Jorge Wagner

Hydrolysates of native and modified soy protein isolates: structural characteristics, solubility and foaming properties

Abstract The effect of enzymatic hydrolysis on structure characteristics (surface aromatic hydrophobicity and molecular size) of both native and modified soy protein isolates was studied. Effects on thermal behavior and functional properties (solubility and foam formation and stabilization at pH 4.5) were also analysed. Hydrolysates were obtained by bromelain digestion at pH 8 of native (N) and of thermally treated isolates at pH 7 (T7) and at pH 1.6 (T1.6). The differential effect of bromelain on the three isolates produced partially hydrolyzed structures, which exhibited an enhancement of their protein solubility ( S TCA and S pH 4.5 ). Bromelain digestion was more effective on isolate T7 resulting hydrolysates with improved capacity to form foams at pH 4.5. The different functional behavior at pH 4.5 of hydrolysates was explained through the changes in thermal behavior, surface aromatic hydrophobicity and molecular mass distribution.

Coalescence of o/w emulsions stabilized by whey and isolate soybean proteins. Influence of thermal denaturation, salt addition and competitive interfacial adsorption

Abstract The ability of both whey and isolate soy proteins to form oil in water (o/w) emulsions and prevent coalescence under controlled shear stress was evaluated. The effect of protein denaturation, salt addition and the simultaneous presence of whey and isolate proteins were studied. Native and denatured whey soy protein (NWSP and DWSP) showed similar emulsifying activity index and droplet size distribution to those of native and denatured soy isolate (NSI and DSI). The NSI emulsions were most resistant to coalescence. When increasing concentrations of NaCl were added to previously formed NSI emulsion, higher coalescence was obtained. Emulsions prepared with mixtures of NSI and either DSI, NWSP or DWSP should have more stability with increasing amounts of NSI in the blend. A small amount of DSI or DWSP in the aqueous phase showed a substantial influence on the coalescence.

Comparative study of foaming properties of whey and isolate soybean proteins

Abstract Whey soy proteins (WSP) is obtained from the soluble fraction separated in the isoelectric precipitation of the major components of native soy isolate (NSI), the 7S and 11S fractions. WSP is composed mainly by Kunitz Trypsin Inhibitor and Lectin. Water solubility of WSP was less affected by pH than that of NSI. Therefore, WSP has good foaming capacity in all pH range (pH 2–10). According to the rate of liquid incorporation into the foam, WSPs have higher tensioctive behavior, and in consequence, they are rapidly adsorbed into the water–air interface. At extreme pHs, with low or high ionic strength, good foams were obtained with WSP and NSI. Mixtures of both in different relations, showed better stabilization and foam formation than that expected for the simple additivity of both preparations. This effect was more evident at high ionic strength. These results indicate that there is a synergistic effect between these proteins in relation to foaming properties.

Relationship Between Interfacial Behaviour of Native and Denatured Soybean Isolates and Microstructure and Coalescence of Oil in Water Emulsions: Effect of Salt and Protein Concentration

The capacity of both native (NSI) and denatured (DSI) soybean isolates to stabilise oil in water emulsions under controlled shear stress was evaluated. The effect of protein concentration, thermal treatment of proteins and salt addition were studied. Sodium caseinate (SC) was used as standard protein. Emulsions prepared with NSI and SC were stable against coalescence in the whole range of protein concentration (1-10 mg/mL) in spite of showing different interfacial behaviour. The interfacial pressure of DSI was higher than NSI, according to its high dissociation degree and aromatic surface hydrophobicity. However, the emulsions prepared with this sample were unstable in the whole range of bulk protein concentrations. When NaCl was added, higher coalescence was obtained with NSI and SC emulsions at low protein concentrations, and stabilisation was reached only by increasing protein concentrations. At high protein concentrations(>5 mg/mL), DSI emulsions were stable in presence of salt, due to the formation of rigid flocs resistant to agitation. Droplet size distribution, microstructure and flocculation tendency of droplets explained the differences in coalescence of NSI, DSI and SC emulsions.

The effects of divalent cations in the presence of phosphate, citrate and chloride on the aggregation of soy protein isolate

Abstract The protein aggregation–dissociation process induced by divalent cations was studied using diluted aqueous dispersions of a native soy isolate. Turbidity, as a protein aggregation indicator, was quantified at a fixed time after addition of increasing amounts of CaCl 2 , MgCl 2 or an equimolar mixture of them. In all cases turbidity values attained with Ca 2+ were higher than these obtained with Mg 2+ . The effects of phosphate, citrate and chloride presence were also determined. Phosphate, becomes incorporated into Ca 2+ -induced aggregates, but not in Mg 2+ ones. Both citrate and chloride ions inhibit protein aggregation and contribute to the dissociation of previously formed aggregates. Low citrate concentrations allowed aggregate formation, even at high CaCl 2 concentrations; this behavior was not observed in the presence of MgCl 2 . A sedimentation based test was developed as a stability parameter of dispersions of soy protein aggregates. Results showed that even though aggregation was induced by divalent cations, competition between phosphate, citrate and soy protein could modify the aggregates properties. Optimum aggregation conditions were established from combinations of these variables.

Thermal behavior and hydration properties of yeast proteins from Saccharomyces cerevisiae and Kluyveromyces fragilis

Abstract High pressure homogenization of yeast cells followed by incubation at 50°C for the dissociation of ribonucleic acid-protein complexes resulted in a high denaturation degree of isolated proteins. Proteins in intact cells exhibited an ample endothermic peak with peak temperatures (TP) at 66.66 and 63.67°C for S. cerevisiae and K. fragilis, respectively. No differences were found with respect to the associated enthalpy changes for both studied species. The isolation of proteins from its biomass shifts TP to values around 50°C. Impurities such as nucleic acid, polysaccharides and other intracellular components seem to play a protective role upon denaturation. Isolated proteins showed solubilities lower than 40% but exhibited water retention properties and wettability from 3.5 to 7.0 ml of water/g of protein.

Thermal Denaturation Kinetics of Yeast Proteins in Whole Cells of Saccharomyces cerevisiae and Kluyveromyces fragilis

The kinetics of thermal denaturation of yeast proteins in intact cells of Saccharomyces cerevisiae instant dry yeast and Kluyveromyces fragilis L-1930 have been studied through differential scanning calorimetry. Maximum deflection-peak temperatures (TP) were greater for S. cerevisiae (66.65°C) than for K. fragilis (63.21 °C). Kinetic parameters showed that the former was slightly more resistant to thermal protein denaturation and exhibited a higher activation energy (Ea) of 63.80 kcal/mol compared to 42.92 kcal/mol of K. fragilis. Reaction rate constants (Kd) demonstrated that the shelf-life of yeast proteins at temperatures above 60°C, in whole cells was very limited. Se estudió la cinética de desnaturalizaciòn térmica de las proteínas de levadura en células intactas de Saccharomyces cerevisiae y Kluyveromyces fragilis mediante calorimetría diferencial de barrido (CDB). Los picos de temperaturas máximas de deflexión (TP) fueron mayores para S. cerevisiae (66.65°C) que para K. fragilis (63.21 °C). Los parámetros cinéticos mostraron que la primera especie fue más resistente a la desnaturalización térmica de las proteínas y exhibió una energía de activación (Ea) de 63.80 kcal/mol frente a 42.92 kcal/mol de K. fragilis. Las constantes de reacción (Kd) demostraron que la vida media de las proteínas de levadura a temperaturas superiores a 60°C, en las células enteras, es muy limitada.