Effect Of Oxidation Of Defatted Soybean Flour On Gelling Properties Of Soybean Protein Isolate

Effects of oxidation induced by lipoxygenase (Lox) in defatted soybean flour on gelling properties of the corresponded soybean protein isolates (SPI) were discussed in this paper. The relationship between markers of oxidized defatted soybean flour and gelling properties of the corresponded SPI was analyzed in order to seek a rapid and simple method to evaluate the gelling properties of SPI from its raw material.Firstly, heating temperature, heating time and water content, three important factors of dry heating, were controlled to get defatted soybean flour containing a series of different activity of Lox. Effects of different activity of Lox on TBA value, carbonyl content, sulfhydryl and disulfide content and surface hydrophobicity of defatted soybean flour were compared.At the same time,SPI was extracted by alkali extraction and the texture of gelling properties was researched.Through the relativity analysis we could get: the correlative coefficient between the gelling intensity and TBA value and carbonyl content were almost zero,the correlative coefficient between the gelling intensity and sulfhydryl content and disulfide bonds content of soybean protein were 0.9473 and -0.9231,respectively,while the correlative coefficient between between the gelling intensity and surface hydrophobicity of of soybean protein was 0.7500.Effect of conservation conditions of defatted soybean flour on the oxidation and gelling properties of SPI were also studied. New-made defatted soybean flour was stored different temperature, as conservation time increased, TBA value and the carbonyl content of defatted soybean flour increased;the sulfhydryl content showed a trend that increased first and decreased after;at the same conditions,the change of disulfide bonds was contrary with the sulfhydryl content;the surface hydrophobicity also showed a trend that rose first to behind after.The relativity analysis was showed that:there were no relativity between the gelling intensity and TBA value and carbonyl content; the correlative coefficient between the gelling intensity and sulfhydryl contentand disulfide bonds content of soybean protein were 0.9260,-0.9270,respectively,while the correlative coefficient between between the gelling intensity and surface hydrophobicity of of soybean protein was 0.9339.Lastly,effects of different factors on the activity of Lox were studied. Low denatured defatted soybean flour which prepared by laboratory was used as material in order to obtain high Lox activity of defatted soybean flour. Effect of dry heating, pH, metal ions, acid radical ions, metallic complexes and antioxidants on the activity of Lox isozyme was compared.The result was that activity of Lox isozyme could be completely inactivated only by drying heat.Based on the obtained facts, we can found that lipoxygenase could promote oxidation of defatted soybean flour. The contents of sulfhydryl and disulfide bonds of defatted soybean flour show significant relativity with gelation strength of SPI, and surface hydrophobicity of defatted soybean flour also shows a good relativity with gelation strength of SPI.