Sara E. Molina Ortiz

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.

Dynamic properties of soy globulin adsorbed films at the air-water interface

In this paper we present surface dilatational properties of soy globulins (beta-conglycinin, glycinin, and reduced glycinin with 10 mM of dithiothreitol (DTT)) adsorbed onto the air-water interface, as a function of adsorption time. The experiments were performed at constant temperature (20 degrees C), pH (8.0), and ionic strength (0.05 M). The surface rheological parameters were measured as a function of protein concentration (ranging from 1 to 1x10(-3)% wt/wt). We found that the surface dilatational modulus, E, increases, and the phase angle, phi, decreases with time, theta, which may be associated with protein adsorption. These phenomena have been related to protein adsorption, unfolding, and/or protein-protein interactions (at long-term adsorption) as a function of protein concentration in solution. From a rheological point of view, the surface viscoelastic characteristics of soy globulin films adsorbed at the air-water interface are practically elastic. The main conclusion is that the dilatational properties of the adsorbed films depend on the molecular structure of the protein.

Amaranth protein isolates modified by hydrolytic and thermal treatments. Relationship between structure and solubility

Abstract Structure and solubility of modified amaranth isolates were studied. Isolates were obtained from proteolytic (cucurbita or papain) and/or thermal treatments of an amaranth isolate. Results showed that 11S-globulin and globulin-P, the main targets for the proteases, were hydrolyzed more efficiently by papain than by cucurbita. Thermal treatment induced both aggregation and dissociation of proteins. Dissociation increased with hydrolysis. It was also shown that polymers and globulin molecules were responsible for the insolubility of a non-treated isolate and that their hydrolytic modification made the isolate more soluble. While a thermal treatment decreased the isolate solubility, a previous hydrolysis with papain increased the solubility. Consequently, the amaranth isolate hydrolyzed with papain is a suitable ingredient in foods submitted to thermal treatment considering that it keeps a high solubility after heating.

Analysis of products, mechanisms of reaction, and some functional properties of soy protein hydrolysates

Soybean protein isolates were hydrolyzed with papain, bromelain, cucurbita, hieronymin, and pomiferin. The highest hydrolysis rate for cucurbita and the lowest for papain was detected at 720 min. Gel filtration, reversed-phase liquid chromatography, and electrophoresis showed that the action of each protease was different. Pomiferin acted on the native structure of β-conglycinin and glycinin, generating a large number of small hydrolysis products with hydrophilic and hydrophobic characteristics. The hydrolysates obtained with cucurbita showed residual structures that were almost intact and very similar to the substrate and contained a low number of small peptides. The hydrolyzates obtained with papain, bromelain, and hieronymin had hydrolysis products with structures similar to the partially hydrolyzed native isolate. The molecular masses of these products were similar to or lower than the controls. Polypeptides of low molecular mass were also detected. The prevalence of one-by-one and zipper mechanisms was suggested for cucurbita and pomiferin, respectively. For the other proteases both mechanisms were likely to coexist. The solubility of hydrolysates and their ability to form and stabilize foams correlated well with the structural properties and with the suggested mechanisms of hydrolysis. The best properties were presented by the hydrolysates prepared with cucurbita. Foaming ability for pomiferin hydrolysates was as high as that for unhydrolyzed soy isolate, but the foams were extremely unstable.