The effect of sugars on the extrusion of maize grits: I. The role of the glass transition in determining product density and shape

The effects of sugars (sucrose, glucose, fructose, xylose, lactose and maltose) on both sectional (radial) and longitudinal expansion and subsequent shrinkage of maize extrudates have been investigated. Sugars reduced the sectional expansion, monosaccharides more than disaccharides. Reduced sectional expansion and increased density with sugar content was interpreted as a combination of a reduction in bubble growth and an increase in the degree of shrinkage on leaving the die. The decrease in bubble growth was considered to be the result of a temperature reduced driving force for bubble growth and reduced bubble wall extension before rupture caused by less starch conversion with increasing sugar content. Shrinkage stops when the temperature decreases to approximately T_g+ 30°C, where T_g is the glass transition temperature. The addition of sugars and water will reduce the glass transition temperature (T_g) of the melt and hence increase the temperature range over which the extrudate will shrink. The driving force for shrinkage is the release of stored elastic energy following bubble rupture and the pressure difference between the interior of closed cells in the product and atmospheric pressure.     

Glass Transition Temperature of Casein as Affected by Transglutaminase

The objective of this study was to determine glass transition temperature (T_g) of a-casein with differential scanning calorimetry (DSC) and to study the effect of microbial transglutaminase (MTG) treatment on T_g of a-casein. MTG-treated a-casein was fractionated following the MTG treatment. After adjusting water content, T measurements were made by DSC and the dependencies of T_g on the water content determined. Oscillation DSC (ODSC) was also used. The determination of T_g of MTG-treated samples by conventional DSC was unreliable. However, by comparing with ODSC data, their T could be detected. The MTG treatment elevated the T_g of a-casein, which maybe the reason for improved stability of dry daily products during storage.     

Physical Properties, Sensory Attributes, and Consumer Preference of Pear Fruit Leather

ABSTRACT: Pear fruit leathers were prepared by drying a mixture of pear juice concentrate, pectin, corn syrup, and water at 70°C for 8 h. The effects of ingredients on texture, color, microbial growth, moisture content (MC), water activity (a_w), glass transition temperature (T_g), and sensory attributes were investigated. Pectin was the most significant independent variable that affected the properties of interest. This was followed by initial moisture content and corn syrup. Glass transition temperature (T_g) was not useful for predicting microbiological attributes but had strong correlations with instrumental and sensory hardness and chewiness. The results from partial least squares analysis (PLS) indicate instrumental hardness, chewiness, and cohesiveness could be used to predict their corresponding sensory attributes. The consumer liking of pear fruit leather could be increased by raising their fruit aroma, sweetness, tartness, and shininess.     

Nonenzymatic Browning in Amorphous Food Models: Effects of Glass Transition and Water

ABSTRACT: The effect of glass transition on nonenzymatic browning (NEB) rates of amorphous, L-lysine, and D-xylose containing maltodextrin (MD)- or polyvinylpyrrolidone (PVP)-based food models, equilibrated into a_w of 0.23, 0.33, or 0.44 was studied. Glass transition temperature, T_g, was measured using DSC and NEB rates, followed spectrophotometrically. Use of citrate buffer as a pH regulator was evaluated. The NEB rates were higher in PVP than in MD models. Arrhenius plots showed nonlinearity in the vicinity of the T_g. Increasing a_w decreased T_g, but did not affect NEB rates around the T_g. The temperature difference (T - T_g) was not a sufficient measure of material stability.     

State Diagram of Freeze-dried Garlic Powder by Differential Scanning Calorimetry and Cooling Curve Methods

ABSTRACT: The state diagram of freeze-dried garlic powder was developed using freezing curve, glass transition line, and maximal-freeze-concentration condition. Freezing points of garlic powder were measured by differential scanning calorimetry (DSC) and the cooling curve method, whereas glass transitions were measured by DSC. The freezing curve and glass transition line were modeled using the Clausius-Clapeyron equation, adjusted with unfreezable water, and the Gordon-Taylor model, respectively. Maximal-freeze-concentrated condition was found as X '_s (characteristic water content) = 0.82 [ X '_g (characteristic solids content) = 0.18] with the characteristic temperature of glass formation being T '_m (characteristic glass transition) =−38.6°C and T '_m (characteristic end point of freezing) =−26.0°C. Other characteristic glass transitions T "_g and T ‴_g equal to −29.3°C and −48.6°C, respectively.     

The Contribution of Water to the Specific Heat Change at the Glass-to-Rubber Transition of the Ternary System BSA-Water-NaCl

ABSTRACT: We studied the glass-to-rubber transition of a BSA-water-NaCl system using DSC. The T_g was 178–187 K and decreased with increasing NaCl content from 1:9 to 6:4 NaCl:BSA. The specific heat change (ΔC_p*, J/((g BSA)K)) at T_g increased as the weight ratio increased from 1:9 to 5:5 NaCl:BSA, as did the change in calculated unfrozen water (UFW). The UFW was also correlated with the amount of NaCl in the glassy region with a correlation coefficient of approx. 6 (mol H₂O)/ (mol NaCl), which is comparable to the hydration number of Na⁺. These results suggest that UFW could contribute to the ΔC at T_g of the BSA-water system.     

Influence of Cooling Rate on Glass Transition Temperature of Sucrose Solutions and Rice Starch Gel

ABSTRACT: The study's objectives were to determine the influence of cooling rate through the primary zone of freezing on T_g in sucrose solutions and rice starch gels. The influence of cooling rate, water content, and annealing on T_g were evaluated. Results indicated that the observed T_g values for sucrose solutions were lower after rapid cooling (70% solids: rapid cooling −66.7°C; slow cooling −64.6 °C; 30% solids: rapid cooling −34.6 °C; slow cooling −33.3 °C). The T_g values of annealed samples are higher than the T_g of both rapidly and slowly cooled samples (70%: −44.2 °C; 30%: −32.7 °C). The T_g of the rice starch gel was −9.0 °C after rapid cooling and −7.5 °C after slow cooling.     

Effect of protective agents on the viability of geotrichum candidum during freeze-drying and storage

ABSTRACT:  The effect of 3 cryoprotective agents (trehalose, sucrose, and maltose) on the survival of concentrated cultures of Geotrichum candidum was studied. Initially, the effect of the carbohydrates at 9% and 23% concentrations or combined with skim milk (16%) was compared to the control (skim milk alone) immediately after freeze-drying. Two freeze-drying shelf temperatures, 25 °C and 35 °C, were investigated. Afterwards, the survival of G. candidum freeze-dried with carbohydrates at 23% concentration (alone or combined with skim milk 16%) was studied during 12 wk of storage at 4 °C, in the darkness, under vacuum, and at 0% relative humidity. The glass transition temperature ( T_g ) of the dehydrated protective agents was measured by differential scanning calorimetry. The results showed that the survival after freeze-drying was proportional to the concentration of the protective agents, with skim milk alone giving poor survival of G. candidum . However, when skim milk was combined with disaccharides, a clear improvement was noted. No general tendency of shelf temperature on the survival of G. candidum was noted immediately after freeze-drying. However, changes in the viability were observed during storage. Glass transition temperature ( T_g ) of protective agents linked to their moisture may contribute to predict the stability of lyophilized G. candidum during freeze-drying and storage.     

Thermal Unfolding of Gelatin in Solids as Affected by the Glass Transition

ABSTRACT: The thermal unfolding or denaturation of solid-state proteins during processing can affect their functionality. This project explored the relation between the glass transition temperature (T_g) and thermal unfolding temperature (T_m) using gelatin as a model system. Freeze-dried gelatin was prepared containing various polyol types at several concentrations. Using differential scanning calorimetry, T_m andT_g were determined. Moisture and polyols (that is, glycerol, xylitol, sorbitol, sucrose, and trehalose) promoted a lowering of both T_g andT_m, the extent of which depended upon the plasticizer type and concentration. For all the data, T_m > T_g, and a plot of T_m against T_g indicated a linear relationship ( R²= 0.95). These results suggest that formulations must be in the rubbery state to have the necessary mobility for hydrogen bond disruption that leads to protein unfolding. Glass transition data should be considered when developing processing parameters for proteinaceous systems.     

Slow Molecular Mobility in the Amorphous Solid State of Fructose: Fragility and Aging

ABSTRACT:  The molecular mobility of β-D-fructose was studied by thermally stimulated depolarization currents (TSDC) in the amorphous solid state. The amorphous solid samples were prepared in such a way that the tautomeric mixture was near the equilibrium composition. A broad secondary relaxation was observed, that merges, at high temperatures, with the alpha relaxation. The alpha relaxation temperature provided by the TSDC technique is  T_g  = 13 °C (at 4 K min⁻¹). The fragility index calculated from TSDC data is  m  = 34, significantly lower when compared with the values reported in the literature obtained from Dielectric Relaxation Spectroscopy (DRS). The physical significance of the fragility obtained by the 2 dielectric techniques is discussed. The influence of physical aging on the secondary relaxation in amorphous fructose was analyzed as the glass structurally relaxes. A complex behavior was observed such that the faster components (lower temperature) of the secondary relaxation are negligibly dependent on aging and may be ascribed to intramolecular modes of motion, while the slower motional modes (higher temperature) show a significant dependence on aging and correspond to the genuine Johari–Goldstein β-relaxation.     

Thiamin Stability in Solids as Affected by the Glass Transition

The stability of thiamin was evaluated in solid polyvinylpyrrolidone model systems as a function of water activity (a_w) and the glass transition. In the glassy state, thiamin degradation rate constants correlated with the glass transition temperature (T_g), decreasing as T_g increased. Above a_w 0.4, rate constants correlated better with a_w rather than T_g. However, rate constants decreased in the rubbery state above a_w 0.4, which could be attributed to glass transition-induced structural collapse. Glass transition effects (mobility considerations, collapse) have a larger effect on thiamin stability than a_w, which should be recognized during the development of fortified food products.     

Properties of Wheat Dough at Sub-Zero Temperatures and Freeze Tolerance of a Baker's Yeast (Saccharomyces cerevisiae)

ABSTRACT: This work studied the relationship between the freeze tolerance of a baker's yeast ( Saccharomyces cerevisiae ) and the physical properties of the frozen wheat dough. The behavior of wheat dough, of the gluten phase, and of the liquid phase at sub-zero temperatures was examined by differential scanning calorimetry (DSC) and cryomicroscopy. The effect on yeast viability of dough water content, freezing and storage temperatures, and prefermentation before freezing was studied. Yeast viability was mostly affected by the freezing and storage temperatures, with temperatures below the glass transition temperature (T_g) giving the highest survival ratios. At temperatures above T_g, viability after freezing treatment and during storage seemed to be governed by different mechanisms, encompassing osmotic and mobility factors. Those factors were also found to influence the freeze tolerance of growing yeast in the presence of ethanol.     

Physical Aging of Maltose Glasses

Physical aging is the alteration in the physical properties of a glass, as a function of time, at temperatures below its glass transition temperature (Tg). The physical aging of maltose glasses, prepared from maltose monohydrate sugar, was studied using differential scanning calorimetry (DSC). The maltose was aged at 30°C for times ranging from 5 min to ～ 10 days. The Tg and change in heat capacity of the unaged and aged maltose glasses were measured. The change in enthalpy relaxation with aging time was determined by integrating the difference between the aged and the unaged DSC thermograms. The enthalpic relaxation data were adequately described by the Williams-Watts equation.     

THE VISCOSITY OF SUPERCOOLED FRUCTOSE AND ITS GLASS TRANSITION TEMPERATURE

The viscosities of supercooled liquid fructose and glucose are measured between a lower viscosity limit corresponding to 95°C and an upper limit of about 10⁶ Pa.s. The glass transition temperature of fructose is estimated from the viscosity data using different extrapolation procedures. The Vogel-Tamman-Fulcher and power-law equations gave estimates of 283K and 289K, respectively and the Williams-Landel-Ferry equation 286K. These values are in agreement with recent calorimetric determinations of the glass transition temperature.     

Effects of inulin/oligofructose on the thermal stability and acid-induced gelation of soy proteins

ABSTRACT:  Differential scanning calorimetry (DSC) and dynamic oscillatory shear testing were performed to study the influence of inulin (Raftiline^® HP-gel and Raftiline^® ST-gel) and oligofructose (Raftilose^® P95) on the thermal stability and gelation (using glucono-δ-lactone [GDL] as a coagulant) of soy protein isolate (SPI) dispersions. Addition of 10% (w/v) inulin/oligofructose or sucrose increased ( P < 0.05) the peak denaturation temperatures ( T_m ) of 7S and 11S soy proteins in SPI dispersion (5%[w/v], pH 7.0) by an average of 1.9 and 2.3 °C, respectively. GDL induced SPI thermal gelation, and the gel rheology was affected by both the pH decline and the specific temperature of heating. Addition of inulin/oligofructose (8%, w/v) improved the gelling properties of preheated SPI dispersion (8%, w/v) coagulated with GDL, showing 14.4 to 45.6% increase ( P < 0.05) in gel rigidity ( G ' value) at the end of heating (81 °C). Microstructural examination revealed a denser protein cross-linking structure and reduced pore sizes in SPI gels containing inulin/oligofructose. In general, inulin was more capable of improving SPI gelation than oligofructose, suggesting that the degree of fructose polymerization in the fructans was of thermal and rheological importance.     

Glass transition behaviour of fructose

The glass transition temperature and the second transition (the endothermic change between the glass transition and melting temperatures) of fructose were studied. The thermal history strongly affected both transitions of fructose. Storage for 10 days at 22 °C increased the dynamic glass transition temperature from 16 to 25 °C and decreased the second transition of fructose from 110 to 98 °C in the first differential scanning calorimetric (DSC) scan. The amplitude of the second transition increased slightly with storage time and reached 260% of the first transition for vacuum oven dried samples. The effect of thermal history on the glass transition temperature of fructose can be removed by scanning the sample in a DSC to 130 °C. The effects of water content, glucose and sucrose on the two transitions were also investigated.     

Rheology and Oxidative Stability of Whey Protein Isolate-Stabilized Menhaden Oil-in-Water Emulsions as a Function of Heat Treatment

ABSTRACT:  Menhaden oil-in-water emulsions (20%, v/v) were stabilized by 2 wt% whey protein isolate (WPI) with 0.2 wt% xanthan gum (XG) in the presence of 10 mM CaCl₂ and 200 μM EDTA at pH 7. Droplet size, lipid oxidation, and rheological properties of the emulsions were investigated as a function of heating temperature and time. During heating, droplet size reached a maximum at 70 °C and then decreased at 90 °C, which can be attributed to both heating effect on increased hydrophobic attractions and the influence of CaCl₂ on decreased electrostatic repulsions. Combination of effects of EDTA and heat treatment contributed to oxidative stability of the heated emulsions. The rheological data indicate that the WPI/XG-stabilized emulsions undergo a state transition from being viscous like to an elastic like upon substantial thermal treatment. Heating below 70 °C or for less than 10 min at 70 °C favors droplet aggregation while heating at 90 °C or for 15 min or longer at 70 °C facilitates WPI adsorption and rearrangement. WPI adsorption leads to the formation of protein network around the droplet surface, which promotes oxidative stability of menhaden oil. Heating also aggravates thermodynamic incompatibility between XG and WPI, which contributes to droplet aggregation and the accumulation of more WPI around the droplet surfaces as well.     

Improved Firmness in Calcified Diced Tomatoes by Temperature Activation of Pectin Methylesterase

ABSTRACT: The effects of temperature and calcium on pectin methylesterase (PME) activity and texture in tomato pericarp material were examined. Heating thin slices of pericarp to temperatures between 50°C and 75°C led to the rapid evolution of methanol from the material, indicating an activation of PME. This activity was further stimulated when CaCl₂ (up to 2.0% w/v) was added. When applied to half-inch diced tomato pericarp, the same conditions that led to the activation of PME also improved firmness. Diced tomatoes treated for 5 min with 0.5% CaCl₂ at 70°C were 2.5 times firmer than diced tomatoes treated with CaCl₂ at room temperature. This improvement in texture by treating with CaCl₂ at elevated temperatures was only apparent when the tomatoes received a subsequent 100°C treatment. Heating tomatoes to 70°C either before or after the CaCl₂ treatment also improved firmness through a subsequent high-temperature treatment, but to a lesser extent than heating during the CaCl₂ treatment. These results are consistent with the model that heating to 70°C greatly increases PME activity, leading to extensive pectin de-esterification and increased calcium cross-linking of the pectins in the middle lamella. Production of thermally processed diced tomatoes with improved firmness should be possible by increasing the temperature during and after calcium treatment.     

Temperature Mapping of Particles During Aseptic Processing with Magnetic Resonance Imaging

Magnetic Resonance Imaging (MRI) was used to obtain 2-dimensional temperature maps in potatoes undergoing aseptic processing. The change in precession frequency of protons served as the temperature indicator. The larger particles (6.9 and 3.84 cm3) showed a δT (T_surface - T_center) of up to 22±0.4°C 45s after exiting the heat exchanger with the δ (T_outlet - T_inlet) of the carrier fluid in the heat exchanger at 30 to 45°C. No AT was measured between the center and the surface of particles <2.05 cm³ pumped at <22.7 L/min. The average fluid to particle convective heat transfer coefficient (h_fp) for the heat exchanger and holding tube was calculated using a finite element method. The hfp was from 600 to 2500 and >3000 W/m²°C for the large (6.9 cm³ cubes) and smaller particles respectively.     

Comparison of Nonenzymatic Browning Kinetics in Spray-dried and Freeze-dried Carbohydrate-based Food Model Systems

ABSTRACT: Nonenzymatic browning (NEB) rates of amorphous, carbohydrate-based, freeze-dried and spray-dried food model systems containing L-lysine and D-xylose as reactants (5% w/w) were investigated at different temperatures (40 °C, 50 °C, 60 °C, 70 °C, 80 °C, 90 °C). Samples were exposed to various relative vapor pressure levels (11%, 23%, and 33%) to adjust water contents. Water sorption was determined gravimetrically, and data were modeled using the Brunauer-Emmett-Teller and Guggenheim-Anderson-deBoer equations. Glass transition, T_g, was observed by differential scanning calorimetry. NEB was followed spectrophotometrically. The surface structures of freeze-dried and spray-dried models were observed by scanning electron microscopy. The freeze-dried samples and the spray-dried samples showed different surface structures and slight differences in thermal behavior. Crystallization of component sugars in the freeze-dried samples was slightly more delayed than in spray-dried samples. The glass transition temperatures in spray-dried samples were higher than those of the freeze-dried samples at the same water activity. The temperature dependence of NEB rate in both systems followed the Arrhenius kinetics, but the activation energies were different. Williams-Landel-Ferry equation could be used to model the NEB kinetics in the freeze-dried system, but for the spray-dried system, negative constant values were not within the allowable range.