Differential Scanning Calorimetry Glass Transition Temperatures of White Bread and Mold Growth in the Putative Glassy State M. P. Buera,1,2 K. Jouppila,3 Y. H. Roos,3 and J. Chirife1,4 ABSTRACT
Cereal Chem. 75(1):64–69
Differential scanning calorimetry (DSC) was used to determine the onset and end temperatures of the glass transition (Tg) for white bread equilibrated between 53 and 84% rh. Calorimetric Tg end values were ≈20°C higher than onset values, indicating that it is probably more correct to refer to a “glass transition range” rather than a glass transition temperature. Slices of white bread inoculated with a mixture of xerophilic molds were equilibrated to 75% rh (equilibrium moisture content of 14.5 g of water/100 g of dry material) and stored at 26°C. In a parallel
experiment, some of the equilibrated bread samples were stored without mold inoculation and subjected to spontaneous contamination from the immediate surroundings. As suggested by measured Tg, bread stored at 75% rh and 26°C appeared to be glassy. After storage, samples of bread (inoculated or not) were spoiled by xerophilic molds, suggesting that Tg, as measured by DSC, cannot be considered as an absolute threshold for mold growth inhibition.
Water is probably the single most important factor governing microbial spoilage in foods, and the concept of water activity (aw) has been very valuable in physiological studies of microorganisms, principally because measured values of aw generally correlate well with the potential for growth and metabolic activity. In recent years, the concept of aw and its utilization to predict microbial stability have been challenged. Slade and Levine (1987, 1991) and Franks (1991) have stated that in many product situations, equilibrium thermodynamic descriptors such as aw are inappropriate because the measured physical properties are time-dependent, thus aw should not be used to describe the attributes of these systems. In many foods and biological materials, the solids (either biopolymers or low molecular weight carbohydrates) are in an amorphous, metastable state that is very sensitive to changes in moisture content and temperature (Slade and Levine 1987, Roos and Karel 1991, Levine and Slade 1992). This amorphous matrix may exist as either a very viscous glass or as a more liquid-like rubbery structure. The characteristic temperature range (Tg) over which the glass-rubber transition occurs, has been proposed as a physicochemical parameter that can define stability and safety of foods (Slade and Levine 1987, 1991a,b; Levine and Slade 1992), allowing the construction of a stability map based on Tg. In a glass state (at T < Tg), the rates of all diffusion-limited processes are much lower than in the rubbery state (at T > Tg). The parameter (T – Tg), the difference between the storage temperature and the glass transition temperature, which was related to the magnitude of changes in physical properties in polymers, was also proposed to describe the stability (physical, chemical, and biological) of foods at >Tg. Slade and Levine (1987) proposed to replace the concept of aw by a water dynamics approach to better predict microbial stability of intermediate moisture foods. High viscosity states, such as glasses, would greatly interfere with the growth of microorganisms, because mobility may be needed in reduced-moisture solid foods for transport of nutrients and metabolites from and to the food matrix. However, the deliberate use of high viscosity states to inhibit microbial growth has not yet been demonstrated (Gould and Christian 1988, Chirife and Buera 1996). The effect of the glassy state on microbial growth inhibition was examined by Chirife and Buera (1996) using literature data for the effect of moisture content on Tg of food systems, and also,
when available, microbial growth under those conditions. They noted that at aw = 0.7, a value at which most microorganisms are not able to develop due to osmotic stress, various foods are in the rubbery state at typical storage temperatures of 20–40°C, which gives a (T – Tg) value of >40°C. If mobility would be the main mechanism controlling growth of microorganisms, these foods should not support microbial growth. However, the opposite was known to be the case at this low aw. Chirife and Buera (1996) concluded that either: 1) this effect plays little role in microbial growth inhibition (aw seems to be the controlling parameter) if molecular mobility is determined by (T – Tg); or 2) molecular mobility is still reduced at high values of (T – Tg) so growth does not occur, in which case (T – Tg) cannot be considered the determinant of microbial growth. It is not easy to separate the individual effects of aw and physical state (glassy or rubbery) on microbial growth, because in practically all foods, the matrix is in the glassy state at moisture contents at which the aw is well below the limiting value for growth of most microorganisms. For example, the growth of pathogenic bacteria is inhibited when aw < 0.86 (Chirife 1993) and at moisture contents corresponding to this aw value, many food systems are in the rubbery state at room temperature (Chirife and Buera 1996). However, due to the high Tg values of starchy systems (Zeleznak and Hoseney 1987), white bread may be in the glassy state in a moisture range over which some aw-tolerant microorganisms (e.g., xerophilic molds) may still grow. Thus, bread may provide a model to investigate microbial stability in the glassy state. For this purpose, the Tg of white bread was determined, and the behavior of xerophilic molds was studied.
1 Departamento
de Industrias, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, (1428) Buenos Aires, Argentina. 2 Member of CONICET, Argentina. 3 Department of Food Technology, PO Box 27 (Latokartanonkaari 7), FIN-00014 University of Helsinki, Helsinki, Finland. 4 Corresponding author. E-mail:
[email protected] Publication no. C-1998-0106-03R. © 1998 by the American Association of Cereal Chemists, Inc.
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MATERIALS AND METHODS White bread was formulated and baked in a commercial bakery; it did not contain added sugar, preservatives, or fat. The approximate formula for the dough was: water 36.2%, wheat flour 60.3%, salt 1%, yeast 1%, and dried milk (whole) 1.3%. Thin slices of white bread were dried in a forced-convection oven at 57°C until aw ≈ 0.5, inoculated (as described below) with a mold suspension, and rehumidified to aw = 0.75 by placing in a closed container over a saturated solution of sodium chloride at 26°C (Resnik and Chirife 1988). After about two weeks of equilibration, the aw of bread slices was measured with a Novasina Thermoconstanter electric hygrometer (following the procedure described by Kitic et al 1986) and found to be aw = 0.75. Inoculation of White Bread Samples A spore suspension of molds (in a 0.1% Tween 80 aqueous solution) isolated from a spontaneously spoiled fruit cake (aw =
0.75) and intermediate moisture bread was inoculated by spraying on the surface of dried (aw = 0.50) white bread slices. It was verified that the low proportion of water incorporated with the inocula did not raise the moisture content of the bread to aw > 0.75. Some of the molds identified included, among others, Aspergillus flavus, A. niger, A. ochraceus, unknown Penicillum spp, Eurotium herbariorum, and E. chevalieri. The inoculated bread slices were rehumidified over a saturated solution of sodium chloride as described above and stored at 26°C. Periodically, the samples were inspected to detect the occurrence of mold spoilage. Other equilibrated bread samples were not inoculated, but rather placed in a closed container over saturated salt solution (aw = 0.75) and exposed to spontaneous contamination from the immediate surroundings. Adsorption Isotherm Samples of dried bread were equilibrated at 26°C in evacuated desiccators over saturated salt solutions that provided constant relative humidities: NaBr (58% rh), NaCl (75.3% rh), (NH4)2SO4 (80.1% rh) , and KCl (84.3% rh). Duplicate samples were used and the average is reported. Moisture content of equilibrated bread was determined by weighing the samples before and after drying in a constant temperature oven (with a desiccant) at 85°C for 24 hr (these conditions were observed to be sufficient to assess constant weight). X-ray Diffraction The amorphous state of powdered, dried white bread was examined using X-ray diffraction. X-ray diffraction patterns were obtained using a Philips powder X-ray diffractometer (PW 1830 generator, PW 1710 diffractometer control, PW 1820 vertical goniometer equipped with graphite reflected-beam monochromator, and PC-APD software for Automatic Powder Diffraction Version 3.0; Philips Analytical, The Netherlands).The X-ray diffractometer was operated in reflection mode at 40 kV and 50 mA. Samples on aluminum trays (sample layer 15 mm × 20 mm × 1.5 mm) were exposed to CuKα radiation (λ = 0.15418 nm) at diffraction angles (2θ) from 5 to 30° (step size 0.02, 2.5 sec/step). Divergence slit for primary beam was 0.25°. Divergence slit and receiving slit for diffracted beam were 0.25° and 0.1 mm, respectively. Glass Transition Temperatures Glass transition temperatures (onset and endset values) for white bread were determined using a differential scanning calorimetry (DSC) (model TA 4000 analysis system with a DSC-30 low temperature cell, a TC10A TA processor, and GraphWare TA72AT.2 thermal analysis software, Mettler-Toledo AG, Switzerland). The DSC equipment was calibrated for temperature using n-hexane (–95°C mp), distilled water (0.0°C mp), and indium (156.6°C mp), and for heat flow using indium (latent heat of melting, 28.5 J/g). The samples were prepared for DSC measurements by grinding dehydrated white bread, using mortar and pestle, and by weighing ≈15 mg of bread powder in standard aluminum DSC pans (inner volume 40 µL) at least in triplicate. The samples in open pans were humidified in vacuum desiccators over saturated salt solutions at room temperature for at least four days, giving conditions of 53, 66.2, 75.3 and 84.3% rh. This time was sufficient to reach steady-state because of the small amount of samples, large surface area of powdered bread, and use of vacuum. Similar experiences with crisp bread, cereal extrudates, etc., indicated that steady-state water contents are generally obtained in four days under these experimental conditions. After storage, the pans were hermetically sealed and scanned twice, using DSC from ≈40°C below the onset of the Tg range reported for amorphous corn starch (Jouppila and Roos 1997) to well above the observed Tg for the sample. Each sample was immediately rescanned over the same temperature range used in the first scan. An average value of three to six replicate samples was taken as the Tg.
Scanning Electron Microscopy The dried (equilibrated to aw = 0.75) bread samples were mounted on aluminum stubs, gold-coated, examined, and photographed, using a JEOL JSM 5300 scanning electron microscope. Stereomicroscopy Some spoiled bread samples were also examined and photographed using a stereomicroscope (Zeiss Stemi SR, Germany). RESULTS AND DISCUSSION The structural matrix of bread is composed of polymers that are either completely amorphous (gluten) or partially crystalline (starch) (LeMeste et al 1992). Starch (the main component of bread) exists as an amorphous or partially crystalline polymer in various food products. The amorphous state of dried white bread was examined using X-ray diffraction (Fig. 1). Only slight crystallinity was observed because peaks characteristic of native cereal starches were absent (Chinachoti and Steinberg 1986, Carrillo et al 1988). This means that the amylopectin crystallinity in native wheat starch disappears as a consequence of the gelatinization process during baking of bread. The moisture contents obtained after equilibration of dried bread at different relative humidities are represented in the adsorption isotherm of white bread (Fig. 2). Figure 3A and B show the first and second DSC scans for white bread samples equilibrated at 75 and 80% rh, respectively. The second scan appears to have eliminated the hysteresis effects of thermal relaxation typical of glass transitions (Roos and Karel 1991). DSC thermograms (first scan) for white bread samples stored at various relative humidities are shown in Fig. 4. The Tg values (onset and end temperatures) for white bread samples decreased with increasing moisture content (Fig. 5), showing the typical behavior of amorphous food materials (Slade and Levine 1991a). The plasticizing effect of water on wheat polymers (e.g., its influence on Tg) has been reported for native and gelatinized wheat starch (Slade and Levine 1987, Zeleznak and Hoseney 1987), wheat gluten (Hoseney et al 1986, Slade and Levine 1987), and hard and soft wheat flour (Doescher et al 1987). It is noteworthy that calorimetric Tg end values for white bread samples are ≈20°C higher than onset values. Thus, in the case of such samples, it is probably more correct to refer to a “glass transition range” rather than a “glass transition temperature”. Peleg (1996) postulated the existence of three states: the glassy state, the transition region, and the fully plasticized state, and suggested that changes in certain properties of foods and biomaterials are determined by whether the material is in the glassy state, is undergoing a transition, or is fully plasticized. LeMeste et al (1992) noted that, because of the
Fig. 1. X-ray diffraction pattern for dried amorphous white bread. Vol. 75, No. 1, 1998
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complexity and heterogeneity of products such as bread, the Tg can be expected to be very broad, and that appears to be the case here. The Tg (onset and end values; average of six replicate samples ) for white bread stored at 75% rh (moisture content 14.6 g of water/100 g of dry material) were 32.8°C (σ = 3.7°C) and 54°C (σ = 3.3°C), respectively. LeMeste et al (1992) noted that DSC may not be sensitive enough to reveal the Tg of a complex food product such as bread, and they used thermomechanical analysis (TMA) to measure the Tg in white bread. They reported a Tg of 44°C for white bread at a moisture content of 14.6 g of water/100 g of dry material. This Tg value falls within the calorimetric transition region (about midway between our onset and end Tg values) determined for the present white bread samples. Published data (Hoseney et al 1986, Zeleznak and Hoseney 1987) on the Tg of wheat ingredients indicated that Tg (DSC) of gelatinized wheat starch and wheat gluten, at the same moisture content as white bread stored at 75% rh (14.6% db), were 66 and 35°C, respectively. It is to be noted that during fermentation of the dough, the amylases of yeast may hydrolyze starch, and hydrolysis would decrease the molecular weight of the starch fraction, which might depress the Tg of bread (M. J. Laine and Y. Roos, unpublished). Below the Tg the amorphous matrix of white bread is hard and rigid, and above the Tg, its viscosity decreases dramatically (LeMeste et al 1992). The rates of diffusion-limited processes would be much more rapid in the rubbery liquid state than in the glassy solid state (Slade and Levine 1991a). The amorphous white bread samples equilibrated to 75% rh were stored at 26°C, and because the measured onset of the Tg was 32.8°C, the bread may have been in the glassy region, below the onset temperature of the calorimetric transition region. Recrystallization of amorphous starch components in bread may occur during storage, but it is
Fig. 2. Adsorption isotherm of white bread at 26°C. 66
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known that this process is kinetically inhibited below Tg (Slade and Levine 1991a; M. J. aine and Y. Roos, unpublished). Thus, glassy bread samples would be expected to remain amorphous throughout storage. Fungal growth on samples of glassy bread slices was observed with the unaided eye after about two months of storage at 26°C. Figures 6 and 7 show micrographs of fungal colonization on inoculated glassy bread samples, as observed with a stereomicroscope and scanning electron microscopy (SEM). Abundant conidiophores from Aspergillus spp. (most likely A. chevalieri and A. herbariorum) are observed in the stereomicrograph (Fig. 6). The SEM micrographs show mycelial development and conidia of Aspergillus spp. (Fig. 7A,B). Mold colonization on glassy white bread slices subjected to spontaneous contamination is observed in Fig. 8 (stereomicrograph) and Fig. 9 (SEM micrographs). Conidiophores (likely Aspergillus spp., anamorphous state of Eurotium) and mycelium development are observed in the stereomicrograph (Fig. 8) and mycelial development, conidia (Fig. 9A), and conidiophores (Fig. 9B) are observed in the SEM micrographs. It is generally accepted in the literature that the strategies employed by different microorganisms (e.g., xerophilic fungi) to enable them to survive and grow at reduced aw rely on a common approach. This is the intracellular accumulation of a solute or solutes to balance the aw inside the cell against the external aw, thus preventing movement of water out of the cell. (Hocking 1988). At low aw, these solutes must be accumulated at very high concentrations, but at the same time, they must not inhibit the cell’s normal functions. Consequently they are known as compatible solutes. The limits of tolerance and ability of a microorganism to adapt to
Fig. 3. Differential scanning calorimetry scans for white bread samples stored at 75% rh (A) and 80% rh (B). Tg = glass transition temperature.
aw stress are due in part to the ability of cells to accumulate compatible solutes and also to the amount of metabolic activity diverted to the production of these solutes. These solutes include polyols such as glycerol, arabitol, and mannitol in fungi (Beuchat 1981). Glycerol has been shown to be particularly important in maintaining the aw balance of a number of xerophilic filamentous fungi including Penicillium spp., Eurotium spp., Xeromyces bisporus and Wallemia sebi (Hocking 1988). Solutes are known to have a low diffusion coefficient below Tg in glassy polymers, and the diffusion of small molecules in glassy polymers depends on the size of the diffusant and polymer-diffusant interactions (Frisch and Stern 1983). Because formation of a glassy solid results in greatly reduced molecular translational mobility, some workers (Levine and Slade 1992) considered that translational diffusion in these systems is virtually nonexistent. If these assumptions are correct, one may hypothesize that glasses would greatly interfere with the growth of microorganisms, because mobility may be needed in reduced-moisture solids for transport of nutrients and metabolites from and to the food matrix. Consistent with this line of thought, van den Berg (1991) noted that, according to Slade and Levine’s (1987) arguments, a food taken below Tg should be perfectly stable, in terms of any ordinary food chain from producer to consumer, and the use of the product Tg-water content relationship for the considered food product could eliminate the necessity of using aw to predict food stability. Although mobility may be needed for microbial growth in reduced-moisture foods, present results suggest that mold growth in white bread may be possible below Tg, and spoilage may not be prevented by keeping the product (bread) below Tg. One may attempt to explain this behavior by saying that, although on average, the bread matrix is glassy (as determined by DSC), molds may grow in some nonglassy microregions. It is known that the Tg for the gluten fraction is lower than that for the starch (Hoseney et al 1986, Zeleznak and Hoseney 1987) and one may speculate that the bread provided a two-phase system in which the starch was glassy and the gluten was rubbery. The same may eventually occur, if small amounts of sugars (e.g., glucose from yeast activity during dough fermentation) are present and provide a separate rubbery phase in which molds may grow. If rubbery microregions exist in a food like bread, even if they could be detected, the question then arises: which Tg serves as a reference condition to predict microbial growth—the Tg of the main structural matrix or the Tg of one or of several minor components (provided that they are not compatible with the main matrix and form separate phases)? In any case, Tg data for bread, as conventionally obtained by DSC,
does not allow one to predict the onset of mold spoilage. Also, we cannot assure that the surface of the bread slice on which mold growth occurred had the same Tg as that measured for a sample of powdered bread. However, we must assume it, otherwise the concept of Tg would be useless for prediction of microbial stability of foods because local conditions are not practical to determine. It is also possible that in certain polymers, the translational diffusion coefficients of small molecules may remain at a relatively high level in the glassy state (Comyns 1985). This is particularly true where the glassy matrix is relatively nondense. Hydrogels and other polymeric matrices may consist of structures that show glass-like behavior with respect to thermal and mechanical properties without substantially hindering diffusion of molecules smaller than the distance between chain segments of the matrix (C. Schebor et al, unpublished).
Fig. 5. Effect of moisture content on differential scanning calorimetry glass transition temperatures (Tg, onset and end values) for white bread samples.
Fig. 4. Differential scanning calorimetry thermograms (first scan) for white bread samples stored at various relative humidity conditions.
Fig. 6. Stereomicroscopic photograph showing mold growth on inoculated glassy white bread slices stored at 26°C and 75% rh. Vol. 75, No. 1, 1998
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In summary, the overall picture that emerges from the present mold growth experiments in the putative glassy white bread is that a Tg measured by DSC cannot be considered as an absolute threshold for mold growth inhibition and does not appear to offer any better alternative than aw as a predictor of stability (growth and inhibition). We are not saying here that mobility factors in addition to aw may not be useful for a better definition and prediction of microbial behavior in foods, particularly as concerns the rate of growth. However, literature data on this subject are scarce. Gervais et al
(1988) intended to compare simultaneously the effects of water content and aw values of a cellulose substrate on the growth rate of Penicillium roqueforti at relatively high aw values (>0.94). They observed that the effect of aw was highly significant, while the water content level (between 0.4 and 1 g/g of dry matter) did not significantly modify mold growth. Gervais et al (1993) showed that the rate of growth of the filamentous fungus Trichoderma viride was somewhat reduced by a decrease in water content (at constant aw) caused by adding silica gel to an agar medium. When water content in a control gel (aw = 0.98) was reduced from ≈60.5 g of water/g of insoluble dry matter to 5.2 or 2.1 g of water/g of insoluble dry matter, the rates of radial growth of T. viride were reduced by ≈20 and 38%, when glucose concentration in the medium was 3 g/L, and by 6 and 20%, when glucose concentration was increased to 20 g/L. Gervais et al (1993) suggested that these results indicated that diffusional phenomena occurred during the growth of fungi, and that the limitation in rate of radial growth was caused by a reduction in solute (glucose) diffusivity. They concluded that diffusional factors induced by low water content may limit growth of microorganisms. Additional research is needed to better define the conditions under which kinetic factors (diffusivity and viscosity), in addition to reduced environmental aw, may effectively inhibit or retard microbial growth in foods. Experimental systems have to be developed in which it is possible to investigate the separate effects of aw and mobility because rates of growth are also reduced due to the amount of metabolic activity diverted to the production of compatible solutes when the environmental aw is reduced.
Fig. 7. Scanning electron microscopy photographs (A and B) of fungal colonization on inoculated glassy white bread slices stored at 26°C and 75% rh.
Fig. 8. Stereomicroscopic photograph showing mold colonization (spontaneous contamination) on glassy white bread slices stored at 26°C and 75% rh. 68
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Fig. 9. Scanning electron microscopy photographs (A and B) of fungal colonization (spontaneous contamination) on glassy white bread slices stored at 26°C and 75% rh.
Water activity is a solvent property, and Tg is a property related to the structure of the solute matrix (van den Berg 1991). However, both are strongly dependent on water content, and so they are related. Thus, knowledge of both aw and Tg may be necessary for a better understanding of the water relationships of microorganisms in foods. ACKNOWLEDGMENTS This study was supported by Universidad de Buenos Aires, the Academy of Finland and Agencia Nacional de Promoción Científica y Tecnológica (Proyecto 344, Préstamo BID 802/OC-AR). We are grateful to Jano Kansikas for his expertise in X-ray diffraction determinations and to J. M. Aguilera and L. Cadoche (Universidad Católica de Chile) for their assistance with some of the SEM micrographs. LITERATURE CITED Beuchat, L. 1981. Microbial stability as affected by water activity. Cereal Foods World 26:345-349. Carrillo, P. J., Gilbert, S. G., and Daun, H. 1988. Starch/solute interaction in water sorption as affected by pretreatment. J. Food Sci. 53:1199-1203. Chinachoti, P., and Steinberg, M. P. 1986. Crystallinity of waxy maize starch as influenced by ambient temperature absorption and desorption, sucrose content and water activity. J. Food Sci. 51:997-1000. Chirife, J., and Buera, M. P. 1994. Water activity, glass transition and microbial stability in concentrated and semimoist food systems. J. Food Sci. 59:921-927. Chirife, J. 1993. Physicochemical aspects of food preservation by combined factors. Food Control 4:210-215. Chirife, J., and Buera, M. P. 1996. Water activity, water/glass dynamics and the control of microbiological growth in foods. Crit. Rev. Food Sci. Nutr. 36:465-513. Comyns, J. 1985. Polymer Permeability. Elsevier Applied Science: London. Doescher, L. C., Hoseney, R. C., and Millken, G. A. 1987. A mechanism for cookie dough setting. Cereal Chem. 64:158-163. Franks, F., 1991. Water activity: A credible measure of food safety and quality? Trends Food Sci. Technol. 2:68-72. Frisch, H. L., and Stern, S. A. 1983. Diffusion of small molecules in polymers. Crit. Rev. Solid State Materials Sci. 11:123-187. Gervais, P., Bensoussan, M., and Grajek, W. 1988. Water activity and water content: Comparative effects on the growth of Penicilllium roqueforti on solid substrate. Appl. Microbiol. Biotechnol. 27:389-392. Gervais, P., Marechal, P. A., and Voirin, T. 1993. Medium hydration effects on the growth of Trichoderma viride. J. Food Sci. 58:1346-1348.
Gould, G. W., and Christian, J. H. B. 1988. Characterization of the state of water in foods—Biological aspects. Pages 43-56 in: Food Preservation by Moisture Control. C. C. Seow, T. T. Teng, and C. H. Quah, ed. Elsevier Applied Science: London. Hocking, A. D. 1988. Strategies for microbial growth at reduced water activities. Microbiol. Sci. 5:280-284. Hoseney, R. C., Zeleznak, K., and Lai, C. S. 1986. Wheat gluten: A glassy polymer. Cereal Chem. 63:285-286. Jouppila, K., and Roos, Y. H. 1997. The physical state of amorphous corn starch and its impact on crystallization. Carbohydr. Polym. 32:95-104. Kitic, D., Favetto, G. J., Chirife, J., and Resnik, S. L. 1986. Measurement of water activity in the intermediate moisture range with the Novasina Thermoconstanter humidity meter. Lebensm. Wiss. Tech. 19:297-301. Le Meste, M., Huang, V. T., Panama, J., Anderson, G., and Lentz, R. 1992. Glass transition of bread. Cereal Foods World 37:264-267. Levine, H., and Slade, L. 1992. Glass transitions in foods. Pages 83-220 in: Physical Chemistry of Foods. H. G. Schwartzberg and R. W. Hartel, eds. Marcel Dekker: New York. Peleg, M. 1996. On modeling changes in foods and biosolids at and around their glass transition temperature range. Crit. Rev. Food Sci. Nutr. 36:49-67. Resnik, S. L., and Chirife, J. 1988. Proposed theoretical literature values at various temperatures for selected solutions to be used as reference sources in the range of microbial growth. J. Food Prot. 51:419-423. Roos, Y., and Karel, M. 1991. Plasticizing effect of water on thermal behavior and crystallization of amorphous food models. J. Food Sci. 56:38-43. Slade, L., and Levine, H. 1987. Structural stability of intermediate moisture foods–A new understanding? Pages 115-147 in: Food Structure–Its Creation and Evaluation. J. R. Mitchell and J. M. V. Blanshard, eds. Butterworths: London. Slade, L., and Levine, H. 1991a. Beyond water activity: Recent advances based on an alternative approach to the assessment of food quality and safety. Crit. Rev. Food Sci. Nutr. 30:115-360. Slade, L., and Levine, H. 1991b. A food polymer science approach to structure-property relationships in aqueous food systems. Pages 29-101 in: Water Relationships in Foods. H. Levine and L. Slade, eds. Plenum Press: New York. Soesanto, T., and Williams, M. C. 1981. Volumetric interpretation of viscosity for concentrated and dilute sugar solutions. J. Phys. Chem. 85:3338-3341. van den Berg, C. 1991. Food-water relations: Progress and integration, comments and thoughts. In: Water Relationships in Foods. H. Levine and L. Slade, eds. Plenum Press: New York. Zeleznak, K., and Hoseney, R. C. 1987. The glass transition in starch. Cereal Chem. 64:121-124.
[Received April 21, 1997. Accepted September 22, 1997.]
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