Water Mobility in Glassy and Rubbery Solids as Determined by Oxygen-17 Nuclear Magnetic Resonance: Impact on Chemical Stability

Although chemical reactivity in solid food systems has been studied as a function of water activity and glass transition, the possible effects of water mobility on chemical reactions have not been investigated. The effect of the glass transition on water mobility at constant temperature and water content was determined. The relationship between the experimentally determined water mobility in polyvinylpyrrolidone (PVP) systems and chemical reactivity data from the same PVP systems was evaluated. Water mobility, as determined via ¹⁷O-NMR, was not affected by the glass transition; PVP systems at constant water activities and water contents, but different physical states (glassy and rubbery), had the same water mobility. An evaluation of four chemical reactions showed no relation between water mobility and kinetic data. The effect of water on chemical reactions is multidimensional and cannot be reduced to a single physicochemical parameter.     

Thermal Stability of Invertase in Reduced-Moisture Amorphous Matrices in Relation to Glassy State and Trehalose Crystallization

The thermal stability of enzyme invertase in reduced-moisture model systems of maltodextrin (MD), polyvynilpyrrolidone (PVP; MW 40,000) and trehalose heated at 90°C was studied. Significant invertase inactivation was observed in heated glassy PVP and MD systems kept well below their glass transition temperature (T_g), but the enzyme was fairly stable in rubbery trehalose systems. However, at moisture contents which allowed trehalose crystallization rapid thermal inactivation of invertase was observed. Invertase inactivation in heated PVP, MD and trehalose systems of reduced-moisture could not be predicted on the basis of glass transition and this was particularly true for trehalose. Conditions which would allow collapse of the systems and crystallization of trehalose were fairly well predicted based on the estimated T_g of model systems.     

Glass Transition and Sticky Point Temperatures and Stability/Mobility Diagram of Fruit Powders

Principal components present in fruits are low molecular weight sugars and some organic acids. They have low glass transition temperature (T _g) and are very hygroscopic in their amorphous state, so the dry product becomes sticky. Water acts as a plasticizer and decreases the glass transition temperature of the product with the increase in moisture content and water activity. To overcome this problem, ingredients having high T _g value, such as maltodextrin, and food grade anti-caking agents were added to prepare vacuum dried fruit powders. The relationship between T _g and a _w provides a simple method for prediction of safe storage temperature at different relative humidities environment. Food powders namely, mango, pineapple, and tomato (3–4% w.b moisture content) were produced by mixing with maltodextrin and tri calcium phosphate at predetermined levels before drying. The relationship among glass transition temperature (T _g), sticky point temperature (T _s), moisture content and water activity of the three powders was represented in a stability/mobility diagram to find out safe storage conditions. Glass transition temperature of the fruit powders were interpreted in terms of the Gordon-Taylor model for verification. Glass transition and sticky point temperatures were compared by plotting them in a graph against moisture content.     

Color formation due to non-enzymatic browning in amorphous,glassy, anhydrous,model systems

Polymeric matrix model systems (gelatinized starch, maltodextrin and polyvinylpyrrolidone (PVP) MW 40 000) containing a low concentration of added Maillard's reactants (glucose and l-lysine) were freeze dried, further desiccated (P₂O₅) to `zero' % moisture, and heated for 48 h at 90°C. Under these conditions, all model systems were well below their measured glass transition temperature (T_g). Skim-milk powder samples (1.7 and 3.9% (dry basis) moisture) were also stored at 45 or 60°C, and browning was recorded. Browning was observed in the glassy polymeric matrices kept well below T_g and in the virtual absence of water. Browning developed in the milk powder samples stored at 60°C at a higher rate than at 45°C, even when both systems were in glassy conditions. This suggests that the T_g parameter should not be considered as an absolute threshold of stability with regard to non-enzymatic browning reactions. ©     

Glass Transition Temperatures Determined using a Temperature-Cycling Differential Scanning Calorimeter

Glass transition temperatures, T_g, of polystyrene, polyvinylpyrrolidone, polydextrose, gelatin, corn flakes, pasta, and aqueous glucose/glycine solutions were determined using a differential scanning calorimeter which cycled the temperature while the net temperature increased at a constant rate. Operating conditions of the modulated differential scanning calorimeter^(tm) (MDSC^(tm)) for optimizing the endothermic baseline shift associated with the glass transition were a scan rate of 5°C/min with an amplitude of ± 1°C over a modulation period of 60 or 100 sec. The MDSC successfully separated the glass transition from other irreversible thermal changes in simple food ingredients. While the MDSC did not distinctly determine T_g of complex food systems at low moisture contents, glass transitions were observed for solutions and food systems at higher moisture contents. T_g values from MDSC were reproducible and similar to those from standard DSC.     

Water Adsorption and Glass Transition of Spray-Dried Soy Sauce Powders Using Maltodextrins as Carrier

Moisture adsorption isotherm and glass transition temperature of various spray-dried soy sauce powders containing different types and concentrations of maltodextrins were studied and compared. Maltodextrins of dextrose equivalent (DE)?=?5, DE?=?10 and DE?=?15, respectively, with concentrations of 20 or 40 % (w/v) were used as carrier agents. The equilibrium moisture content was reduced with increased maltodextrin concentration, whereas it was not apparently influenced by the value of maltodextrin DE. Both the Brunauer–Emmett–Teller (BET) and Guggenheim–Anderson–de Boer (GAB) models could be applied to simulate the moisture adsorption behaviour of the soy sauce powders. The monolayer moisture content of the powders was determined by fitting experimental data to the BET/GAB models with a _w up to 0.53, although both models could fit satisfactorily with the experimental data to a higher water activity level_. The glass transition temperatures (T _g) of the powders equilibrated under various water activities were determined using a differential scanning calorimeter. Increasing moisture adsorption of the soy sauce powders resulted in a T _g reduction, and the experimental T _g values fitted the Gordon–Taylor model well. The BET and Gordon–Taylor models were applied together to predict the critical moisture contents (i.e. 0.0464–0.0777 g water/g dry matter) and water activities (i.e. 0.032–0.241), above which the soy sauce powders become vulnerable to degradation and changes in their physicochemical properties.     

Application of Glass Transition in Food Processing

The phenomenon of glass transition has been employed to food products to study their stability. It can be applied as an integrated approach along with water activity and physical and chemical changes in food in processing and storage to determine the food stability. Also associated with the changes during agglomeration crystallization, caking, sticking, collapse, oxidation reactions, nonenzymatic browning, and microbial stability of food system. Various techniques such as Differential Scanning Calorimetry, Nuclear Magnetic Resonance, etc. have been developed to determine the glass transition temperature (T_g) of food system. Also, various theories have been applied to explain the concept of T_g and its relation to changes in food system. This review summarizes the understanding of concept of glass transition, its measurement, and application in food technology.     

Glass Transition and State Diagram for Jujube Powders With and Without Maltodextrin Addition

Both water activity and glass transition concepts were used for stability evaluation of jujube powder in this study. The water sorption behaviour at 25, 35 and 45 °C and the relationship between water content (X _w), glass transition temperature (T _g) and water activity (a _w) were investigated for jujube powder with and without the maltodextrin addition. Moisture sorption isotherms of different jujube powders were of type III and BET model was the best for fitting the experimental data. The equilibrium moisture content of jujube powder containing 20% maltodextrin at a given a _w was lower than that of pure jujube powder. However, temperature had no significant effect on the moisture sorption of jujube powders. Values of T _g of different jujube powders were obtained using differential scanning calorimetry and the glass transition line was fitted to the Gordon-Taylor model. A depression in T _g with increasing water content was observed. State diagrams of different jujube powders were established based on the moisture sorption isotherm and the glass transition curve. Results showed that jujube powders with and without maltodextrin addition were stable at 25 °C when the water content was lower than 0.0673 and 0.0566 g/g solid, respectively.     

Amorphous lactose crystallisation kinetics

Amorphous lactose crystallisation kinetics were investigated at different relative humidity and temperature combinations. Amorphous lactose was equilibrated to a water activity of 33%, then placed in sealed pans with a saturated magnesium chloride solution to maintain a constant relative humidity (33%). The temperature was raised to 10–40 °C above the glass transition temperature (T_g). The degree of crystallisation was measured using isothermal microcalorimetry. Crystallisation was shown to be an all-or-nothing event, such that direct measurement of the kinetics was not possible. This was not expected from the Avrami model. The rapid crystallisation could be an autocatalytic effect, as moisture is released during crystallisation, or a showering event as seen in highly supersaturated lactose solutions. Experiments using a blend of crystalline and amorphous lactose, produced by spray drying a lactose crystal slurry, showed crystallisation occurring at lower T_g conditions than was required for the crystallisation of 100% amorphous lactose.     

Effect of oligomeric or polymeric additives on glass transition,viscosity and crystallization of amorphous isomalt

The main purpose of this study was to further improve the stability of the amorphous state of isomalt by the addition of high molecular weight compounds. Differential scanning calorimetry (DSC) was used to determine the glass transition temperature T_g of a series of polyol mixtures containing isomalt as a function of water content. As expected, for each mixture T_g decreased with increasing moisture. Only with the addition of more than 75% of higher molecular weight compounds to isomalt an increase of T_g could be achieved. Moisture content and time dependent phase transitions in the metastable amorphous state of the mixtures strongly affect the storage stability of isomalt hard candies. Storage tests indicated a markedly accelerated water absorption and crystallization when oligomeric or polymeric compounds were added to amorphous isomalt. Rheological experiments showed that in contrast to pure isomalt melts, the viscosity of melts containing oligomeric or polymeric additives deviated from the model curve predicted by the Williams–Landel–Ferry-kinetics (WLF) with increasing water content.     

Solid-state tyrosinase stability as affected by water activity and glass transition

The effects of water activity (a_W) and the state of the system as dictated by the glass transition temperature (T_g) on tyrosinase storage stability were evaluated in a model food system. Tyrosinase was incorporated into low and high molecular weight polyvinylpyrrolidone (PVP-LMW and PVP-K30, respectively). Samples were equilibrated and stored in desiccators from a_W 0.33 to 0.77 at 20°C. Tyrosinase activity was monitored for a week, and pseudo-first order rate constants for activity loss were calculated. Residual activity after equilibration correlated with T_g rather than a_W. An apparent rate constant increase slightly above T_g was observed and rate constants were the same at constant T_g, suggesting that molecular mobility as dictated by T_g was influencing tyrosinase storage stability.     

Moisture Sorption of Amorphous Sugar Products

Moisture sorption into sugar glasses led to significant changes in the physico‐chemical properties. Increased moisture content reduced the glass transition temperature, T_g, although the effect was dependent on type of corn syrup. In general, higher corn syrup content and addition of a high‐maltose corn syrup resulted in slightly higher T_g for a given moisture content. Higher corn syrup concentration increased hardness, but also resulted in increased stickiness. Stickiness increased as a function of moisture until a maximum was reached, whereupon further moisture sorption reduced stickiness. A maximum in stickiness occurred when T^g values were still above room temperature, indicating that surface moisture was considerably higher than bulk moisture content.     

Effect of water content and molecular weight on the moisture isotherms and glass transition properties of inulin

Inulin is being used more and more in foods due to its ability to add functionality either as a sweetener or fat replacer and also for its nutritional benefits. In this research, moisture sorption isotherms and glass transition temperature (T_g) as a function of moisture content and molecular weight for four inulins were determined. The T_g of the inulins was related to moisture content and also molecular weight. As with maltodextrins, the extent of lowering T_g was directly related to the average chain length (lower T_g with lower chain length). The sorption properties are also similar to maltodextrins, thus inulin may be useful in providing similar functionality as maltodextrins with almost no calories (1.5 kcal/g).     

Glycine Loss and Maillard Browning as Related to the Glass Transition in a Model Food System

The effects of water activity and the glass transition on the rate constants for glycine loss and brown pigment formation due to the Maillard reaction were evaluated in a model food system. Equimolar glucose and glycine were incorporated into amorphous polyvinylpyrrolidones of various molecular weights and moisture contents. Glycine loss and brown pigment formation were quantified during storage at 25°C. At constant water activity, rate constants were higher in systems with lower glass transition temperatures. Glycine rate constants decreased upon matrix collapse, but browning rate constants were not affected by collapse. Changes associated with the glass transition influence bimolecular reactions and should be considered during product formulation and shelf-life testing.     

Water and Molecular Weight Effects on Glass Transitions in Amorphous Carbohydrates and Carbohydrate Solutions

The effects of water, freeze-concentration and effective molecular weight (M_e) on glass transition (T_g) of maltose and maltodextrins were studied, and methods to predict T_g were used to establish state diagrams. T_g of maximally freeze-concentrated solutes (T′_g) and onset of ice melting (T′_m) increased with M_e, and for high molecular weight polysaccharides T′_g and T′_m were predicted to have the same temperature value. Ice formation at T′_gm was time dependent. Unfrozen water in maximally freeze-concentrated matrices was about 20% independently of M_e. The state diagrams can be used to evaluate physical state of frozen and dehydrated foods.     

Glass Transition State Diagram of a Baked Cracker and Its Relationship to Gluten

The polymer science approach can be applied to the understanding of physical changes in foods such as loss of texture upon moisture gain. The glass transition temperature (T_g) of a commercial cracker was measured by Dynamic Mechanical Thermal Analysis (DMTA) as a function of water content. The final state diagram showed that the cracker was plasticized by water with a glass transition temperature that decreased as water content increased. When compared with published glass transition data on cereal ingredients, gluten may be the main component responsible for the mechanical changes and loss of crispness of such products when moisture content is increased.     

The glassy state phenomenon in applications for the food industry: Application of the food polymer science approach to structure–function relationships of sucrose in cookie and cracker systems

This review of the glassy state phenomenon in applications for the food industry comprises two main parts. The first is a broad but brief overview of the so-called ‘food polymer science’ approach and its importance to food R&D studies of glassy solid and rubbery liquid states and glass transitions in food products and processes. The following elements of this approach are discussed: (i) the glass transition temperature (T_g) and methods for its measurement in foods; (ii) plasticization by water and its effect on T_g; (iii) the concepts of ‘water dynamics’ and ‘glass dynamics’ in non-equilibrium food systems; (iv) Williams–Landel–Ferry kinetics in the rubbery state above T_g, (v) state diagrams; and (vi) the effect of molecular weight on T_g. A comprehensive and up-to-date listing of more than 400 literature references on the glassy state phenomenon and glass transitions in food materials and systems is featured in that part of the paper, and these references are also compiled and tabulated according to specific subject headings. The second part of this review highlights the application of the food polymer science approach in recently reported studies on the structure–function relationships of sucrose in cookie and cracker systems. This part describes (i) the sucrose–water state diagram as a tool in understanding cookie and cracker baking; (ii) shortcomings of the traditional AACC sugar-snap cookie method as a test-baking system, in contrast to a new test system based on a model commercial-type wire-cut cookie formula; and (iii) a revealing illustration of sucrose functionality in cookie baking. The review concludes with a word about future prospects.     

WATER PLASTICIZATION EFFECTS ON CRYSTALLIZATION BEHAVIOR OF LACTOSE IN A CO-LYOPHILIZED AMORPHOUS POLYSACCHARIDE MATRIX AND ITS RELEVANCE TO THE GLASS TRANSITION

ABSTRACT

Crystallization of lactose in a co-lyophilized amorphous polysaccharide matrix was investigated under various hydration conditions to test the possible relation between the ability of the polymer to raise the glass transition temperature (T_g) of the lactose-pullulan blend, relative to pure lactose, and the crystallization kinetics. Both calorimetric (DSC) non-isothermal measurements and x-ray diffraction analysis of samples stored at a constant temperature revealed a marked retardation in lactose crystallization in the presence of pullulan; i.e., the rate constant values, as determined by the Avrami analysis, declined and the ‘half-time’ (t_1/2c) for lactose crystallization increased with decreasing ratio of lactose/pullulan. The inhibitory action of pullulan on crystallization of lactose could not be solely attributed to T_g-related effects on molecular mobility of the composite systems. At a pullulan weight fraction range of 0.25–0.33 (w/w of total solids) the influence of the polymeric additive on T_g was marginal over the entire water content range examined, although crystallization was delayed as compared with pure lactose. Modeling of the temperature-dependence of t_1/2c for the combined lactose-pullulan systems with the Williams-Landel-Ferry (WLF) equation was feasible only when the coefficients C₁ and C₂ were allowed to vary instead of assuming their ‘universal’ values.     

Plasticizing Effect of Water on Thermal Behavior and Crystallization of Amorphous Food Models

Dehydrated sugar solutions were used as models of thermal behavior of amorphous foods, and of the effect of temperature, moisture content and time on physical state of such foods. The transition temperatures determined were glass transition (T_g), crystallization (T_cr) and melting (T_m) which all decreased with increasing moisture. T_g of a sucrose/ fructose model had a slightly lower value than the empirical “sticky point,” at all moisture contents studied. Crystallization of sucrose was delayed by addition of fructose or starch. Crystallization above T_g was time-dependent, and the relaxation time of this process followed the WLF equation.     

Kinetics of non-enzymatic browning in amorphous solid systems: distinguishing the effects of water activity and the glass transition

Debates have emerged recently on whether water activity or the state of the system as dictated by the glass transition temperature (T_g) impacts the rates of chemical reactions in reduced-moisture solid systems. The objective of this study was to evaluate the kinetics of brown pigment formation in polyvinylpyrrolidone model systems of different molecular weights so that the effects of water activity and the glass transition could be distinguished. Browning rates at different T_gs, but constant water activity, were significantly different except when all were in the glassy state. As the system changed from a glassy state to a rubbery state, the rate of browning increased 7-fold. The rate of browning also increased as water activity increased from 0.33 to 0.54, but then appeared to plateau with further increases in water activity. Thus, the rate of brown pigment formation is influenced significantly by the glass transition temperature of the system and less by the water activity. In addition, the concentration of reactants in the aqueous microenvironment had a significant impact on the rate of brown pigment formation.