Enzymatic hydrolysis as a means of expanding the cold gelation conditions of soy proteins

Effect of soy peptides with different hydrolysis degrees on the rheological, tribological, and textural properties of soy protein isolate gels

This study was performed to examine the effect of two soy peptides addition with hydrolysis degrees of 90% and 30% (hydrolysis degree (DH)90, DH30) at various concentrations (1-10 mg/mL) on soy protein isolate (SPI) gel behavior and pure SPI gel was set as control. SPI gels with adding peptides were prepared, and their rheological, textural, and tribological properties, as well as water-holding capacity, zeta potential, and particle size, were determined. During the rheological measurement, adding peptides reduced storage modulus (G') compared to the control, with larger particles formed. However, peptide addition could significantly reduce gelation time, showing a more significant effect with DH30. The gels' firmness, adhesiveness, and water-holding capacity decreased as peptide concentration increased. Syneresis was observed in gels with peptides, whereas the control sample showed no syneresis. Based on the rheological results, the shear stress in the control sample was higher than in the gels containing peptides indicating more resistance to shear. The gels with DH30 showed greater G' and G″ than DH90 at all studied concentrations. Nevertheless, there was an improvement in the lubrication behavior of SPI gels with peptide addition. DH30 showed a relatively more significant friction reduction than DH90, indicating their slightly better lubrication properties.

Soy peptide aggregates formed during hydrolysis reduced protein extraction without decreasing their nutritional value

Upon enzymatic hydrolysis, soy protein isolates showed a strong tendency to aggregate, presenting a significant loss of valuable proteins. This study mainly focused on the large insoluble aggregates formed during proteolysis, and the influence of heating was further explored for a better understanding of the mechanism involved. The results from SDS-PAGE and amino acid analysis clearly showed that the insoluble aggregates formed upon hydrolysis were aggregated peptides, mainly attributed to the hydrophobic interactions between peptides with hydrophobic amino acids (Val, Ala, Leu, Ile, Tyr, Phe, and Pro) and sulfur-containing (Met and Cys) residues. Heating of the hydrolysates further enhanced the peptide-protein interactions through hydrophobic forces and disulfide bonds, accelerating the aggregation, where fractions from the basic subunits of glycinin were particularly involved. Furthermore, taking into consideration the fact that aggregates had a high proportion of essential amino acids, the in vitro digestion properties of the aggregates were also investigated. Interestingly, the relatively pepsin-resistant aggregates showed a high degradability toward pancreatin, releasing low molecular weight peptides possessing a higher proportion of antioxidative amino acids, which therefore had a better antioxidant activity. These results indicated a potential use of the insoluble peptide aggregates as protein supplements or active delivery systems for human consumption.

Peptides: Production, bioactivity, functionality, and applications

Production of peptides with various effects from proteins of different sources continues to receive academic attention. Researchers of different disciplines are putting increasing efforts to produce bioactive and functional peptides from different sources such as plants, animals, and food industry by-products. The aim of this review is to introduce production methods of hydrolysates and peptides and provide a comprehensive overview of their bioactivity in terms of their effects on immune, cardiovascular, nervous, and gastrointestinal systems. Moreover, functional and antioxidant properties of hydrolysates and isolated peptides are reviewed. Finally, industrial and commercial applications of bioactive peptides including their use in nutrition and production of pharmaceuticals and nutraceuticals are discussed.

Novel antioxidative peptides from the protein hydrolysate of oysters (Crassostrea talienwhanensis)

The antioxidative activity of hydrolysate peptides from oysters (Crassostrea talienwhanensis) was investigated. After hydrolysis with subtilisin, the yields of the peptides that were soluble in trichloroacetic acid (TCA-soluble) and the antioxidant activities of the resulting hydrolysate were determined using an orthogonal design and a hydroxyl radical scavenging reaction. The hydrolysate was fractionated using Sephadex G-15 gel filtration chromatography, and the two resulting bioactive peptides were subsequently purified by RP-HPLC with a Kromasil C18 (ODS) column. The amino acid sequences were analyzed by nano-ESI-MS/MS. The critical reaction temperature, pH, hydrolysis time and enzyme-to-substrate (E/S) ratio were determined for the optimum hydrolysis with subtilisin, and the E/S ratio was found to be the most critical reaction condition. The amino acid sequences of the peptides (518 and 440 Da) were proline-valine-methionine-glycine-aspartic acid (PVMGA) and glutamine-histidine-glycine-valine (QHGV), respectively. These two novel peptides exhibited high antioxidative actions based on their hydroxyl and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activities.

Morphology of molecular soy protein fractions in binary composite gels

We investigate the structural properties of gels of binary mixtures of the three major soy protein fractions: 11S, 7S, and 2S. Gels are formed at 25 degrees C in the presence of glucono-delta-lactone and studied using a combination of dynamic rheology and scanning electron microscopy. The theological data was then modeled using a blending-law approach that yields insights into the solvent distribution between the gelled protein fractions and first-order reaction kinetics that follow the gelation process of the single fractions and their mixtures. Gelled mixtures of 11S and 7S yielded enhanced network strength with increasing solid content; in these gels, 50% more solvent partitioned into the 11S phase as compared to that in the 7S phase. In contrast, the addition of small-molecular-weight counterpart 2S to either 11S or 7S results in a catastrophic drop in the values of the overall strength of the mixture. The unexpected phase behavior has been rationalized on the basis of the high water-holding capacity of 2S; 450% more solvent partitions preferentially into the 2S phase as compared to that in the 11S phase. As the concentration of 2S is increased relative to that of 11S or 7S, it becomes the dominant phase and entraps the polymeric segments of 11S (or 7S), thus preventing them from becoming the structural knots of the gel. In addition to the solvent distribution in the gel, the rates of gelation differ markedly between 11S and 2S (with the 11S rate of gelation being up to 2 orders of magnitude greater); a fixed 11S concentration, the rate of gelation decreases with increasing amounts of 2S, further confirming that the latter essentially becomes the dominant phase in the composite gel.

Effects of thermal treatment on the coagulation of soy proteins induced by subtilisin Carlsberg

The effects of thermal treatment on the subtilisin Carlsberg-induced coagulations of soy protein isolate (SPI) and soy proteins in 7S and 11S fractions, most of which are beta-conglycinin and glycinin, respectively, were examined by measuring the turbidity (OD(660)) of the reaction solutions. With the treatment at 37-60 degrees C, the turbidity did not increase at all by the proteolysis, while with the treatment at 70-96 degrees C, it drastically increased. The degree of the coagulation is the highest for the treatment at 80 degrees C and the most remarkable for 11S soy protein. Changes in the sodium dodecyl sulfate-polyacrylamide gel electrophoresis pattern of the digests during the proteolysis were in good agreement with those in the turbidities for SPI and 7S and 11S soy proteins. Circular dichroism analysis revealed that the amounts of nonstructured protein in SPI and 7S and 11S soy proteins were initially 40-50%, increased to 55-60% by the treatment at 80 degrees C, and further increased to 65-75% by the proteolysis. The maximum fluorescence intensity of SPI and 7S and 11S soy proteins increased with an increase in the incubation temperature up to 80 degrees C. These findings suggest that the thermal treatment at 80 degrees C most effectively changes the secondary structure of soy proteins and renders them coagulate when hydrolyzed by subtilisin Carlsberg.

Legumin allergens from peanuts and soybeans: effects of denaturation and aggregation on allergenicity

Legumin proteins Ara h 3 from peanuts and glycinin from soybeans are increasingly described as important allergens. The stability of an allergen's IgE binding capacity towards heating and digestion is considered an important characteristic for food allergens. We investigated the effects of heating and digestion on the IgE binding of Ara h 3 and glycinin. Both proteins are relatively stable to denaturation, having denaturation temperatures ranging from 70 to 92 degrees C, depending on their quaternary structure and the ionic strength. Aggregates were formed upon heating, which were partly soluble for glycinin. Heating slightly decreased the pepsin digestion rate of both allergens. However, heating did not affect the IgE binding capacity of the hydrolyzates, as after only 10 min of hydrolysis no IgE binding could be detected any more in all samples. Peanut allergen Ara h 1, when digested under equal conditions, still showed IgE binding after 2 h of hydrolysis. Our results indicate that the IgE binding capacity of legumin allergens from peanuts and soybeans does not withstand peptic digestion. Consequently, these allergens are likely unable to sensitize via the gastro-intestinal tract and cause systemic food allergy symptoms. These proteins might thus be less important allergens than was previously assumed.

Identification of strong aggregating regions in soy glycinin upon enzymatic hydrolysis

Upon hydrolysis with chymotrypsin, soy glycinin has a strong tendency to aggregate. The regions of glycinin from which the aggregating peptides originate were identified by accumulative-quantitative peptide mapping. To this end, the aggregating peptides were further hydrolyzed with trypsin to obtain peptides of which the sequence can be identified using RP-HPLC-MS/MS. This resulted in a hydrolysate in which 90% of the proteinaceous material was dissolved. The soluble fraction was analyzed using the method of accumulative-quantitative peptide mapping: fractionation using ion exchange chromatography, followed by identification of peptides by RP-HPLC-MS/MS, quantification based on the absorbance at 214 nm, and finally peptide mapping. For the peptide mapping the proportions in which each of the five glycinin subunits are present, as determined by Edman degradation, were taken into account. The results showed that mainly the basic polypeptide and a part of the acidic polypeptide, close to the location of the disulfide bridge between the basic and acidic polypeptides, are present in the aggregating peptide fraction. On the basis of the results obtained, an aggregation mechanism was proposed. The hydrophilic acidic polypeptides shield the hydrophobic basic polypeptides, and the former are preferentially degraded upon hydrolysis. This results in a net increase in hydrophobicity of the remaining material, which mainly consists of the basic polypeptide fragments. This increase in hydrophobicity is proposed to be the driving force in the aggregation of chymotrypsin-derived peptides of glycinin.

Phase model interpretation of the structural properties of two molecular soy protein fractions

The structural properties of mixtures of two molecular soy fractions, 11S (glycinin) and 2S, were investigated in the presence of glucono-delta-lactone (GDL) during an isothermal run at 25 degrees C for 1500 min. Analytical methodology included small-deformation dynamic oscillation, visual observations, scanning electron microscopy, and blending-law modeling. The aim of the work was to identify the state of phase separation and the pattern of solvent distribution between the two constituent polymers. It was found that the high molecular weight distribution of 11S supported rapid kinetics of structure formation, with this continuous matrix being concentrated and, hence, further reinforced in the presence of small additions of 2S. Blending-law modeling suggested that 2S was able to retain disproportionate volumes of solvent within its phase. A consequence of such property was that high additions of 2S leading to equal concentrations of the two molecular fractions in the blend resulted in a catastrophic drop in the values of the overall network strength. This behavior is rather unexpected for the structural properties of a phase-separated system, and it has been rationalized on the basis of the high water-holding capacity of the small molecular weight fraction. 2S entraps in its phase the polymeric segments of glycinin, which are then unable to become structural knots of a cohesive three-dimensional morphology observed in single 11S preparations.

Micrometer-sized fibrillar protein aggregates from soy glycinin and soy protein isolate

Long, fibrillar semiflexible aggregates were formed from soy glycinin and soy protein isolate (SPI) when heated at 85 degrees C and pH 2. Transmission electron microscopy analysis showed that the contour length of the fibrils was approximately 1 microm, the persistence length 2.3 microm, and the thickness a few nanometers. Fibrils formed from SPI were more branched than the fibrils of soy glycinin. Binding of the fluorescent dye Thioflavin T to the fibrils showed that beta-sheets were present in the fibrils. The presence of the fibrils resulted in an increase in viscosity and shear thinning behavior. Flow-induced birefringence measurements showed that the behavior of the fibrils under flow can be described by scaling relations derived for rodlike macromolecules. The fibril formation could be influenced by the protein concentration and heating time. Most properties of soy glycinin fibrils are comparable to beta-lactoglobulin fibrils.

Functional region identification in proteins by accumulative-quantitative peptide mapping using RP-HPLC-MS

A new method was developed to identify regions in proteins from which peptides are derived with specific functional properties. This method is applicable for systems in which peptides of a hydrolyzed protein possess specific functional properties, but are too large to be sequenced directly and/or the peptide mixture is too complex to purify and characterize each peptide individually. In the present work, aggregating peptides obtained by proteolytic hydrolysis of soy glycinin were used as a case study. The aggregating peptides are isolated and subsequently further degraded with trypsin to result in peptides with a mass <5000 Da to enable sequence identification using RP-HPLC-MS in combination with MS/MS. Prior to RP-HPLC the peptides are fractionated using anion and cation exchange chromatography. The fractions obtained are analyzed with RP-HPLC-MS. The peptides, with identified sequences, were quantified using the peak areas of the RP-HPLC chromatograms measured at 214 nm. Next, the peak areas were corrected for the molar extinction coefficient of the individual peptides, followed by accumulative-quantitative peptide mapping. The results show that in complex systems, based on the method described, the regions in the parental protein from which the functional peptides originate can be properly identified.

Comparison of the aggregation behavior of soy and bovine whey protein hydrolysates

Soy-derived proteins (soy protein isolate, glycinin, and beta-conglycinin) and bovine whey-derived proteins (whey protein isolate, alpha-lactalbumin, beta-lactoglobulin) were hydrolyzed using subtilisin Carlsberg, chymotrypsin, trypsin, bromelain, and papain. The (in)solubility of the hydrolysates obtained was studied as a function of pH. At neutral pH, all soy-derived protein hydrolysates, particularly those from glycinin, obtained by hydrolysis with subtilisin Carlsberg, chymotrypsin, bromelain, and papain showed a stronger aggregation compared to the non-hydrolyzed ones. This increase in aggregation was not observed upon hydrolysis by trypsin. None of the whey-derived protein hydrolysates exhibited an increase in aggregation at neutral pH. The high abundance of theoretical cleavage sites in the hydrophobic regions of glycinin probably explains the stronger exposure of hydrophobic groups than for the other proteins, which is suggested to be the driving force in the aggregate formation.