Oxidation Of Soy Protein Isolate Powder And Its Effect On The Solubility During Storage

During storage, the solubility of soy protein isolate (SPI) usually decrease, which hasnegative effect on its applications significantly. Illustration of the cause for solubility decreasewill provide theoretical and technical basis for its extensive applications. On the basis ofprevious research, we have proposed a hypothesis that oxidation may be one of the causes forsolubility decrease. By investigating the relationship between oxidation induced condition(oxidized lipid) as well as storage environment conditions (water and oxygen) and solubility,the cause for solubility decrease has been primarily discussed.First, the effects of lipid oxidation degrees on oxidation degree, structure and solubilityof SPI were discussed. The results have demonstrated that oxidized lipid increased thedecrease of solubility significantly, and the sulfydryl content significantly decreased (P<0.05)for SPI with lipid. Lipid with higher oxidation degree can accelerate the oxidation anddecrease of solubility of SPI.SPI with moisture content between3.5%and7.0%was oxidized and the solubility alsodecreased. Solubility of SPI with3.5%moisture was lower than that of SPI with7%moisture,and decrease rate of solubility of these two samples was almost at same speed, this indicatesthat water content and process of spray dry could influence solubility of final products.Furthermore, effects of oxygen content during storage on structure, oxidation degree andsolubility of SPI were discussed. The results show that protein carbonyl content of SPI invacuum package was lower than SPI in normal package. The solubility of SPI in normalpackage decrease extent was higher than that of in vacuum package at first three weeks,which indicated that protein oxidation and decrease of solubility can be partly inhibited byeliminating environment oxygen.The correlation analysis of structure and solubility of SPI shows that carbonyl and freeamino contents correlate positively with solubility of SPI (p<0.05), while sulfydryl contentscorrelate negatively with the solubility of SPI (, p<0.05), and the results imply that oxidationis one of the causes for solubility decrease.In order to explore the cause for solubility decrease of SPI during storage, differentantioxidants were added during the preparation of SPI. The results have demonstrated thatsodium sulfate can inhibit the increase of sulfydryl content, and slow down solubility declineduring storage of SPI. Other antioxidants such as free radical scavengers, singlet oxygenquenchers and metal ion chelating agents were not able to inhibit decrease of solubility of SPI.The above results have demonstrated that solubility decrease of SPI during storage probablyhad relationship with insoluble aggregates which induced by disulfide bond, and it is possibleto prevent the SPI’s solubility decrease by inhibiting the aggregate of SPI and formation ofdisulfide bond.