Glass Transition and Food Technology: A Critical ... - Lorentz Center

Concise Reviews in Food Science

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Concise Reviews and Hypotheses in Food Science.

Glass Transition and Food Technology: A Critical Appraisal M. 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 will discuss 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 main conclusions are provisionally proposed. Firstly, glass transition cannot be considered as an absolute threshold for molecular mobility. Transport of water and other small molecules takes place even in the glassy state at a significant rate, resulting in effective exchange of water in multi-domains foods or sensitivity to oxidation of encapsulated materials. Texture properties (crispness) also appear to be greatly affected by sub-Tg relaxations and aging below Tg. Secondly, glass transition is only one among the various factors controlling the kinetics of evolution 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 regards the effects of temperature and water content. On the contrary, chemical/biochemical reactions are frequently observed 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

T

HE HUGE VARIETY OF POSSIBLE APPLICATIONS OF GLASS TRANSITION

in food science and technology was highlighted in the 1980s by Levine and Slade. Stimulated by an extensive series of papers and presentations from these authors (Levine and Slade 1986; Slade and Levine 1988, 1991) and by theoretical and experimental progress in materials science, a constantly increasing number of studies in the food area refer to glass transition. It seems timely to review current literature, in order to discuss the extent to which it is possible to understand and predict the behavior of foods during processing and storage on the basis of glass transition-related phenomena. Most food products with reduced moisture content are partly or totally amorphous. Depending on the storage temperature and their composition (mainly water content), they are glassy and may be expected to be rigid (eventually crispy) and stable, or contain a rubbery or liquid phase and then be soft and prone to physical and chemical changes. Processes such as baking, air- and freeze-drying, extrusion, and flaking may also operate through the glass transition range. Since the theoretical basis of glass transition that are of interest to food science were already reviewed (Angell and others 1994; Perez 1994; Simatos and others 1995b) a few points will only briefly be mentioned here. We will focus on experimental data dealing with food related materials.

Glass transition and molecular mobility Definition Glass transition (or glass-liquid transition GLT) is the name given to phenomena observed when a glass is changed into a supercooled melt during heating, or to the reverse transformations during cooling. Both are non-crystalline states; but while the glass is a rigid solid, the supercooled melt, which is observed between the GLT and the melting point, can be a viscoelastic “rubber” in the case of a polymeric material, or a mainly viscous liq2444

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uid, for low molecular weight materials. The GLT is a kinetic and relaxation process associated with the so-called relaxation of the material. At temperatures above the GLT the material, if submitted to a perturbation, can recover after a characteristic relaxation time (): the supercooled melt is in a metastable state. The liquid-like structure of the melt is “frozen” in the glass, which is an out-of-equilibrium state (Figure 1). The GLT region is the temperature range where the relaxation time of the material is similar to the experimental time scale.

Mobility above GLT A characteristic feature of mechanical properties of the supercooled melt is the strong temperature dependence in the temperature range above the glass transition temperature (Tg): the apparent activation energy (Ea) commonly attains 200 to 400 kJ.mole-1; it decreases when temperature increases. The most popular expressions to describe this behavior are the Vogel-Tammann-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 expressions can be inter-converted. Expressions similar to Eq 1 and 2 can be written with the obtained for example with mechanical spectroscopy (Figure 2). C1g and C2g can fluctuate (Ferry 1980) around the “universal” values given by Williams and others (1955) (17.4 and 51.6 respectively) as a function of the considered material. The variations of C2g and of B correspond to the classification proposed by Angell (Angell and others 1991, 1994) of strong/fragile materials according to the variation of their dynamic properties through the glass © 2002 Institute of Food Technologists

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transition. The fragility parameter m was introduced to differentiate fragile systems (m between 100 and 200) which are highly sensitive to temperature changes above Tg, from strong ones (m between 16 and 100) which are less disturbed when passing through the glass transition. By definition, m is the slope of the scaled Arrhenius plot of the viscosity when the temperature approaches Tg from above, Ea being the apparent activation energy (kJ.mole-1): m = Ea/ (RTg ln10)

(3)

This parameter m can be calculated with the VTF and WLF coefficients: 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 m for food materials can be found in Simatos and others (1995b). Low molecular weight sugars can be classified as rather fragile materials. They seem to be located in a narrow domain of the fragility diagram (Figure 3). For proteins, the scarce experimental data seem to indicate a strong behavior: m » 40.5 for poly-L-asparagine (15 to 25% water) (Angell and others 1994) and similar values for elastin and gluten (Simatos and others 1995b). Pullulan-starch blends were found to show strong behavior (m » 42 to 51) increasing with water content (Biliaderis and others 1999). Fragility was reported to increase in the order: pullulanTg’. From Blond and others 1997


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Encapsulation

ting parameters to other data would be necessary.

In the food industry there is a growing interest for encapsulation technologies, which are designed to protect the encapsulated material (or “active”) and to allow only controlled release. This is particularly the case for flavoring components, which are prone to loss by evaporation, oxidation or ingredient interactions. Although a variety of methods have been proposed to encapsulate flavors, spray-drying and extrusion are still the most common techniques. A great number of studies have been devoted to the retention of aroma during drying, freeze-drying, extrusion, or storage of dried products. It is well known that in favorable conditions, aroma can be retained to a much larger extent than expected from their volatile character (see Flink 1975 for a review). At low water content the diffusion coefficient for organic compounds drops rapidly, even more rapidly than that for water. This “selective diffusion” process (Thijssen 1971), with some modifications, is generally accepted as the main process explaining the retention of aroma being entrapped in glassy materials (carbohydrates or proteins) (King 1988). Release of volatiles is promoted by temperature and water content conditions that bring the dry glassy material to the rubbery state ( To and Flink 1978; Whorton and Reineccius 1995). The potential interest of glass transition in view of controlled release from glasses was the basis of a patent (Levine and others 1991). The study of release kinetics is complicated by the structure of dried/extruded products and changes induced by plasticization. The release of propanol from freeze-dried sucrose and sucrose-raffinose matrices was shown to increase with (T-Tg) following WLF kinetics with the “universal” coefficients, in a range of (T-Tg) approximately 10 to 30 °C (Levi and Karel 1995b). Actually the release kinetics appeared to be controlled by crystallization (in the sucrose matrix) or by collapse (in the sucroseraffinose matrix). The release of volatiles was also associated with the collapse of spray-dried maltodextrins ( Whorton and Reineccius 1995). Volatiles loss during rehumidification of freeze-dried foods models was interpreted in terms of a diffusion-based model where the web thickness within a sample increases with time because of structure collapse (Omatete and King 1978). The introduction of an effective diffusion coefficient, which should be time-dependent, was suggested to obtain a realistic description of processes above Tg (Karel and others 1993). While the completion of collapse results in a decline of the release rate, crystallization is accompanied by a total loss of the volatiles (Levi and Karel 1995b; Senoussi and others 1995). For flavors encapsulated in carbohydrate matrices by an extrusion process, it was concluded that release (at low water contents) due to diffusion through the matrix was relatively slow as compared to release due to matrix cracking (Gunning and others 1999). With undercooled maltose-water mixtures where a more regular macroscopic structure could be expected, and with steady water content, the release of volatiles could be modeled as a Fickian diffusive process (Gunning and others 2000). Close to the glass transition of the material, the temperature dependence of the apparent diffusion coefficient showed a decoupling from viscosity similar to the one described for probes in sucrose-water mixtures. The dependence on water content of the effective diffusion coefficients of water and ethanol in maltodextrins films could be fitted to the model developed originally by Fujita (1961) from the free volume theory to express the concentration dependence of the diffusion coefficient of a solvent in a synthetic polymer (Furuta and others 1984). However, the comparison of the curve fit-

Edible films and barriers Edible films and coatings are expected to be concerned by glass transition, as both their mechanical and barrier properties are strongly affected by temperature, ambient humidity and plasticizer content. There is an abundant literature on glass transition temperature of biopolymer films and on the influence of water and other plasticizers. Nevertheless, quantitative data correlating their functional properties to glass transition phenomena are scarce. In an extensive study on polyol-plasticized pullulan-starch blends (Biliaderis and others 1999), the permeability to O2 and CO2 could be fitted to Arrhenius behavior both below and above Tg. The Arrhenius plots showed a distinct break in the temperature range of Tg (DSC) and T (drop in E’). The apparent activation energy for permeability (Ea P) was larger above glass transition than below, amounting to 74 to 113 kJ.mole -1; it decreased for increasing water contents. These values, however, are small when compared to the apparent activation energy for the relaxation, which was between 226 and 296 kJ.mole-1. Even smaller values of EaP above Tg were reported for O 2, CO2, and ethylene permeability in gluten films (20 to 56 kJ.mole-1) (Mujica and others 1997). Similar features were described for the permeability or diffusion coefficients of gases in natural and synthetic polymers. Consistent with the predictions from the free-volume theory of diffusion, experimental observations showed that as

Figure 6—NMR relaxation time T2 (a) and second moment M2 (b) versus (T-Tg) for freeze-dried maltodextrins (DE 2, 21, 40) equilibrated at a water activity of 0.4. T2 represents the rotational mobility of the more mobile protons (mainly water). M2 is considered to be inversely related to mobility of the less mobile protons (matrix protons + possibly some water protons). Data from Grattard and others 2002 Vol. 67, Nr. 7, 2002—JOURNAL OF FOOD SCIENCE

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Glass transition in food . . .


Glass transition in food . . . the weight fraction of small molecules increases (for instance, increase in water content in the above examples) the apparent activation energy of diffusion above Tg decreases and the inflexion at Tg becomes less apparent (Duda 1999; Ramesh and Duda 2001). Although the free-volume theory for diffusion allows, at least qualitatively, to explain the temperature and water content dependence of permeability, it should be remembered that permeability (P) is the product of diffusivity (D) and solubility (S) of the diffusant in the matrix: P=DS The activation energy for P is: EaP = EaD + HS where HS is the dissolution enthalpy. Depending on the respective hydrophilic/hydrophobic character of the film and the permeant, EaP may be variously affected.

Oxidation kinetics The relatively high mobility of water and oxygen in glassy matrices is responsible for the limited shelf-life of encapsulated materials or dried food products. The permeation rate of oxygen into a freeze-dried (sucrose-maltodextrin-gelatin) matrix was found to show Arrhenius behavior (EaP = 74 kJ.mole-1) below and above Tg and to control the oxidation kinetics of the encapsulated oil (Andersen and others 2000). Evidently, the permeation rate is influenced by the matrix structure (crystallinity, porosity, tortuosity) and the distribution of oil in the case of an emulsion. Glass transition appears to only have a weak direct impact on the diffusivity of small molecules such as gases and water. A number of studies however show that the structural modifications induced by glass transition significantly affect the actual permeation. Crystallization of the matrix was shown to induce the release of the encapsulated oil, thus enhancing its oxidation (Shimada and others 1991). Oxidation of saffron carotenoids and of the beetroot pigment betanin encapsulated in polymer matrices was observed in conditions of water content and temperature where the matrices were glassy, confirming the permeation of O2 in the glass. The lower degradation rates were actually observed when the matrix was collapsed (Selim and others 2000; Serris and Biliaderis 2001). Depending on the experimental conditions, partial or total release of the oil may result from collapse; while the released oil is oxidized, the fraction remaining entrapped appears to be protected (Labrousse and others 1992; Grattard and others 2002). Glass transition knowledge may be beneficial to encapsulation, edible films, and coating technologies by helping to define processing parameters and especially the matrix formulation. More investigations are necessary however, to better understand the mobility of small solutes in glassy and rubbery matrices and particularly the influence of water. Moreover, it must be kept in mind that in practical situations release or permeation kinetics may be affected by events such as collapse or crystallization.

geot and Blond 1991), dried white bread and extruded flat bread (Fig. 9). As a first explanation, the crispness loss was directly related to glass transition (Ablett and others 1986, Slade and Levine 1993, Roos 1995). Crispness is associated with a low-density cellular structure that is brittle and generates a high- pitched noise when fractured. The loss of brittleness was attributed to the drop in rigidity modulus that is characteristic of the glassrubber transition in polymers. Due to the plastifying effect of water, the temperature of glass transition was assumed to be decreased to the ambient temperature where crispness was assessed. A glass transition could indeed be recognized in a real food product such as bread using thermomechanical analysis (TMA) and later DMTA (Le Meste and others 1992, 1996). It was shown, however, that the water content at which the loss of sensory crispness, as well as the brittle to ductile transition observed in mechanical tests, occurred in white or extruded breads, corresponded to a glass transition temperature (T)much higher than the testing temperature (Le Meste and others 1996, Roudaut and others 1998) (Figure 9). Similar results were obtained with corn cakes (Li and others 1998) and with starch extrudates (Attenburrow and others, 1992, Nicholls and others 1995). This most important texture change thus takes place while the material is in the glassy state. It was suggested that it could be best modeled without any relation with glass transition, by empirical expressions such as the Fermi equation (Peleg 1996) (Fig 9). The underlying microstructural events are not elucidated yet. Secondary relaxations are evidenced by DMTA and dielectric spectroscopy below glass transition in dry and extruded breads (Le Meste and others 1996, Roudaut and others 1998, 1999) (Fig 10). It is difficult to definitely relate these features with the texture changes, because the latter result from a variation in water content at ambient temperature, whereas the former are detected in a temperature / frequency scanning at constant water content. The values of tan (DMTA),however were shown to increase from the water content (9%) at which the acoustic emission drop was initiated, which allows to conclude to a significant increase in mobility in the glassy state, coinciding with the crispness loss

Structure and Texture Texture of low moisture products Crispness, a popular texture attribute of various low moisture foods, is lost when their water content is raised above a threshold, which was found between 6 to 9% for crackers, popcorn, and potato chips (Katz and Labuza 1981), breakfast cereals (Sauva2450

Figure 7—Translational diffusion coefficient (FRAP) of fluorescein in sucrose solutions. The curve is the prediction according to the Stokes-Einstein relation (Eq. 7), with viscosity predicted from the WLF model (parameters in Figure 2 caption). Water contents indicated in the inset. Champion and others 1997b


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(Figure 11). The loss tangent increase could result from sub-Tg relaxations becoming possible at 25 °C when the water content is raised above 9%, or could be associated with motions just preceding the onset of glass transition ( relaxation) (Roudaut and others 1998). Fracture tests evidence another important event in glassy cereal products: an hardening effect is observed, beginning at about 5% and maximum between 9 and 11% water content with dried and extruded breads, which is also detected in sensory analysis and which is followed by a softening at higher hydration (Fontanet and others 1997, Roudaut and others 1998). Similar observations have been reported for corn cakes (Li and others 1998) and “fat-free apple chips” (Konopacka and others 2002). It may be noted that similar effects have also been described for starch extrudates (Attenburrow and others 1992, Nicholls and others 1995) and films (Chang and others 2000). The first change in texture could, therefore, be an increase in fracture stress: being less easily fractured, the product is perceived as less crispy (Roudaut and others 1998). This hardening effect was ascribed to antiplasticization (Roudaut and others 1998; Chang and others 2000). The addition of a diluent to a polymer, although decreasing Tg, can hinder the polymer chain motions, resulting in an increased rigidity (Vrentas and others 1988). This antiplastifying effect of water in starch systems has been attributed to a short-range reorganization (Fontanet and others 1997) resulting in a density increase by filling the defects in the glass structure (Benczedi 1999, Chang and others 2000). It is worth noting that this antiplastifying effect was observed under high deformation conditions. Water at low concentration may therefore act as plasticizer under low deformation conditions and as antiplasticizer under high deformation conditions (Chang and others 2000).

Collapse and caking The changes in mechanical properties related to glass-liquid (or rubber) transition are the governing factors in many food processing operations. Structure collapse of the product during airor freeze-drying, or during the storage of dried products, is responsible for the reduction in volume and porosity, which results in the loss of desirable appearance and volatile substances and in poor rehydration. It is generally considered that structure collapse should be avoided, although it may be credited of some beneficial aspects such as a reduced sensitivity to oxidation. Powder stickiness and caking are phenomena related to collapse.

Figure 8—Translational diffusion coefficient of fluorescein in sucrose solutions (Cf. Fig. 7) to show the decoupling of Dtrans from viscosity for Tg/T>0.86. (T/Dtrans␩ =constant when the DSE law is obeyed: solid line). Champion and others 1997b

This imposes important constraints on drying and storage in the dry state of products with a high relative content of low molecular weight solutes such as sugars, minerals, and protein hydrolysis products. Here again, a controlled caking process (or agglomeration) is used to improve the appearance and handling of powders and their dispersion in water. It is well known that the factors controlling structure collapse, stickiness, and caking are temperature and water content and that these processes are time dependent. The collapse temperature 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 increases sharply) has been shown to decrease when water content increases. During freeze-drying, structure collapse occurs when, as a result of an increase in water pressure for instance, the heat input exceeds the drying needs, inducing a rise in temperature at the sublimation interface. Structure collapse in the course of freeze-drying was shown to happen when the viscosity of the cryo-concentrated phase had fallen to the range of 107 to 104 Pa.s (Bellows and King 1973). Below this limit, the interstitial network could not withstand the collapsing effects of the capillary forces. The same critical viscosity range was shown to determine collapse in freeze-dried materials (Tsourouflis and others 1976). A similar mechanism was proposed to explain caking phenomena: the formation of interparticle bridges between adjacent particles and then aggregation take place when the surface viscosity reaches the critical range 108 to 106 or 107 to 105 Pa.s for sucrosefructose-maltodextrin mixtures or coffee extract respectively (for particle diameters 3 to 4 µm and 30 to 40 µm respectively and contact time 1 to 10 s.) (Downton and others 1982; Wallack and King 1988). The connection of collapse and caking with glass transition is demonstrated by the parallel evolution of agglomeration temperature and Tg as a function of water content. The sticky point (Ts) and glass transition temperature of the sucrose-fructose mixture model were observed to be similarly affected by increasing moisture content, with Ts close to the Tgend values, that is about 20°C above Tgonset (Roos and Karel 1991a). Similar results were reported for the collapse temperature (Tc) of freeze-dried maltodextrins, but with Tc values about 30 to 70°C above Tgonset (Roos and Karel 1991b). It must be stressed that the critical viscosity levels, and then the (T-Tg) values are dependent on the characteristic times of the methods used to monitor the changes. The link between collapse and caking and glass transition is further supported by many observations showing that collapse and caking temperatures are raised as Tg is, when the average molecular weight of the product increases, for instance with maltodextrins (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 of sugar solutions following the addition of maltodextrins was reported to be correlated with an increase in drying yield (thanks to the reduction of product sticking on the spray-dryer walls) (Busin and others 1996). Both collapse and caking have been demonstrated to obey WLF kinetics. The collapse of a freeze-dried sucrose-raffinose model, measured by the decrease in specific volume, was reported to change exponentially with time. The relaxation time values could be fitted to the WLF equation with the “universal constants”, for a (T-Tg) range between 15 and 30 °C (Levi and Karel 1995). Caking of a spray-dried fish protein hydrolyzate was also found to follow a first order kinetic model. The temperature dependence of the relaxation time was characterized by a WLF relationship, with adjustable C1 and C2 coefficients, for (T-Tg) beVol. 67, Nr. 7, 2002—JOURNAL OF FOOD SCIENCE

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Glass transition in food . . . tween about 20 and 80 °C (Aguilera and del Valle 1995). Because of 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 (>200400 kJ.mole -1), which points to the large temperature dependence of the phenomenon. Prevention of collapse or caking is 1st based on low temperature and/or low water content. This means, for instance, cooling the walls of the spray-dryers and the design of towers with dimensions large enough so that the droplets would not reach the walls before being in the non-sticky domain. The beneficial action of anticaking agents, which has been recognized for long, is explained by several mechanisms, the rise in glass transition temperature by increasing the average molecular weight of the amorphous phase being only one of them (Aguilera and others 1995). It may be concluded from the presently available evidence that structural collapse, stickiness and caking/agglomeration, being primarily dependent on flow rate, are successfully explained using the glass transition concept. From the current literature, it appears that these phenomena are never observed in the temperature/water content domain below glass transition. Even in cases where some chemical evolution in the glassy state was reported, collapse could not be observed at TTg : 50 to 100 kJ.mole-1 (Karmas and others 1992), 130 to 140 kJ.mole -1 (Craig and others 2001). These values are quite smaller than the activation energies commonly observed for dynamical properties in the glass transition range. In some systems, reactions in the glassy state may have been rendered possible by a phase separation, resulting in microregions with a higher water content (and then a lower Tg) than the whole matrix. It should be also noted that in most studies, reactants were mixed together during the preparation step, limiting the need for translational diffusion. A decoupling of reactants mobility from the matrix viscosity may also be an explanation for the occurrence of finite reaction rates at or below Tg. Some observations suggest that the temperature itself rather than the difference (T-Tg) controls the non-enzymatic browning kinetics (Roos and Himberg 1994; Schebor and others 1999a). As was recalled by Karel and Saguy (1991) physical chemistry books state that, for reactions that are influenced by diffusion of reactants, the reaction rate constant kapp may be described by the relation: kapp = kact / (1 + kact / D) where kact is the reaction constant observed in well-stirred solutions, which recognizes the fact that only a fraction of potential reactants coming into contact are activated and can react. D is the diffusivity of the reactants (equal to the sum of the individual diffusion coefficients), is a coefficient depending on a collision distance. Both parameters k act and D are temperature-dependent. The reaction can be fully controlled by diffusion if the reaction constant kact is much larger than D (then kapp » D), the temperature dependence is the same as for D (possibly WLF kinetics). In the case of reactions that have a high activation energy (as could be for non-enzymatic browning), kact remains low as long as the temperature is not high enough, and consequently kapp is controlled by activation of reactants rather than by diffusion (k app » k act). The temperature dependence is of Arrhenius type. The temperature dependence of the alkaline phosphatase activity in frozen concentrated sucrose solutions represented a favorable situation for modeling according to WLF kinetics (with variable Tgs : Fig. 5) probably because the enzyme exhibits a relatively high kact even in concentrated sucrose solutions and at low temperature (Champion and others 2000).

Microbial stability Slade 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 intermediate-moisture food products. From an extensive review of the then available knowledge, Chirife and Buera (1996) demonstrated that these expectations were not supported by experimental evidence. Among the arguments that were presented was the observation that many foods, including fruits, vegetables, and milk, are in the rubbery state for humidity conditions (a w) where these foods are known to be resistant to microbial growth. (T-Tg) values, the difference between a typical incubation temperature of 30 °C and the product Tg, were estimated to be between 60 °C and 100 °C for a w values between 0.60 and 0.85. To the contrary, molds were reported to grow in maize and wheat flour in moisture conditions where the products were likely to be glassy. It was concluded that, although mobility factors, in addition to aw, may be useful for a better prediction of microbial behavior in foods, glass transition concepts do not provide any better alternatives than aw as a predictor for this. More recently, germination of Aspergillus niger spores was observed in starch samples for water content/temperature conditions just above Tg, but not in samples below Tg (Kou and others 1999). More research is needed to specify the importance of molecular mobility and glass transition phenomena in controlling the microbial stability of foods, dealing with different types of microorganisms and substrates, not only spores germination but also growth and metabolic activity.

Conclusion

O

NE IMPORTANT ISSUE HINDERING THE APPLICATION OF GLASS

transition concepts to food technology is the lack of an unique Tg. The reasons for that are related to the influence of the experimental time and of the stress nature. Moreover, “the” glass transition temperature is not an absolute frontier for molecular mobility. Even mechanical properties allow the observation of some mobility below glass transition, which is in connection with sub-Tg relaxations or physical aging, and which may be of prime importance as regards texture properties of some food products. Transport of small molecules also makes high mobility evident in the vicinity, or below, the glass transition. One should further be aware that the glass transition temperature is not sufficient in itself to characterize the behavior of a material in the GLT range. Other parameters, such as width of the transition, fragility, non-linearity, non-exponentiality, have to be collected for food materials, related to chemical or structural attributes and studied as regards their relevance to food technology issues. These problems, however are not specific to food materials. They have to be also overcome for non-food materials, where GLT and related phenomena have proved to be such useful concepts. More particular to the food area may be difficulties resulting from the extreme chemical and structural complexity of many food products. The ubiquitous presence of water may also be an important factor: the large variations in water content and the specific interactions with other food constituents make special investigations and theoretical models necessary. Moreover, at high water content, GLT is no longer relevant. Based on presently available information, glass transition concepts do not seem useful to predict with confidence the microbial stability of foods; further research is needed to assess the influence of water and solutes mobility for controlling growth and metabolic activity of microorganisms in intermediate moisture foods. Vol. 67, Nr. 7, 2002—JOURNAL OF FOOD SCIENCE

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Glass transition in food . . . In situations where visco-elastic properties play a dominant role, GLT based technology has proved truly efficient. Both the Tg (or T) values and WLF kinetics show good capabilities for the understanding and prediction of the effects of temperature, water content and product formulation and can be successfully used to control texture, processes such as collapse and agglomeration, and various technological operations including drying and extrusion. There are circumstances however, where molecular mobility, which may be under the dependence of GLT in low moisture and frozen products, is only one of the factors that control the evolution kinetics. Crystallization and chemical/biochemical reactions offer good examples where glass transition-linked mobility combined with other concepts can provide a satisfactory understanding of the observed kinetics. Important progress may be expected along this line, however much more experimental work is needed to gain a clearer view on molecular mobility, particularly in the vicinity of Tg and below, and its relationships with secondary relaxations or aging. It seems highly desirable to establish “mobility maps” for food materials showing the characteristic relaxation time for the different types of molecular motions, as a function of temperature and water content. Improved understanding of mobility in the vicinity of Tg should result from the rapidly growing knowledge on heterogeneity in glasses and supercooled liquids (Sillescu 1999). Recent theoretical developments and experimental evidence are leading to a clearer view on heterogeneity in (chemically homogeneous) glass-forming liquids and polymers. Heterogeneity at the nm scale, connected with the distribution of relaxation times, is being studied by various spectroscopic methods and by experiments with colloidal glasses (Ediger 2000). These studies should particularly help in understanding and predicting molecular mobility immediately above glass transition temperature. More attention should also be given to the behavior in the GLT range of complex food products: respective mobility of individual components according to their molecular size (and other physico-chemical characteristics), different glass transition temperatures for various components resulting from the distribution of water, contribution of possible phase transitions (lipids) to the observed macroscopic behavior, and so on. Glass transition in proteins should deserve further research in the food technology context, particularly addressing the questions of glass transition in frozen products, protective actions in the dry state and plasticizing effects in films of sugars and polyols.

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


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