| Soybean protein isolate(SPI)has a protein content of more than 90% and nearly 20 amino acids and is characterized by low cost and easy digestion and absorption.In addition,SPI is also an important source of bioactive peptides.However,its application in the food field is limited by its less-than-ideal functional properties,the presence of flatulence factors and undesirable flavor components.Ultrasound technology can improve the structure of soybean isolates,but the currently reported ultrasonic pretreatment of soybean isolates has a low concentration,resulting in a long subsequent enzymatic digestion time,which is not conducive to improving the end product concentration and production efficiency.Therefore,the feasibility of treating high concentrations of soybean isolate protein by ultrasound is an important research direction for the food processing industry.Bioactive peptides possess a variety of physiological functions,are easy to absorb,have high biological activity,and have few toxic and side effects.Among them,antioxidant peptides can improve the health of the body by alleviating the oxidative stress caused by lipid peroxidation and excess free radicals in the body,thus acting as an antioxidant.The antioxidant peptides prepared from soybean isolate have small molecular weight,easy absorption,high activity and rich amino acid composition.In this paper,we obtained soybean peptides by enzymatic digestion after sonication of high concentration of SPI and evaluated the antioxidant function of the enzymatic digestion products.However,the bitterness of soybean peptides obtained by ultrasound-assisted enzymatic preparation has a direct impact on the flavor of the enzymatic solution.Therefore,a study on its debittering by lactic acid bacteria fermentation was conducted to provide theoretical basis and technical support for the preparation of soybean peptide high-value foods with low bitterness,good flavor and strong antioxidant activity.The specific research is as follows:In this paper,the preparation of soybean peptides by ultrasonic pretreatment of soybean protein isolate was investigated and the process was optimized.The SPI was enzymatically digested by different proteases using soybean peptide isolate as raw material,and alkaline protease was screened as the best enzyme in terms of hydrolysis degree and molecular weight.The optimal process parameters were determined based on the single-factor combined response surface optimization of ultrasonic pretreatment enzymatic digestion: ultrasonic power 300 W,ultrasonic time 20 min,enzyme addition0.14 g,and enzymatic digestion time 4 h.The final peptide content was verified to be73.69%±0.96%.In this thesis,the effect of ultrasound on the structure of high concentration SPI was explored under optimal ultrasonic conditions.SDS-PAGE showed that the SPI primary structure could not be altered by sonication,regardless of the protein concentration.The secondary structure showed that as the concentration increases,the ratio of β-fold and α-helix in the protein also changes.Among them,the β-fold content of the samples reached a maximum of 43.20% and the α-helix content decreased at 16%high concentration of SPI by sonication,which made the protein secondary structure tend to unfold.The increase of fluorescence spectral intensity and surface hydrophobicity further indicated that the increase of concentration was the reason for the looseness of protein internal structure caused by ultrasound and the exposure of more hydrophobic groups.Therefore,the effect of ultrasound on protein modification was more obvious.In this thesis,the enzymatic digestion of soybean isolate was performed at the same concentration(4%)after ultrasonication modification.The results showed that the highest peptide content of the enzymatic digestion product was 84.27% at 16% concentration of SPI by sonication,and the free amino acid content of the product was significantly higher than other concentrations by sonication.Meanwhile,the analysis of the molecular weight of the enzymatic digestion products showed that the generation of low molecular weight peptides from 200 to 1 000 Da was favored after sonication at 8-16% concentration of SPI.In the antioxidant analysis of the enzymatic digestion products,it was found that the highest antioxidant activity was achieved at a high concentration of 16% SPI treated with ultrasound.In this thesis,different lactic acid bacteria were used to analyze the enzymatic digestion of soybean peptides for debittering.Lactobacillus casei B-2 and Lactobacillus plantarum B-17 were found to have the highest combined score at 4 h,with a better taste.Lactobacillus fermentum B-6 could achieve the same de-bittering effect at 2 h.This resulted in different times for the fermentation of different lactic acid bacteria strains to achieve the same effect on the debittering of soybean polypeptides.The hydrophobicity analysis revealed that the hydrophobicity of different lactic acid bacteria was positively correlated with the variation of bitterness.Molecular weight analysis revealed that the content of enzymatic solution with molecular weight <500 Da increased about 27% after de bittering compared with that before de-bittering.FAA analysis revealed that the content of sweet and fresh amino acids increased and the content of hydrophobic amino acids decreased to different degrees among the free amino acids after debittering.GC-MS detected that the relative contents of glutaraldehyde,which showed bitter almond flavor,and acetaminophen,which showed bitter flavor,were significantly reduced in the B-2,B-17 and B-6 groups after de-bittering.At the same time,with the fermentation of lactic acid bacteria,acids and esters would be produced,so that the bitterness and beany taste of soybean polypeptide enzymatic solution decreased,and had a refreshing lactic acidic taste. |
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