| Soybean protein isolate(SPI) has been widely used in food processing because it has better functional properties and high nutritional value. However, SPI contains 1~2% phytate which has been considered as an anti-nutrition factor. Phytate will chelate with minerals to form insolubles, lowering mineral bioavailability. Besides it has also been reported to form complex with protein, making protein digestibility decreased. Therefore, hydrolyzing the phytate in SPI, the functional and nutritional properties of SPI will be fully utilized. In this study, we proposed that phytase-assisted treatment method during the process of preparation of SPI, so as to select the proper step to conduct phytase treatment, and obtain SPI with low phytate content and better functional and nutritional properties. This article mainly studied the selection of conducting phytase treatment and the optimization of process conditions, then we have a comprehensive research about the physicochemical properties,structure,functional and nutrition characteristic of SPI with different phytate content after phytase treatment. Finally, we also studied the effects and relationship of phytate and SPI on dialysability of fortified mineral. All of these can provide a theoretical basis to promote the industrialization and widely application of functional SPI.Firstly, this article studied the selection of conducting phytase treatment during the process of preparation of SPI and the optimization of enzyme hydrolysis process conditions, mainly including the effects of temperature, time, enzyme added amount and p H on phytate content and protein solubility. The results show that performing phytase treatment during acid precipitation is better than that of extracting. The optimum conditions were determined as follows: the enzyme temperature: room temperature(25 °C), time: 60 min, enzyme added amount:20 U/g soybean meal, p H 5.0. Under this conditions, phytate content is reduced to 0.11 mg/g and the degradation rate is up to 99.56%, the solubility of SPI is 95.57%. In this process, High temperature and low p H will lead to protein denaturation and cause serious damage to the functional properties of protein, which is the reason of not choosing the optimum temperature(55 °C) for phytase. Therefore, in order to obtain SPI with low phytate and better functional properties, the phytase treatment was chosen under ambient temperature conditions.Secondly, the preparation of different phytate content of SPI was conducted by the optimal enzymatic hydrolysis conditions(25 °C, 20 U/g soybean meal, p H 5.0) under different time(5~60 min), then we study the influence of different phytate content on the physicochemical properties, structure,functional and nutrition characteristics of SPI. The results show that ash content(R2 = 0.940), solubility(R2 = 0.983), ζ-potential value(R2 = 0.793), denaturation temperatures(β-conglycinin, R2 = 0.941; glycinin, R2 = 0.977), emulsifying activity index(R2 = 0.983), foaming capacity(R2 = 0.955), the total flavour compounds and trypsin inhibitor activity(R2 = 0.821) of SPI were positively correlated with phytate content, while protein content(R2 = 0.876), protein recovery(R2 = 0.781), emulsifying stability index(R2 = 0.953), foaming stability(R2 = 0.919), gel hardness(R2 = 0.893), surface hydrophobicity of protein(R2 = 0.954), viscosity of protein solution(15%)and in vitro digestibility(R2 = 0.969) were negatively correlated with phytate content. Amino acid analysis showed that low phytate protein has higher total amino acid content, which is consistent with the increase of protein content, but no significant change between samples can be observed using SDS-PAGE and the percentage of every amino acid varies very little. Circular dichroism spectra showed that secondary structure of SPI was changed by phytase. Simulated gastrointestinal digestion showed that percentages of dialyzable Zn and Ca were increased with decreasing phytate content, while the amounts of dialyzable Zn and Ca revealed different behaviors: the former was increased and the latter was decreased.Lastly, the effects and relationship of phytate and soybean protein isolate(0.11 mg/g phytate) on dialysability of fortified mineral were studied. The results showed that either phytate or SPI have significant inhibitory effect on dialysability of fortified mineral(Ca and Zn) when Ca or Zn was fortified with Ca Cl2 or Zn SO4.7H2 O separately. And the inhibition on Zn dialysability becomes stronger when a combination phytate with SPI. So phytate and SPI have coordinate inhibitory on Zn dialysability. When Ca Cl2 and Zn SO4.7H2 O was fortified together, we can observed that phytate and SPI also have coordinate inhibitory on Zn dialysability, while for Ca dialysability, is higher than that of phytate, lower than that of SPI.This study has important theoretical guidance and practical significance on the preparation of low phytate, better gels, high protein digestibility and mineral bioavailability of soy protein isolate. |
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