Physicochemical properties of β and α'α subunits isolated from soybean β-conglycinin

Soy protein has shown great potential for use in biobased adhesives. β-Conglycinin is a major component of soy protein; it accounts for 30% of the total storage protein in soybean seeds. β-Conglycinin was isolated and purified, and its subunits' (β, α'α) physicochemical and adhesive properties were characterized. Crude β-conglycinin was isolated from soy flour and then purified by the ammonium sulfate precipitation method. The α'α and β subunits were isolated from the purified β-conglycinin by anion exchange chromatography. Yields of α'α subunits and β subunits from 140 g of soy flour were 1.86 g (1.3%) and 0.95 g (0.67%), respectively. The minimum solubility for α'α subunits, β subunits, and β-conglycinin occurred in pH ranges of 4.1-5.4, 3.5-7.0, and 4.8-5.3, respectively. Transmission electron microscopy showed that the β subunits existed as spherical hydrophobic clusters, whereas α'α subunits existed as uniformly discrete particles at pH 5.0. Differential scanning calorimetry showed that β subunits had higher thermal stability than α'α subunits. The pH had a lesser effect on adhesion strength of the β subunits than on that of the α'α subunits. The adhesives made from β subunits also showed greater water resistance than those from α'α subunits and β-conglycinin. Soy protein rich in β subunits is likely a good candidate for developing water-resistant adhesives.     

Heat-induced soy-whey proteins interactions: formation of soluble and insoluble protein complexes

The aggregation behavior during heating of a solution containing soy protein and whey protein isolate (WPI) was studied using rheology, confocal microscopy, gel filtration, and electrophoresis. Soy/WPI mixtures formed gels at 6% total protein concentration with a high elastic modulus (G') and no apparent phase separation. The ratio of soy to WPI was fundamental in determining the type of network formed. Systems containing a high soy to WPI ratio (>70% soy protein) showed a different evolution of the elastic modulus during heat treatment, with two apparent stages of network development. Whey proteins formed disulfide bridges with soy proteins during heating, and at low ratios of soy/WPI, the aggregates seemed to be predominantly formed by 7S, the basic subunits of 11S and beta-lactoglobulin. Size exclusion chromatography indicated the presence of high molecular weight soluble complexes in mixtures containing high soy/WPI ratios. Results presented are the first evidence of interactions between soy proteins and whey proteins and show the potential for the creation of a new group of functional ingredients.     

Study of the effect of soy proteins on the acid-induced gelation of casein micelles

The objective of this research was to understand whether addition of soy protein to milk protein affects the properties of acid-induced casein gels. Different samples were prepared by suspending casein micelles pellets in milk serum containing soy proteins or whey proteins as well as mixtures of the two proteins. Glucono-delta-lactone was added, and the changes in apparent size (in diluted systems) as well as the viscoelastic properties of the mixtures were measured. Size exclusion chromatography was also carried out to characterize the soluble phase of the various mixtures before and after heating. Soy protein affected the gelation of the mixtures; however, not to the same extent as whey proteins, which dominated formation of the network in soy-whey-casein systems. It was concluded that, up to a critical ratio of soy/whey proteins, soy proteins can be incorporated in the mix without a significant change in structure of the casein gels.     

Growth kinetics of amyloid-like fibrils derived from individual subunits of soy β-conglycinin

The amyloid-like fibrillation of soy β-conglycinin subunits (α, α', and β) upon heating (0-20 h) at 85 °C and pH 2.0 was characterized using dynamic light scattering, circular dichroism (CD), binding to amyloid dyes (Thioflavin T and Congo red), and atomic force microscopy. The fibrillation of all three subunits was accompanied by progressive polypeptide hydrolysis. The hydrolysis behaviors, fibrillation kinetics, and morphologies of amyloid-like fibrils considerably varied among α, α', and β subunits. Faster hydrolysis rates and special fragments were observed for the α and α' subunits compared to the β subunit. However, the order of the fibrillation rate and capacity to form β-sheets was α' > β > α, as evidenced by CD and Thioflavin T data. Moreover, sequential growth of twisted screw-structure fibrils, leading to macroscopic fibrils with distinct morphological characteristics, was observed for β-conglycinin and individual subunits. The different fibrillation kinetics and morphologies of α, α', and β subunits appear to be associated with the differences in the amino acid composition and typical sequence of peptides. Besides, the disruption of ordered structure of fibrils occurred upon further heating (6-20 h) due to extensive hydrolysis. These results would suggest that all subunits are involved in the fibrillation of β-conglycinin, following multiple steps including polypeptide hydrolysis, assembly to amyloid structure, and growth into macroscopic fibrils with a fibril shaving process.     

Effect of soy milk characteristics and cooking conditions on coagulant requirements for making filled tofu

The amount of coagulant added to soy milk is a critical factor for tofu-making; particularly it affects the textural properties of tofu. Earlier research indicated that the critical point of coagulant concentration (CPCC) is a characteristic parameter of soy milk and could be used as an effective indicator of optimal coagulant concentration (OCC) for making filled tofu. The objective of this study was to investigate the possible correlations between CPCC and the characteristics of soy milk made from various soybean samples and the effect of soy milk cooking and dilution conditions on CPCC. CPCC was determined by a titration method. Calcium chloride and magnesium chloride were used as coagulants. Soy milk characteristics including solid, protein, phytate, pH, titratable acidity, mineral content, and 11S/7S protein and these characteristics as affected by heating rate, heating time, and sequence of dilution and heating were studied. The results showed that the CPCC was significantly (p < 0.05) positively correlated with phytate content (grams per gram of protein), pH, and 7S protein content but negatively correlated with protein content, 11S protein content, 11S/7S ratio, titratable acidity, and original calcium content. Within the same soybean material, more proteins required more coagulant, but higher protein concentration during cooking resulted in less coagulant required by each gram of protein during coagulation. The CPCC decreased with increasing soy milk heating time or decreasing heating rate. The sequence of heating and diluting for preparing soy milk also had an effect on CPCC.     

Effects of ultrasound pretreatment on the enzymatic hydrolysis of soy protein isolates and on the emulsifying properties of hydrolysates

Soy protein isolate (SPI) was modified by ultrasound pretreatment (200 W, 400 W, 600 W) and controlled papain hydrolysis, and the emulsifying properties of SPIH (SPI hydrolysates) and USPIH (ultrasound pretreated SPIH) were investigated. Analysis of mean droplet sizes and creaming indices of emulsions formed by SPIH and USPIH showed that some USPIH had markedly improved emulsifying capability and emulsion stabilization against creaming during quiescent storage. Compared with control SPI and SPIH-0.58% degree of hydrolysis (DH), USPIH-400W-1.25% (USPIH pretreated under 400W sonication and hydrolyzed to 1.25% DH) was capable of forming a stable fine emulsion (d43=1.79 μm) at a lower concentration (3.0% w/v). A variety of physicochemical and interfacial properties of USPIH-400W products have been investigated in relation to DH and emulsifying properties. SDS-PAGE showed that ultrasound pretreatment could significantly improve the accessibility of some subunits (α-7S and A-11S) in soy proteins to papain hydrolysis, resulting in changes in DH, protein solubility (PS), surface hydrophobicity (H0), and secondary structure for USPIH-400W. Compared with control SPI and SPIH-0.58%, USPIH-400W-1.25% had a higher protein adsorption fraction (Fads) and a lower saturation surface load (Γsat), which is mainly due to its higher PS and random coil content, and may explain its markedly improved emulsifying capability. This study demonstrated that combined ultrasound pretreatment and controlled enzymatic hydrolysis could be an effective method for the functionality modification of globular proteins.     

Nutritional and health benefits of soy proteins

Soy protein is a major component of the diet of food-producing animals and is increasingly important in the human diet. However, soy protein is not an ideal protein because it is deficient in the essential amino acid methionine. Methionine supplementation benefits soy infant formulas, but apparently not food intended for adults with an adequate nitrogen intake. Soy protein content of another essential amino acid, lysine, although higher than that of wheat proteins, is still lower than that of the milk protein casein. Adverse nutritional and other effects following consumption of raw soybean meal have been attributed to the presence of endogenous inhibitors of digestive enzymes and lectins and to poor digestibility. To improve the nutritional quality of soy foods, inhibitors and lectins are generally inactivated by heat treatment or eliminated by fractionation during food processing. Although lectins are heat-labile, the inhibitors are more heat-stable than the lectins. Most commercially heated meals retain up to 20% of the Bowman-Birk (BBI) inhibitor of chymotrypsin and trypsin and the Kunitz inhibitor of trypsin (KTI). To enhance the value of soybeans in human nutrition and health, a better understanding is needed of the factors that impact the nutrition and health-promoting aspects of soy proteins. This paper discusses the composition in relation to properties of soy proteins. Also described are possible beneficial and adverse effects of soy-containing diets. The former include soy-induced lowering of cholesterol, anticarcinogenic effects of BBI, and protective effects against obesity, diabetes, irritants of the digestive tract, bone, and kidney diseases, whereas the latter include poor digestibility and allergy to soy proteins. Approaches to reduce the concentration of soybean inhibitors by rearrangement of protein disulfide bonds, immunoassays of inhibitors in processed soy foods and soybean germplasm, the roles of phytoestrogenic isoflavones and lectins, and research needs in all of these areas are also discussed. This integrated overview of the widely scattered literature emphasizes general concepts based on our own studies as well as recent studies by others. It is intended to stimulate interest in further research to optimize beneficial effects of soy proteins. The payoff will be healthier humans and improved animal feeds.     

Degradation behavior of soy protein-wheat gluten films in simulated soil conditions

Films containing soy protein and wheat gluten were exposed to simulated farmland soil mix over a period of 30 days and monitored for degradation. The simulated farmland soil mix (topsoil/sand/Sunshine compost/vermiculite, 59:6:25:10, wt %) was mixed and stored at ambient humidity (48-55%) and temperature (20-24 degrees C); the soil mix was constantly maintained at 15% moisture by weight. Research focused on evaluating the effectiveness of gluten and cysteine additions on biodegradable behavior in the simulated farmland soil conditions. The four types of films, soy protein (S:G 1:0); soy protein with cysteine addition (S:G 1:0 + CYS); soy protein-wheat gluten (S:G 4:1); and soy protein-wheat gluten with cysteine addition (S:G 4:1 + CYS), were prepared at pH 7. 0 for degradation studies. Soy protein-gluten film rapidly degraded with 50% weight loss in about 10 days and with up to 95% weight loss in 30 days. Tensile strength and elongation of all soy protein-gluten films significantly decreased in 3 days. However, cysteine addition delayed the degradation rate of soy protein-gluten films. Soy protein-wheat gluten film disintegrated after 20 days in the simulated farmland soil environment. These results suggest that wheat gluten and cysteine addition to soy protein-based films could delay degradation rates due to their high disulfide contents.     

Lunasin concentration in different soybean genotypes, commercial soy protein, and isoflavone products

Lunasin is a unique and novel cancer preventive peptide originally isolated from soy. Information on lunasin concentration of soybean cultivars and commercial soy proteins would be useful in developing lunasin-enriched cultivars and soy products. We report the development of an enzyme-linked immunosorbent assay (ELISA) method to identify lunasin and quantify the variations in concentration in 144 selected, diverse soybean accessions from the U.S. Department of Agriculture Soybean Germplasm Collection, several commercially available soy protein fractions and isoflavone-enriched products. With synthetic lunasin and monoclonal antibody, ELISA shows a linear concentration range of 24-72 ng/mL, good reproducibility, a detection limit of 8 ng/mL, and a recovery of 90% on spiked soy samples. Lunasin concentrations in the tested materials range from 0.10 to 1.33 g/100 g flour. Differences that exceeded 100% have been observed among accessions of similar maturity that were grown in the same environment, indicating that genetic differences in soybeans exist for lunasin. The mean of 23 major ancestral lines of U.S. cultivars is similar to the mean of 16 modern cultivars selected to represent the current diversity of the crop, but the highest values were found within the ancestral and exotic accessions. Soy protein concentrate, isolate, and hydrolyzate contain 2.81 +/- 0.30, 3.75 +/- 0.43, and 4.43 +/- 0.59 g lunasin/100 g flour, respectively, while soy flour and soy flakes contain 1.24 +/- 0.22 g lunasin/100 g flour. Isoflavone-enriched products contain very little or no lunasin. The relative mass (M(r)) of lunasin in the samples is 5.45 +/- 0.25 kDa. The wide range of lunasin concentrations within the Glycine max species indicates that the levels of this important bioactive peptide can be genetically manipulated. Furthermore, soy isolates and hydrolyzed soy proteins contain the highest concentrations of lunasin.     

Fast characterization of industrial soy protein isolates by direct analysis with matrix-assisted laser desorption ionization time-of-flight mass spectrometry

Industrial soy protein isolates (SPIs) due to differences in their processing conditions may differ both in composition and in degree of hydrolysis. As a result, they display different performance in food production and final food properties like consistency and taste. To address this issue, a fast, cheap, and simple method for screening and characterization is required. In this article, the successful analysis of soy protein isolates, a complex mixture of proteins with glycinin and beta-conglycinin as major components, by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) is demonstrated. The preparation implements a fast extraction of the proteins from the raw SPI either under neutral or reducing conditions. The extracts are analyzed subsequently by MALDI-TOF-MS without further purification. Results of the two conditions are compared. Finally, different SPIs from different suppliers are analyzed and compared concerning their consistency. The method could be applied to other plant proteins and mixtures thereof. Since the composition and intactness of different subunits play important roles in functional properties of soy proteins, rapid methods for fingerprinting of different industrial soy protein sources will be valuable tools for successful product formulation.     

Emulsifying properties of acidic subunits of soy 11S globulin

The emulsifying properties of the acidic subunits (AS11S) isolated from soy glycinin (11S) have been studied. The isolated AS11S existed in solution mainly as a dimer species. Circular dichroic analysis indicated only a slight increase in aperiodic structure and no significant difference in beta-sheet structure when compared with those of soy 11S. At similar experimental conditions, the emulsifying properties of AS11S were superior to those of soy 11S and heat-denatured 11S. Emulsions prepared with 1% AS11S remained very stable without any visible oil separation for more than a month under gentle agitating conditions, whereas those prepared with 1% 11S collapsed and separated into phases within 2-3 days. The AS11S-stabilized emulsions were very stable below 0.15 M ionic strength. Studies on the rate of adsorption and surface tension reduction at the air-water interface showed that AS11S was significantly more surface active than soy 11S. It is proposed that, because the mass fraction of acidic subunits in soy 11S is approximately 60% and it is relatively easy to separate the acidic subunits from soy 11S, it may be industrially feasible to develop an economical process to isolate functional acidic subunits for use in emulsion-based food products.     

微波对大豆蛋白氧化聚集体结构及功能特性的影响

为了探究不同时间微波处理对大豆蛋白氧化聚集体的结构和功能性质的影响,由偶氮二异丁脒盐酸盐(2,2′-azobis (2-amidinopropane) dihydrochloride,AAPH)诱导构建大豆蛋白氧化反应体系,采用功率为350 W的微波对其照射不同时间(0、10、20、30、40、50、60、70 s),探究微波处理对氧化聚集大豆蛋白的结构特性和加工特性的影响。结果表明,氧化可诱导形成粒径、分子量更大,结构更致密的蛋白质聚集体,同时对加工特性造成损害。适当时间(<30 s)的微波处理会导致氧化聚集体的分子结构打开、粒径降低和浊度降低,无序结构减少,进而改善了起泡性、乳化性和持水、持油性。长处理时间(>30 s)的微波处理导致已解聚的大豆蛋白分子重新形成更大的分子聚集体,降低功能性质。这表明微波物理场可以通过改变大豆蛋白氧化聚集体的结构和聚集行为调节其功能性质,为大豆蛋白功能性质的改善及微波在大豆蛋白氧化聚集体行为调控的应用方面提供参考。     

Oxalate and phytate of soy foods

The consumption of foods made from soybeans is increasing because of their desirable nutritional value. However, some soy foods contain high concentrations of oxalate and/or phytate. Oxalate is a component of calcium oxalate kidney stones, whereas phytate is an inhibitor of calcium kidney stone formation. Thirty tested commercial soy foods exhibited ranges of 0.02-2.06 mg oxalate/g and 0.80-18.79 mg phytate/g. Commercial soy foods contained 2-58 mg of total oxalate per serving and 76-528 mg phytate per serving. Eighteen of 19 tofu brands and two soymilk brands contained less than 10 mg oxalate per serving, defined as a low oxalate food. Soy flour, textured vegetable soy protein, vegetable soybeans, soy nuts, tempeh, and soynut butter exhibited greater than 10 mg per serving. The correlation between oxalate and phytate in the soy foods was significant (r = 0.71, P < 0.001) indicating that oxalate-rich soy foods also contain higher concentrations of phytate. There also was a significant correlation, based on molar basis, between the divalent ion binding potential of oxalate plus phytate and calcium plus magnesium (r = 0.90, P < 0.001) in soy foods. Soy foods containing small concentrations of oxalate and moderate concentrations of phytate may be advantageous for kidney stone patients or persons with a high risk of kidney stones.     

Formulation engineering can improve the interfacial and foaming properties of soy globulins

In this contribution, we have analyzed the effect of different strategies, such as change of pH (5 or 7) or ionic strength (at 0.05 and 0.5 M), and addition of sucrose (at 1 M) and Tween 20 (at 1 x 10(-4) M) on interfacial characteristics (adsorption, structure, dynamics of adsorption, and surface dilatational properties) and foam properties (foam capacity and stability) of soy globulins (7S and 11S at 0.1 wt %). We have observed that (1) the adsorption (presence of a lag period, diffusion, and penetration at the air-water interface) of soy globulins depends on the modification in the 11S/7S ratio and on the level of association/dissociation of these proteins by varying the pH and ionic strength (I), the effect of sucrose on the unfolding of the protein, and the competitive adsorption between protein and Tween 20 in the aqueous phase. The rate of adsorption increases at pH 7, at high ionic strength, and in the presence of sucrose. (2) The surface dilatational properties reflect the fact that soy globulin adsorbed films exhibit viscoelastic behavior but do not have the capacity to form a gel-like elastic film. The surface dilatational modulus increases at pH 7 and at high ionic strength but decreases with the addition of sucrose or Tween 20 into the aqueous phase. (3) The rate of adsorption and surface dilatational properties (surface dilatational modulus and phase angle) during adsorption at the air-water interface plays an important role in the formation of foams generated from aqueous solutions of soy globulins. However, the dynamic surface pressure and dilatational modulus are not enough to explain the stability of the foam.     

Lateral phase separation in adsorbed binary protein films at the air-water interface.

Lateral phase separation in two-dimensional mixed films of soy 11S/beta-casein, acidic subunits of soy 11 (AS11S)/beta-casein, and alpha-lactalbumin/beta-casein adsorbed at the air-water interface has been studied using an epifluorescence microscopy method. No distinct lateral phase separation was observed in the mixed protein films when they were examined after 24 h of adsorption from the bulk phase. However, when the soy 11S/beta-casein and AS11S/beta-casein films were aged at the air-water interface for 96 h, phase-separated regions of the constituent proteins were evident, indicating that the phase separation process was kinetically limited by a viscosity barrier against lateral diffusion. In these films, beta-casein always formed the continuous phase and the other globular protein the dispersed phase. The morphology of the dispersed patches was affected by the protein composition in the film. In contrast with soy 11S/beta-casein and AS11S/beta-casein films, no lateral phase separation was observed in the alpha-lactalbumin/beta-casein film at both low and high concentration ratios in the film. The results of these studies proved that proteins in adsorbed binary films exhibit limited miscibility, and the deviation of competitive adsorption behavior of proteins at the air-water interface from that predicted by the ideal Langmuir model (Razumovsky, L.; Damodaran, S. J. Agric. Food Chem. 2001, 49, 3080-3086) is in fact due to thermodynamic incompatibility of mixing of the proteins in the binary film. It is hypothesized that phase separation in adsorbed mixed protein films at the air-water and possibly oil-water interfaces of foams and emulsions might be a source of instability in these dispersed systems.     

Beta-conglycinin embeds active peptides that inhibit lipid accumulation in 3T3-L1 adipocytes in vitro

Obesity is a worldwide health concern because it is a well-recognized predictor of premature mortality. The objective was to identify soybean varieties that have improved potential to inhibit fat accumulation in adipocytes by testing the effects of soy hydrolysates having a range of protein subunit compositions on lipid accumulation and adiponectin expression in 3T3-L1 adipocytes. The results showed that differences in the protein distribution of 15 soy genotypes led to different potentials for the reduction of fat accumulation. The inhibition of lipid accumulation of soy alcalase hydrolysates in 3T3-L1 adipocytes ranged from 29 to 46%. Soy hydrolysates made from genotypes with 45.3 +/- 3.3% of total protein as beta-conglycinin, on average, showed significantly higher inhibition of lipid accumulation compared to those with 24.7 +/- 1.5% of extracted total protein as beta-conglycinin. Moreover, after in vitro simulated digestion with pepsin-pancreatin of the soy alcalase hydrolysates, 86% of the original activity remained. Adiponectin expression was induced in 3T3-L1 adipocytes treated with 15 soy hydrolysates up to 2.49- and 2.63-fold for high and low molecular weight adiponectin, respectively. The inhibition of lipid accumulation calculated from a partial least squares (PLS) analysis model correlated well with experimental data (R(2) = 0.91). In conclusion, it was feasible to differentiate soy varieties on the basis of the potential of their proteins to reduce fat accumulation using a statistical model and a cell-based assay in vitro. Furthermore, beta-conglycinin embeds more peptides than glycinin subunits that inhibit lipid accumulation and induce adiponectin in 3T3-L1 adipocytes. Therefore, soy ingredients containing beta-conglycinin may be important food components for the control of lipid accumulation in adipose tissue.     

Relationships between different hydration properties of commercial and laboratory soybean isolates

Functional properties related to water protein interactions of soy protein isolates depend on the structural and aggregation characteristics of their major components (storage globulins 7S and 11S) that could be modified by the preparation procedure, thermal and/or chemical treatments, and drying methods. Commercial and laboratory isolates with different functionalities resulting from their structural modifications were compared. Isolates with high solubility or excessive thermally induced insolubilization or compact calcium-induced aggregates caused low water-imbibing capacity (WIC) values. The highest WIC results from the balance between intermediate solubility and the formation of aggregates with good hydration properties. The apparent viscosity of dispersions of commercial (spray dried) and laboratory (lyophilized) isolates depends on the WIC, the morphology and size of the particles, and the interaction of the hydrated particles. The hydration properties and viscosity of protein isolate suspensions were strongly determined by the amount and properties of the insoluble fraction.     

Distribution of protein oxidation products in the proteome of thermally processed milk

During thermal milk processing, severe oxidation can occur, which alters the technological and physiological properties of the milk proteins. Due to differences in composition and physicochemical properties, it can be expected that the particular milk proteins are differently affected by oxidative damage. Therefore, the protein-specific distribution of oxidation products in the heated milk proteome was investigated. Raw and heated milk samples were separated by one-dimensional gel electrophoresis. Protein oxidation was visualized by Western blot after derivatization of protein carbonyls with 2,4-dinitrophenylhydrazine. Thus, α-lactalbumin displayed enhanced oxidation compared to β-lactoglobulin, despite its lower concentration in milk. Highly selective oxidation was detected for a previously unassigned minor milk protein. The protein was identified by its peptide mass fingerprint as a variant of α(S1)-casein (α(S1)-casein*). Similar oxidation patterns were observed in several commercial milk products.     

Characterization and quantification of proteins in lecithins

Several methods for extraction and quantification of proteins from lecithins were compared. Extraction with hexane-2-propanol-water followed by amino acid analysis is the most suitable method for isolation and quantification of proteins from lecithins. The detection limit of the method is 15 mg protein/kg lecithin, and the quantification limit is 50 mg protein/kg. The relative repeatability limits for samples containing 0-500 and 500-5000 mg protein/kg sample were 12.6 and 7.5%, respectively. The protein recovery ranged between 101 and 123%. The protein content has been determined in different kinds of lecithins. The results were as follows: standard soy lecithins (between 232 and 1338 mg/kg), deoiled soy lecithin (342 mg/kg), phosphatydylcholine-enriched soy lecithins (not detectable and 163 mg/kg), sunflower lecithins (892 and 414 mg/kg), and egg lecithin (50 mg/kg). The sodium dodecyl sulfate-polyacrylamide gel electrophoresis protein patterns of the standard soy and sunflower lecithins are very similar to those of soy flour. The protein profile of the egg lecithin shows several bands with a broad range of molecular masses. The molecular masses of the main proteins of soy lecithins and soy flour have been determined by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) and ranged from 10.5 to 52.2 kDa. Most of the major proteins from soy and sunflower lecithins identified by MALDI-MS and electrospray tandem MS belong to the 11S globulin fraction, which is one of the main fractions of soy and sunflower seeds. In addition, the seed maturation protein P34 from the 7S globulin fraction of soy proteins has also been identified in soy lecithins. This protein has been reported as the most allergenic protein in soybean.     

Interactions of soy protein fractions with high-methoxyl pectin

A protein-binding technique was employed to visualize, using scanning electron microscopy, the soy protein as well as the association between HMP and soy protein fractions. Image analysis indicated that at pH 7.5 and 3.5 soy protein isolate showed a bimodal distribution of sizes with an average [ d(0.5)] of about 0.05 microm, but at pH 3.8 the proteins formed larger aggregates than at high pH. Addition of HMP at pH 3.8 changed the surface charge of the particles from +20 to -15 mV. A small addition of HMP caused bridging of the pectin between soy protein aggregates and destabilization. With sufficient HMP, the suspensions showed improved stability to precipitation. The microscopy images are the first direct evidence of the interactions between soy proteins with high-methoxyl pectin (HMP).