Study On The Coagulation Behavior And Mechanism Of Soymilk Induced By Fermented Soy Whey

Soy whey,rich in protein,sugar,soy isoflavones and other nutrients,is a by-product in the process of tofu production.Soy whey fermented naturally can be used as a tofu coagulant.Soymilk induced by fermented soy whey（FSW）can not only decrease the production cost,but also realize the recycling of by-products of soy products processing and reduce the pressure of sewage treatment.It is a green and environmentally-friendly food processing method.However,the productions of FSW and FSW-tofu depend on artificial experience,and it cannot meet the requirements of industrial production of soybean products due to lack of systematic study on the behavior and mechanism of soymilk coagulation induced by FSW.This paper mainly analyzes microbial diversity and physicochemical indexes of FSW,and texture and flavor characteristics of tofu prepared by FSW and other coagulants（Mg Cl₂,Ca SO₄,lactic acid,acetic acid and glucono‐δ‐lactone）.In addition,the changes of subunit migration,intermolecular force and secondary structure of soy protein during the coagulation of soymilk induced by FSW were studied by proteomic methods.This study aims to provide a theoretical basis for the industrial production of FSW tofu.The main conclusion of this study are as follows:1.Effect of temperature on bacterial floras and physicochemical indexes of FSWHigh throughput sequencing technology and solid phase microextraction coupled with gas chromatography-mass spectrometry（SPME-GC-MS）were used to analyze the microbial community diversity and volatile flavor compounds of soy whey fermented at different temperatures.The results showed that the fermentation temperature（simulated natural temperature,25℃and 37℃）had a significant effect on the bacterial floras and physicochemical indexes of FSW.A total of 84 genera were detected in all FSW.The dominant microorganisms in simulated natural fermented soy whey（NFSW）were Lactobacillus、Acetobacter、Pelosinus、Clostridium sensu stricto 1 and Lactococcus,and Lactobacillus accounted for more than 80%.Redundancy analysis（RDA）showed that bacterial floras of FSW had significant correlation with p H value,lactic acid,ketones and esters（P<0.01）.2.Analysis of quality characteristics and flavor components of FSW tofuThe effects of soy whey fermented by Lactobacillus plantarum D1031 and other coagulants on the quality characteristics and flavor components of tofu were analyzed by texture profile,scanning electron microscopy analysis and SPME-GC-MS.The results showed that the moisture content and yield of the FSW tofu and GDL tofu were significantly higher than those of other coagulants tofu（P<0.05）.The acetic acid tofu had the highest hardness（244.04 m N）and chewiness（176.34 m J）and the lowest L^*value（82.77）.Ca SO₄tofu and GDL tofu had dense network structure,while lactic acid tofu had rough surface and pores.In addition,a total of 100 volatile flavor compounds of soymilk,FSW and tofu made with different coagulants,including alcohols,aldehydes,ketones,esters,acids and phenols,were detected,and FSW tofu had higher numbers of volatile compounds and lower total amount of the compounds of beany flavor.Sensory evaluation showed that FSW tofu and Ca SO₄ tofu were superior to other coagulants tofu.FSW and Ca SO₄can make tofu with better quality and flavor,and their tofu have higher consumption appeal.3.Effects of FSW on intermolecular forces and secondary structure of soybean protein isolate（SPI）The changes in intermolecular forces and secondary structure of soy protein induced by FSW and other coagulants,while the SPI was used as the research object.The results showed that FSW and other coagulants could reduce the p H values of SPI solution,and the final p H value is 5.25～6.14.Compared with heated SPI solution without coagulant,the addition of coagulants caused increases in the surface hydrophobicity and free SH content,and a decrease in Zeta potential in SPI solution.The surface hydrophobicity of SPI induced by coagulants increased first and then decreased,and free SH content decreased at 0～45 min.The Zeta potential of SPI solution added with FSW was significantly higher than that of other coagulants（P<0.05）,ranging from 9.19 m V to 9.90 m V.Mg Cl₂ and Ca SO₄ induced a shift ofα-helix andβ-turn intoβ-sheet of soy proteins,and FSW,lactic acid,acetic acid and GDL destroyedβ-sheet of SPI.4.FSW-induced aggregation of protein in soymilk:A proteomics approachProteomics and high performance liquid chromatography（HPLC）were used to analyze the effects of soy whey fermented by Lactobacillus plantarum D1031 on the distribution of soy protein and isoflavones.The results showed that the p H value of soymilk decreased and the content of protein and subunit in precipitation fraction increased with the addition of FSW.With the addition of FSW increased,the 7Sβ,11S A3 and 11S A1a（spots 17-19）subunits and sucrose binding protein（SBP）,Bd 30 k,lectin and trypsin inhibitor A（TA）aggregated the earliest,and later did the 11S A4,other 11S A1a（spots 14-16）,11S A2 and 11S A1b subunits.The 7Sαandα’subunits and trypsin inhibitor B（TB）showed an obvious delay in aggregation.Moreover,isoflavones in the form of aglycones were more likely to coprecipitate with proteins,compared with glycosides.