The Effects Of High-pressure Treatment On Functional Properties And Structure Of SPI-Polysaccharide Mixed System

Soy protein isolate, as a nutritious, inexpensive vegetable protein, has always been widely used because of its special functional properties. Studies on its role in other edible polymer hav-e been carried out in order to increase the added value of soy protein. In this experiment, a p-rotein-polysaccharide mixed system was established with carboxymethyl cellulose, konjac glauco-mannan, flaxseed gum and soy protein isolate. The technique of dynamic high pressure homog-enization and static pressure were adopted in processing protein-polysaccharides mixture system, and in studying the structure and functional properties of the hybrid system, by setting differe-nt pressure conditions.The main observations are as follows:(1) Affects of dynamic high pressure homogenization on SPI and its polysaccharide mixed system features: significant improvements were noted of the emulsification, solubility, foaming, foam stability and oil retention of soy protein isolate under different homogenization pressures, while its water holding capacity reduced. Compared with pure SPI, the emulsification and solubility of SPI and polysaccharide(CMC, KGM, FG) showed notable improvements after the high-pressure homogenization. The affections on emulsification and solubility of these three kinds of polysaccharides follow the order: the flaxseed gum > konjac gum > CMC. At higher homogenization pressures, no better foamability than SPI was observed of the SPI- FG, however, a positive correlation was found between the SPI-FG foam stability and homogenization pressures. Among these three SPI-polysaccharide systems, only SPI-FG showed slightly better water holding capacity under the homogenization, and it was found that the addition of flax seed gum inhibited the drop of water holding capacity of SPI. The foaming ability and foam stability of SPI-KGM were maximized at 30 MPa, and decreased when the pressures got higher. Under the homogenization pressure, the oil retention of SPI- polysaccharide did not increase as significantly as SPI. With the increase of homogeneous pressure, the emulsification stability of soybean protein was reduced, and this property did not get improved with the addition of several polysaccharides.(2) Affects of dynamic high pressure on the structure of SPI and its polysaccharide mixed systems: Taking not high pressure homogenized samples as comparison, after high pressure homogenization, the total sulphur content of soybean protein isolated and expose sulphur content decreased drastically, the existence of several polysaccharides reduced the extent of oxidation of protein thiol groups. Surface hydrophobicity and circular dichroism analysis indicated that soy protein denaturation took place after the high pressure homogenization, and the presence of the polysaccharide affected the extent of protein denaturation. Amino acid analysis pointed out that high pressure homogenization produced slight affect on the amino acid content of the protein. According to SDS-PAGE electrophoresis, the high-pressure homogenization had certain influence on soy protein isolate subunits composition, and the protein subunit composition was also affected by the existence of polysaccharide. As it was demonstrated in the observation of scanning electron microscopy, after high pressure homogenization, the structures of SPI and SPI-polysaccharide turned relatively looser, and the particles became finer.(3)Affects of static high pressure on functional properties of SPI and its polysaccharide system: The emulsification, solubility and foaming properties of SPI were greatly improved with high static pressure adopted, while its emulsion stability, foam stability and water holding capacity deteriorated. After high hydrostatic pressure treatment, the emulsification, solubility, oil retention of SPI-FG improved appreciably comparing to pure SPI, the foaming and foam stabilities of SPI-KGM and oil retention changes were not notable. The emulsification and the oil retention of SPI-CMC were meliorated considerably by high hydrostatic pressure treatment. Its solubility increased to a certain extent, but was still lower than the solubility of SPI. The addition of flaxseed gum helped meliorate the oil retention of SPI, and the addition of CMC contributed substantially to the improvement of the water holding capacity of SPI.(4)Affects of static high pressure on the structure of SPI and its polysaccharide system: After high hydrostatic pressure treatment, the total sulfhydryl content and exposed sulghydryl content decreased considerably, and their decreases were influenced to a certain extent by the participation of several polysaccharides. Based on the statistic of amino acid analysis, high hydrostatic produced slight affect on the amino acid content of the protein, the participation of polysaccharide promoted the mutual conversion of amino acids. The results of surface hydrophobicity and circular dichroiosm analysis indicated that, high hydrostatic caused the expansion of SPI and its polysaccharide mixed system structure, and the exposure of radical groups. High hydrostatic had slight effect on isolated soy protein subunits, and the trips of SPI-polysaccharide system barely presented changes. Observed through scanning electron microscopy, after high hydrostatic, the large granular structure of soy protein isolate and its polysaccharide mixed system turned into small sheet structure, which was relatively loose.(5)By the comparison of the two hyperbaric treatments above, and their affects on the functional properties of SPI and polysaccharide system, the following conclusions were drawn: High hydrostatic process showed better performance in the amelioration of the emulsification and solubility of SPI and its polysaccharide system compared to dynamic high pressure homogenization. These two hyperbaric treatments influenced several other properties of SPI and its polysaccharide system to different extents. Such differences might be caused by the fact that the cavitation effect and thermal effect of the dynamic high pressure homogenization are more intense, while in the process of the high static pressure, the reaction tended to be relatively moderate under the uniformly effect on the entire sample.