kact is the reaction constant observed in well-stirred solu-tions, which recognizes the fact that only a fraction of potentialreactants coming into contact are activated and can react. D isthe diffusivity of the reactants (equal to the sum of the individu-al diffusion coefficients), � is a coefficient depending on a colli-sion distance. Both parameters kact and D are temperature-de-pendent. The reaction can be fully controlled by diffusion if thereaction constant kact is much larger than �D (then kapp � �D),the temperature dependence is the same as for D (possibly WLFkinetics). In the case of reactions that have a high activation en-ergy (as could be for non-enzymatic browning), kact remains lowas long as the temperature is not high enough, and consequentlykapp is controlled by activation of reactants rather than by diffu-sion (kapp � kact). The temperature dependence is of Arrheniustype. The temperature dependence of the alkaline phosphataseactivity in frozen concentrated sucrose solutions represented afavorable situation for modeling according to WLF kinetics (withvariable Tgs : Fig. 5) probably because the enzyme exhibits a rela-tively high kact even in concentrated sucrose solutions and at lowtemperature (Champion and others 2000).

Microbial stabilitySlade and Levine (1987, 1991) claimed that water dynamics

related to glass transition may be used instead of water activity(aw) to predict the microbial stability of concentrated and inter-mediate-moisture food products. From an extensive review ofthe then available knowledge, Chirife and Buera (1996) demon-strated that these expectations were not supported by experi-mental evidence. Among the arguments that were presentedwas the observation that many foods, including fruits, vegeta-bles, and milk, are in the rubbery state for humidity conditions(aw) where these foods are known to be resistant to microbialgrowth. (T-Tg) values, the difference between a typical incuba-tion temperature of 30 °C and the product Tg, were estimated tobe between 60 °C and 100 °C for aw values between 0.60 and0.85. To the contrary, molds were reported to grow in maize andwheat flour in moisture conditions where the products were like-ly to be glassy. It was concluded that, although mobility factors,in addition to aw, may be useful for a better prediction of microbi-al behavior in foods, glass transition concepts do not provide anybetter alternatives than aw as a predictor for this. More recently,germination of Aspergillus niger spores was observed in starchsamples for water content/temperature conditions just aboveTg, but not in samples below Tg (Kou and others 1999). More re-search is needed to specify the importance of molecular mobilityand glass transition phenomena in controlling the microbial sta-bility of foods, dealing with different types of microorganismsand substrates, not only spores germination but also growth andmetabolic activity.

Conclusion

ONE IMPORTANT ISSUE HINDERING THE APPLICATION OF GLASS

transition concepts to food technology is the lack of anunique Tg. The reasons for that are related to the influence ofthe experimental time and of the stress nature. Moreover, “the”glass transition temperature is not an absolute frontier for mo-lecular mobility. Even mechanical properties allow the observa-tion of some mobility below glass transition, which is in connec-tion with sub-Tg relaxations or physical aging, and which maybe of prime importance as regards texture properties of somefood products. Transport of small molecules also makes highmobility evident in the vicinity, or below, the glass transition.One should further be aware that the glass transition tempera-ture is not sufficient in itself to characterize the behavior of amaterial in the GLT range. Other parameters, such as width ofthe transition, fragility, non-linearity, non-exponentiality, haveto be collected for food materials, related to chemical or struc-tural attributes and studied as regards their relevance to foodtechnology issues.

These problems, however are not specific to food materials.They have to be also overcome for non-food materials, whereGLT and related phenomena have proved to be such useful con-cepts. More particular to the food area may be difficulties result-ing from the extreme chemical and structural complexity of manyfood products. The ubiquitous presence of water may also be animportant factor: the large variations in water content and thespecific interactions with other food constituents make specialinvestigations and theoretical models necessary. Moreover, athigh water content, GLT is no longer relevant. Based on presentlyavailable information, glass transition concepts do not seem use-ful to predict with confidence the microbial stability of foods; fur-ther research is needed to assess the influence of water and sol-utes mobility for controlling growth and metabolic activity ofmicroorganisms in intermediate moisture foods.


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In situations where visco-elastic properties play a dominantrole, GLT based technology has proved truly efficient. Both theTg (or T�) values and WLF kinetics show good capabilities forthe understanding and prediction of the effects of tempera-ture, water content and product formulation and can be suc-cessfully used to control texture, processes such as collapse andagglomeration, and various technological operations includingdrying and extrusion. There are circumstances however, wheremolecular mobility, which may be under the dependence ofGLT in low moisture and frozen products, is only one of the fac-tors that control the evolution kinetics. Crystallization andchemical/biochemical reactions offer good examples whereglass transition-linked mobility combined with other conceptscan provide a satisfactory understanding of the observed kinet-ics. Important progress may be expected along this line, howev-er much more experimental work is needed to gain a clearerview on molecular mobility, particularly in the vicinity of Tg andbelow, and its relationships with secondary relaxations or aging.It seems highly desirable to establish “mobility maps” for foodmaterials showing the characteristic relaxation time for the dif-ferent types of molecular motions, as a function of temperatureand water content. Improved understanding of mobility in thevicinity of Tg should result from the rapidly growing knowledgeon heterogeneity in glasses and supercooled liquids (Sillescu1999). Recent theoretical developments and experimental evi-dence are leading to a clearer view on heterogeneity in (chemi-cally homogeneous) glass-forming liquids and polymers. Heter-ogeneity at the nm scale, connected with the distribution ofrelaxation times, is being studied by various spectroscopicmethods and by experiments with colloidal glasses (Ediger2000). These studies should particularly help in understandingand predicting molecular mobility immediately above glasstransition temperature.

More attention should also be given to the behavior in theGLT range of complex food products: respective mobility of indi-vidual components according to their molecular size (and otherphysico-chemical characteristics), different glass transition tem-peratures for various components resulting from the distributionof water, contribution of possible phase transitions (lipids) to theobserved macroscopic behavior, and so on. Glass transition inproteins should deserve further research in the food technologycontext, particularly addressing the questions of glass transitionin frozen products, protective actions in the dry state and plasti-cizing effects in films of sugars and polyols.

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MS20020079 Submitted 2/6/02, Accepted 4/25/02, Received 4/30/02

Authors are with the Laboratoire d’Ingénierie Moléculaire et Sensorielle,ENSBANA. Université de Bourgogne, 21000 Dijon, France. Direct inquiriesto author Simatos (E mail: [email protected]).


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