Graft Reaction Of Soybean Protein Isolate Under Ultrasonic And Thermal Treatment

The glycosylation of protein was simple to operate, without any chemical additives, and which could significantly improve the functional properties of protein. So glycosylation became a popular way to modify protein now. At present, most graft reaction were realized through the ways of dry or wet heating. Unfortunately, the way of dry heating took a long time, while the way of wet heating was hard to control and always led to darker color products. At the same time, they both confronted the thorny problem of the dissolution of protein. Most vegetable protein were hard to dissolve, and the traditional way to extract protein(Alkali soluble and sinking) had several processing steps and a lower extraction ration of protein. So the widespread use of glycosylation of protein was largely limited. It was of great significance to search a convenient and fast modification method of high efficiency. In this paper, in order to study the effect of ultrasonication on the wet heating way , we chose a low-solubility commercial soybean protein isolate as material to graft with glucose and dextran.Firstly, the effects of ultrasonic power, ultrasonic time, reacting temperature and weight ratio of protein to saccharide on the dissolution rate of protein , degree of graft and chroma of graft products were studied. With the improvement of ultrasonic power, the time of graft reaction between soybean protein isolate and saccharides (especially dextran ) was shorten and the chroma of graft product was lighten. Under the conditions of ultrasonic power 450W, reacting temperature 80℃,weight ratio of protein to dextran 1/1, ultrasonic time 30min, we obtained a lighter chroma graft product with a graft degree at 31.71%, and the dissolution rate of protein was 59.65%.Secondly, we investigated the amino acid content, surface hydrophobicity, secondary structure, denaturation temperature, enthalpy changes, subunit structure, and particle size distribution of soybean protein isolate and its graft products using the methods of HPLC, ANS fluorescent probe, CD spectra, differential scanning calorimeter, SDS-PAGE and Laser light particle size analyzer. With the increase of treatment time of ultrasonication , the surface hydrophobicity and the content ofα-helix andβ-turns structure of protein were gradually increased, while the denaturation temperature of protein was decreased. The SDS-PAGE showed a high molecular aggregate was formed under ultrasonic and wet-heating treatment, while the particle size distribution revealed that dissociation and aggregation of protein took place at the same time but aggregation occupied the leading status. When protein grafted with saccharides, the surface hydropobicity ,enthalpy changes and aggregation of protein were lower, while the denaturation temperature maintained at nearly 57.40℃.At last, the solubility under different pH, emulsify ability, emulsify stability and antioxidant ability of soybean protein isolate and its graft products were determined. Compared with soybean protein isolate, the graft products had a better emulsify stability, and the solubility of the graft products obtained from SPI and glucose had improved in the wide pH range from 3 to 6. The isoelectric point of SPI moved from pH4.3 to pH4 after its grafted with dextran. Moreover, the graft products had outstanding antioxidant ability, especially the Fe²⁺ chelating ability.