Effects of transglutaminase treatment on the thermal properties of soy protein isolates

The effects of covalent cross-linking of microbial transglutaminase (MTGase) on the thermal properties of soy protein isolates (SPI), including the thermal denaturation and glass transition were investigated by conventional and modulated differential scanning calorimetry (DSC). The MTGase treatment significantly increased the thermal denaturation temperatures (including the on-set temperature of denaturation, T_m and the peak temperature of denaturation, T_d) of glycinin and β-conglycinin components of SPI (P ⩽ 0.05), and the thermal pretreatment of SPI further increased the extent of this improvement. The MTGase treatment also improved the ability of SPI to resist the urea-induced denaturation. Modulated DSC analysis showed that there were two glass transition temperatures (T_g) in the reversible heat flow signals of native SPI (about 5% moisture content), approximately corresponding to 45 and 180 °C, respectively. These T_g values of SPI were significantly decreased by the MTGase treatment (at 37 °C for more than 2 h) (P ⩽ 0.05). The improvement in the hydration ability of protein and the formation of high molecular biopolymers may account for the changes of thermal properties of soy proteins caused by the MTGase cross-linking.     

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.     

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.     

Effect of protein concentration on the surface composition,water sorption and glass transition temperature of spray-dried skim milk powders

The effects of dilution of protein content in skim milk (34–8.5% protein content), by lactose addition, on the surface composition, water sorption property and glass transition temperatures of spray-dried powders were investigated. The X-ray photoelectron spectroscopy (XPS) study of spray-dried powders showed preferential migration of proteins toward the surface of the milk particles whereas the lactose remained in the bulk. Sorption studies showed that the lower protein concentration in milk powders is linked to an increased water adsorption property and lowering of water activity (a_w) for lactose crystallization. Analysis of glass transition temperature (T_g) of the powders sorbed at different humidities showed no distinct change in T_g values, indicating the dominant effect of lactose on the glass transition temperature of all the powders.     

Effects of methylcellulose,xanthan gum and carboxymethylcellulose on thermal properties of batter systems formulated with different flour combinations

The functionalities of hydrocolloid–flour mixtures in terms of the thermal properties of their resulting batter systems were investigated, and the effects of different thermal processes such as cooking–freezing–thawing (CFT) and freezing–cooking (FC) on thermal properties of the various batter systems were determined in this study. Differential scanning calorimetry (DSC) was used to determine thermal property parameters including gelatinization temperature (T_G), total enthalpies of gelatinization (ΔH_G), glass transition temperature (T_g), melting peak temperature (T_m), and total melting enthalpies (ΔH_m). The different thermal processes did not significantly affect either T_G or ΔH_G of batter systems, but they influenced the glass transition behavior and the ΔH_m of batter systems. The thermal processes also showed different effects on the batter systems containing different hydrocolloids such as methylcellulose (MC), carboxymethylcellulose (CMC), and xanthan gum (XG). The hydrocolloids shifted T_G upwards, depressed T_g, and increased T_m of batters. The effect of these hydrocolloids on glass transition temperature was more pronounced in raw samples (FC process) than in cooked samples and increased with increasing levels of CMC and MC used in the formulations. Batters with MC showed increased ΔH_m for all the thermal processes. CMC only showed significant effect on ΔH_m for cooked samples (CFT process). MC and CMC showed more pronounced effects on T_g for raw uncooked rice- and corn flour-based batters than on raw uncooked wheat flour-based batters. However, this special effect was not obvious in the batters containing 0.2% XG.     

Enzymatic catalysis in a whey protein matrix at temperatures in the vicinity of the glass transition

The current study evaluates the effect of temperature on α-glucosidase activity, following incorporation of the enzyme into a whey protein matrix through spray drying. Thermomechanical characterization of the matrix was achieved using the techniques of modulated temperature differential scanning calorimetry and small-deformation dynamic mechanical analysis. As the concentration was raised from 75 to 94% (w/w), denaturation of the protein occurred at increasing temperatures. In contrast, denaturation was not observed in calorimetric scans after spray drying. The glass transition temperature (T_g) measured in the dried particles using dynamic mechanical analysis was approximately 40 °C. An optimized procedure was developed whereby α-glucosidase and its substrate p-nitrophenyl α-d-glucopyranoside were incorporated into the whey matrix. The effect of temperature on enzymatic catalysis was investigated and, below 40 °C, activity was low and relatively independent of temperature. However, the rates of product formation markedly accelerated as temperatures were increased beyond T_g. These novel observations strongly emphasize the pronounced effect of mechanical T_g of the protein matrix on enzymatic activity.     

Changes in Structural and Functional Attributes of Fish Mince Proteins in Presence of Cosolvent During Frozen Storage

The effect of polyhydric alcohol, mannitol, on the properties of proteins from Sardinella longiceps has been investigated. Calcium ATPase activity of the proteins decreased during frozen storage. Ca²⁺ATPase activity of mannitol treated samples was higher up to 60 days of storage as compared to untreated samples. Emulsion capacity of proteins decreased as a function of storage. Thermal denaturation profile of proteins revealed no change in apparent thermal transition temperature (app.T_m) from a control value of 57.5°C, in presence of mannitol. Scanning electron micrographs showed an increase in porosity of muscle tissues in the presence of mannitol. These results indicate the stabilizing effect of mannitol at low temperature on the fish muscle protein by retaining structure and functional properties of the muscle during frozen storage.     

Physicochemical basis for cosolvent modulation of β-lactoglobulin functionality: Interfacial tension study

The impact of neutral cosolvents on the thermal stability of globular proteins in aqueous solutions depends on the nature of the cosolvent, e.g., sorbitol causes a pronounced increase in the thermal denaturation temperature (T_m) of β-lactoglobulin (β-lg), whereas glycerol does not. When a protein unfolds there is a change in the exposed surface area and in the type of molecular interactions that occur at the protein–solvent–cosolvent interface. These changes contribute to the free energy change associated with protein denaturation and depend on cosolvent type. In this study we measured the equilibrium interfacial tensions of aqueous glycerol (0–70% w/w) and sorbitol (0–55% w/w) solutions as a function of temperature to provide insights into the role of the interfacial energy on the thermal stability of β-lg. There was a slight increase in interfacial tension with increasing sorbitol concentration, indicating its preferential exclusion from the oil–water interface. On the other hand, there was an appreciable decrease in interfacial tension with increasing glycerol concentration, indicating its preferential accumulation at the oil–water interface. These changes were largely independent of the measurement temperature (30–80 °C). Our results suggest that sorbitol increases the T_m of β-lg mainly through a steric exclusion effect, whereas glycerol has little effect on T_m because the steric exclusion effect is counter-balanced by a differential interaction effect.     

Glass Transition Studies of Baked Tortilla Chips Using Dynamic Mechanical Thermal Analysis

The objectives were to determine the glass transition temperature (T_g) of baked tortilla chips and find a statistical model that represents the change in T_gwith respect to baking conditions. The dynamic mechanical thermal analysis (DMTA) was used to determine the glass transition temperature of samples. TheT_g of samples was in the range of 59 to 76°C with respect to baking temperature and time. The results indicated that the glass transition temperature increased with increasing baking temperature and at longer baking times. TheT_g of samples with longer baking periods increased up to 1.2 times those at smaller baking periods. Statistical analysis showed a linear relationship for T_gvs. baking time and temperature. A statistical model was obtained using a linear regression to describe the change in T_gin terms of baking temperature and time (R²=0.74).     

Physical State Study of (Sugar Mixture)-Polymer Model Systems

The physical state of components in the unfrozen solute phase of frozen solutions was determined by using differential scanning calorimetry (DSC) for mixtures of lactose, sucrose, and trehalose with albumin, gelatine, or cornstarch. An equal weight ratio (1:1) of lactose-sucrose, lactose-trehalose and sucrose-trehalose mixtures with polymer systems (sugar mixture-albumen (1:1:1), gelatine (1:1:1), and cornstarch (1:1:1), as well as cornstarch-gelatine (1:1:1:1) was used. Mixed sugar mixture (lactose/sucrose)-polymer systems were further studied for maximum freeze-concentration in complex systems. A comparative thermal study between sugar-polymer and mixed sugar-polymer systems was conducted. The sugar-polymer and mixed sugar-polymer systems showed similarities and differences in thermal behaviour. The similarities included maximum freeze-concentration of a frozen system at an annealing temperature (T_m´-1) °C, constant initial concentration independent onset of glass transition T_g´, and onset of ice melting temperature T_m´, and increased in temperature difference between T_g´ and T_m´ or increase in broadness of transition with the addition of polymeric compound.     

Physicochemical, mechanical and thermal properties of brown rice grain with various moisture contents

The effects of moisture content on the mechanical and thermal properties of either a short‐grain variety (Akitakomachi) or two long‐grain varieties (Delta and L201) of brown rice were studied. Total starch contents of the three varieties were comparable, but the amylose content of L201 was significantly higher than that of the other two varieties. The maximum compressive strength of brown rice grain was much higher than the maximum tensile strength. L201 showed the highest maximum compressive and tensile strengths. The phase transition temperatures (glass transition temperature T_g and melting temperature T_m) were examined by differential scanning calorimetry. The T_g and T_m for L201 were higher than those for Delta and Akitakomachi. The maximum compressive strength, maximum tensile strength, T_g and T_m for the three varieties of brown rice grains decreased with increasing moisture content. The T_g of individual brown rice kernels decreased from 53 to 22 °C as moisture content increased from 12 to 25% wet basis. A statistical model was calculated by using linear regression to describe the change in T_g in terms of moisture content.     

Thermal denaturation and gelation of vicilin-rich protein isolates from three Phaseolus legumes: A comparative study

The thermal denaturation and gelation properties of vicilin-rich protein isolates from red bean, red kidney bean and mung beans (further denoted as RPI, KPI and MPI) were investigated by differential scanning calorimetry (DSC) and dynamic oscillatory measurements. The relation between the properties of these proteins to their free sulphydryl (SH)/disulfide bond (SS) contents was also evaluated. DSC analyses showed that many DSC characteristics of major endothermic peak (corresponding to vicilin component), including its denaturation temperature (T_d), enthalpy changes (ΔH) and width at half-peak height (ΔT_1/2), significantly varied with the type of protein isolates. Furthermore, the heat-induced gelation, including onset of gelation and the development of mechanical moduli, was also dependent on the type of protein isolates. The thermal denaturation of these proteins was nearly unaffected by the presence of reducing agent dithiothreitol (DTT), while the presence of DTT weakened the gel formation. The T_d of vicilin components and the mechanical moduli of corresponding formed gels were positively related to their SS contents. Additionally, the formed gels were thermo-irreversible, and heat pretreatment (carried out at temperatures higher than or close to the T_d of these vicilins) could improve the gel network formation. These results confirm that vicilin-rich protein isolates from various legumes show different patterns of thermal denaturation and gelation, and these properties are to a great extent related to their SH and/or SS contents.     

Roles of Water and Solids Composition in the Control of Glass Transition and Stickiness of Milk Powders

Abstract: Plasticization and glass transition of amorphous components in food powders often result in stickiness and caking. The glass transition temperature (T_g) of milk powders was measured by differential scanning calorimetry (DSC) and a viscometer method was used to determine sticky-point temperatures. Water sorption isotherms were established for varying solids compositions. Lactose contents were analyzed by high-performance anion exchange chromatography with pulsed amperometric detection (HPAE-PAD) and proteins were identified using SDS-PAGE gel electrophoresis. Solids composition and water affected both the T_g and stickiness behavior. Stickiness was governed by carbohydrates and water plasticization. At low protein contents, precrystallization of lactose decreased the sticky point temperature, but increasing protein content in all milk powders decreased stickiness at all water activities. The results showed that glass transition can be used to describe time-dependent stickiness and crystallization phenomena, and it can be used as a parameter to control and reduce stickiness of dairy solids with various compositions. Practical Application: Glass transition of component sugars in milk powders with various water contents was responsible for a solid-liquid transformation which resulted in their viscous flow at particle surfaces and stickiness of the powders. Stickiness leads to wall deposition in dehydration and caking of powders in storage when the amorphous carbohydrate-rich components gain liquid characteristics. High protein contents in milk powders decreased stickiness, but precrystallization of lactose prior to spray drying increased stickiness at low protein content. Milk powders in storage gained higher water contents with increasing protein contents, but stickiness was reduced and lactose crystallization was delayed which improved storage stability.     

Thermal Denaturation of Hake (Merluccius hubbsi) Myofibrillar Proteins. A Differential Scanning Calorimetric and Electrophoretic Study

Denaturation of hake (Merluccius hubbsi) proteins was studied with DSC by monitoring T_max of transitions and denaturation enthalpies. Whole muscle free of connective tissue showed two transitions (T_max 46.5°C and 75.3°C), and ΔH of 4.27 cal/g. The exudative sarcoplasmic fraction showed three transitions (T_max 45.2°C, 59.0°C and 75.5°C) and ΔH of 3.92 cal/g. The sarcoplasmic proteins from whole hake muscle contributed to both denaturation peaks. Muscle depleted of sarcoplasmic proteins by chloride extraction showed a higher thermal sensitivity and a diminished denaturation enthalpy on the second transition. This suggested an additional effect of chloride upon actin in addition to sarcoplasmic protein extraction. pH had an effect upon the native conformation of thick filament proteins, specifically myosin.     

Rheological and Thermal Properties of Extruded Mixtures of Rice Starch and Isolated Soy Protein

Many foods gain new mechanical, thermal and textural properties after being processed due to interactions between carbohydrates and proteins. This effect is characteristic for each foodstuff. The properties of extruded isolated soy protein (ISP) and rice starch were studied considering the following extrusion variables: starch proportion with respect to ISP (0–100%), pH (3–9), moisture content (20–30%) and temperature (140–180ºC). The following characteristics were measured: Water absorption index (WAI), water solubility index (WSI), glass transition temperature (T_g), melting temperature (T_m), viscosity at 90ºC and at 50ºC, storage (G′), loss modulus (G′′) and tan δ. The results indicate that the extruded starch exhibits higher WAI and WSI values than untreated starch. For extruded ISP these values are much lower than for untreated ISP. Extrudates with higher starch proportion had higher T_g and T_m values; pH has a significant effect (p<0.05), at pH 3 higher T_g values were observed, and at pH 9 higher values of T_m. The highest viscosities at 90ºC and 50ºC were observed for extrudates with a higher starch proportion and pH 9. Extruded mixtures showed a more elastic than viscous behavior and an extruded 1:1 blend of starch‐ISP exhibited the behavior of a viscous liquid.     

Enrichment of ice cream with dietary fibre: Effects on rheological properties,ice crystallisation and glass transition phenomena

In the present study, the effects of four dietary fibre sources (oat, wheat, apple and inulin) on the rheological and thermal properties of model sucrose–polysaccharides solutions and ice cream mixes were investigated. The content of fibre in insoluble compounds increased significantly the viscosity and the shear thinning behaviour of the model solutions and ice creams, due to the increase of total solids and the formation of networks comprised of hydrated cellulose and hemicellulose. The increase of soluble material did not alter significantly the rheology of the samples but limited the freezing point depression and elevated the glass transition temperatures, indicating a potential cryoprotective action. The use of oat and wheat fibre favoured viscosity development due to water-binding, whereas inulin caused a remarkable increase of glass transition temperature (T_g) in model solutions and ice cream mixes, indicating the reduction of water molecule mobility from the bulk aqueous phase to the ice crystals’ surface. Apple fibre addition greatly increased viscosity and elevated the T_g values, particularly in the presence of proteins. Thus, our results suggest the potential use of dietary fibres as crystallisation and recrystallisation phenomena controllers in frozen dairy products.     

Studies of Thermal Denaturation of Oat Globulin by Differential Scanning Calorimetry

Thermal denaturation of oat globulin was studied by differential scanning calorimetry (DSC). Prior heat treatments at 100°C and 110°C resulted in a progressive decrease in enthalpy (ΔH) indicating partial denaturation. Marked increase in denaturation temperature (T_d) and onset temperature (T_m) and decrease in width at half peak height (ΔT_1/2) suggest that the preheated protein assumed a more compact conformation or associated to a complex structure with higher thermal stability and cooperativity. The heated globulin was segregated into soluble and insoluble fractions containing native and denatured protein respectively. The denaturation kinetics of oat globulin was studied and results show a reaction order of 2.5 and an activation energy of 505 KJ/mol. Heat treatments caused a pronounced increase in activation energy and pre-exponential factor.     

Thermal Properties of Proteins in Chicken Broiler Tissues

The thermal behavior of breast and thigh muscles, blood and skin tissues of chicken broilers was evaluated by differential scanning calorimetry (DSC). Onset temperature of transition (T_o), maximum thermal transition (T_max) temperatures, and denaturation enthalpy (ΔH) were evaluated. Breast muscle exhibited a complex thermogram with five endothermic transitions at 57°C, 63°C, 67°C, 73°C, and 78°C at a heating rate of 10°C/min. Thigh muscle exhibited only three major transitions at 60°C, 66°C, and 76°C. Thermal curves of isolated protein fractions indicated that the thermal transitions in muscle corresponded to the denaturation of myosin, sarcoplasmic proteins, collagen and F-actin. An increase in the heating rate from 1.0° to 40°C/min significantly elevated the onset temperature of transition and major transition temperatures, as well as the enthalpy of denaturation. Enthalpy of the muscle system heated to various end-point temperatures, cooled and reheated, showed that myosin was completely denatured at 60°C, sarcoplasmic proteins at 70°C and actin at 80°C.     

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.     

GLASS TRANSITION TEMPERATURE (Tg) AND WATER ACTIVITY (aw) OF DEHYDRATED APPLE PRODUCTS

The onset glass transition temperature (T_go) of Granny Smith apple products was evaluated as a function of drying conditions (air drying and freeze-drying) and moisture and water activity (a_w) levels. T_go was determined in the 0–0.75 a_w range, and it was found to have a maximum value at a_w= 0 (T_go= 4.5 ± 1.6C) for freeze-dried apple and a minimum value at a_w= 0.75 (T_go= -81.4 ± 0.1C) for freeze-dried apple juice. A linear relationship between T_go and a_w was obtained in all cases. Theoretical T_go values at a_w= 0 were calculated using these equations, and were within the range of 3.0C (freeze-dried apple) and 10.5C (apple dehydrated at 30C). When Gordon and Taylor's linear model was applied to the experimental data of all the apple products, K values within the range 2.4 (freeze-dried apple juice) and 3.6 (apple dehydrated at 60C), and the theoretical T_g of solids values at a_w= 0 between the range -16.3C (freeze-dried apple juice) and -1.9C (apple dehydrated at 60C) were obtained. No effect of the type of drying on the value of T_g was detected.