| Effect of dissociation induced by acid treatment and aggregation induced by heattreatment, high-pressure homogenization and the combination treatment on the structure andfunctional properties of soybean proteins were studied, in order to find an optimalmodification measure to improve the functional properties of soybean proteins. Lowtemperature defatted soy meal was used as the raw material.Result showed that the subunit dissociation was mainly due to the intermolecularelectrostatic repulsion, and the extent of the dissociation of glycinin was much greater thanthat of β-conglycini. The result of zeta potential, solubility after ultra-centrifugation,SDS-PAGE, secondary structure and average diameter indicated that the subunits dissociatedwhen the pH value was2.0and3.0and aggregated at pH1.0and7.0.The suitable protein concentration was4%for heat treatment in pH3.0condition.50℃heat treatment had little effect on the SPI.70℃heat treatment induced the β-conglycininform the soluble aggregates when heated for0-90min.90℃heat treatment induced theglycinin and β-conglycinin form the soluble aggregates when heat for0-30min. In pH7.0condition,90℃heat treatment induced the dissociation of the SPI when heated for0-60min,and as the incubation time increased, aggregates were formed. Because more hydrosulphonylexposed when heated in pH3.0condition, the gel properties of samples heated in pH3.0condition was better than heated at pH7.0condition. But due to the aggregation, theemulsibility of samples heated in pH3.0condition was worse.The suitable protein concentration was4%for high-pressure homogenization in pH3.0condition. When the pressure was0-20MPa, SPI would further unfold, inducing the increaseof average diameter and λmax. As the pressure increased, the aggregates were formed.High-pressure homogenization in pH7.0condition would induce the unfolding of the SPIwhen the pressure was0-80MPa. For the functional properties, due to the low solubility ofthe samples which were homogenized in pH3.0condition, the emulsibility was worse thansamples treated in pH7.0condition. The suitable pH value for pepsin was2.0. Hydrolysates incubated for60min exhibitedthe best solubility, emulsibility and the ability of resisting freezing/thawing. SDS-PAGEproved that pepsin could degrade glycinin but had little effect on the α subunit ofβ-conglycinin. The structure unfolding reached the largest extent after incubation for60minand the soluble and flexible aggregates were formed. After120min, glycinin was degradedtotally and β-conglycinin formed insoluble aggregates. Moreover, two methods were appliedfor pepsin deactivation to obtain hydrolysates at pH7.0and2.0and the structure changeswere analyzed. It was found that the unfolding extent and structure characteristic weredifferent in these two conditions. Moreover, some of the insoluble aggregates formed in pH2.0became soluble in pH7.0.Single acid treatment could improve functional properties of SPI, but was not aseffective as single or combined pepsin hydrolysis and high-pressure homogenization in acidcondition. Emulsibility, the ability of resisting freezing/thawing and foaming capacity ofsoybean proteins were remarkably improved by the combination treatment, but noimprovement of foaming stability was detected. Changes of structures were detected bysurface hydrophobicity, and hydrodynamic diameter. It was found that aggregates existed inall treated samples. Besides, more flexible and soluble aggregates were observed in samplestreated by limited pepsin proteolysis, high-pressure homogenization and the combinationtreatments, which might contribute to their functional properties. |
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