Interaction Of Selected Volatile Flavor Compounds And Soy Protein Isolate

Soy proteins are full of nutrition and have an widespread use,which is equivalent to animal proteins and have a health-care effect on the kidney.However,due to the high content of unsaturated fatty acids,like linoleic acid and linolenic acid in soybeans,soybean products are prone to oxidation during producing,processing and storage,and oxidation has a significant effect on the structure,functional properties,and flavor characteristics of soybean protein.In particular,the smell of beany flavor seriously affected consumer's acceptance.This thesis systematically studyed the interaction mechanism of soybean protein isolate(SPI)binding to the key flavor compounds in traditional soybean milk(including beany flavor and non-beany flavor).The structure of SPI was modified by oxidation and the effects of protein oxidation on the retain and release of beany and non-beany flavor compounds by SPI were explored using Solid-phase Microextraction(SPME)combined with Gas Chromatography and Mass Spectrometry(GC-MS)techniques.In addition,the molecular forces involved in binding of key flavor compounds with different functional groups to SPI were studyed.Furtherly,fluorescence quenching,isothermal titration calorimetry(ITC)and circular dichroism(CD)were used together to quantitative analyze the reaction type and binding strength of trans-2-nonenal to SPI and its two major components,7S and 11 S,aimed to provide theoretical guidance for the rational control of oxidation and flavor regulation in the processing of soybean products.The experimental results show that the malondialdehyde(MDA)is used as the representative of secondary metabolites of lipid peroxidation of lipids to induce the oxidation of SPI,significant changes happended in protein surface and structure characteristics,and also significantly affected the retain and release of beany and non-beany flavor compounds.Upon low MDA concentration(?1mM),the SPI structure was unfoled with hydrophobic groups exposed,which is beneficial to the hydrophobic interactions between flavor compounds and SPI.And the increase in the binding of the beany flavors was greater than that of the non-beany flavors,which is good to improve overall flavor.Whereas,high concentration of MDA(?2.5mM)induced aggregates.The binding ability of saturated aldehydes to SPI decreased,but alkenals did not change much,which indicated that covalent reactions of double bonds with some specific amino acids might be readily accessible in the aggregates.The interactions between SPI and different types of key flavor compounds were quite different.By comparing the changes in the binding amount after the hydrophobic interaction was destroyed and the hydrogen bond was enhanced,the binding of SPI to benzaldehyde only involved hydrophobic interactions,and the binding to hexanal and nonenal were mainly by hydrophobic interactions,and hydrogen bond had some contribution.For trans,trans-2,4-octadienal,contribution of hydrophobic interactions and hydrogen bond was an equivalent.For trans-2-nonenal,it was likely that the covalent interaction of the double bond is mainly involved.SPI could not bind 1-octen-3 alcohol,but after the hydrophobic interaction was destroyed,they combined,it might attribute to the role double bond.The interaction mechanism of 7S and 11 S with trans-2-nonenal were completely different.7S had various types of binding sites for trans-2-nonenal,and its binding involved hydrogen bond and van der Waals as well as hydrophobic interaction,but the binding was very weak.While for 11 S,the conformation was not conducive to the binding with trans-2-nonenal,but the ?H in the bonding process was much higher than that of 7S.And the thermodynamic parameters of the bonding process of 11 S indicated that The non-covalent binding of 11 S to trans-2-nonenal was predominantly enthalpy-driven,that is,electrostatic interaction driven.Compared to the binding of SPI,7S and 11 S to trans-2-nonenal,SPI was very similar to 11 S,and the binding was much stronger than 7S,showing that the contribution to the binding of SPI to trans-2-nonenal was mainly 11 S.