Study On Functional Properties Of Soybean Fermented By Lactobacillus Delbrueckii Subsp. Bulgaricus

Soybean is an important plant protein source. Use of Microbial Fermentation can reduce the content of antinutritional factors in soybean and produce polypeptides of different molecular weights in the same time, therefore, to improve nutritional value. In this study, soybeans were used as raw materials to be fermented by lactic acid bacteri,to change the nutritional components of soybeans and to prepare soybean peptides with different functions. This study mainly focused on the screening of fermentation strains, the mutagenesis of strains, the optimization of fermentation medium and fermentation conditions, the changes of nutrients in the fermentation process and the determination of various functional properties of soybean peptides. The results are as follows:(1)Four kinds of lactic acid bacteria were screened by comparing protease activity, the order of protease activity was: Lactobacillus delbrueckii subsp. bulgaricus > Lactobacillus plantarum> Lactobacillus casei> Lactobacillus acidophilus. Lactobacillus delbrueckii subsp. bulgaricus, used as the starting strain, was mutagenized by atmospheric pressure plasma mutagenesis system. Lactobacillus delbrueckii subsp. bulgaricus BC5, which had the highest protease activity in the mutant strains, was isolated through preliminary screening and secondary screening. The hereditary stability of Lactobacillus delbrueckii subsp. bulgaricus BC5 was confirmed through serial subcultivation. The protease activity was 286.58 U/mL at the 6th generation, which was 15.3% higher than that of the non-mutagenized strains.(2)The fermentation of soybean was carried out by the strain of Lactobacillus delbrueckii subsp. bulgaricus BC5. The changes of the hydrolysis degree of the soybean protein which increased from 4.70% to 48.96% and pH which reduced from 6.7 to 3.91 during the fermentation were determined, which indicated that Lactobacillus delbrueckii subsp. bulgaricus BC5 had a strong fermentation and hydrolysis capacity.(3)The optimal medium and culture conditions, obtained by response surface analysis,were as follows(peptides content as an indicator): glucose 25 g/L, soybean meal 107.53 g/L,K2HPO4 5.68 g/L, initial pH 6.7, time 47.6 h, temperature 37.65 ? and rotation speed 118 rpm. The actual peptide content, which was 22.3% higher than that before optimization and near to predicted yield(9.31 mg/mL),was 9.25 mg / mL,which proved the model parameters were reliable and useful.(4) In a certain fermentation time range, the highest digestibility of soybean protein increased by 47.5% was 68.11%, the activity of trypsin inhibitor and phytic acid were reduced by 91.57% and 75.37% and the urease enzyme was substantially completely removed. The molecular weight of soybean protein determine by SDS-PAGE electrophoresis changed from main 35 KD into 25 KD. In pH 2.5-8.5, The solubility of soybean peptides, which remained essentially unchanged, was more stable than soybean protein isolate. In the concentration of 1% to 5%, The emulsification index of soybean protein isolate and soybean peptide increased with increasing concentration, and at the same concentration, the emulsifying index of soybean protein isolate was larger.(5) The highest ABTS scavenging rate of soybean peptides was 30.6% at 36 h. 4 peaks were obtained by Sephadex G-25 gel chromatography from the soybean peptides at 36 h.The ABTS free radical scavenging rate of P1 was the highest at 37.84%. The molecular weight of the polypeptide of P1 determined by MALDI-TOF-MS was about 5000 Da. The highest inhibitory rate of cholesterol was 39.14% and the peak of P2, molecular weight of which was about 2000 Da, was 46.52% molecular weight was about 2000 Da. The highest ACE inhibition rate of peptides and chelating rate of peptide with Ca2+ was 42.65% and 30.8%, respectively and only one elution peak, peptides molecular weight distribution of which was 800 Da to 1000 Da, was obtained by Sephadex G-25 gel chromatography.(6) The optimum chelating conditions optimized by single factor and orthogonal experiments were as follows: pH 5, reaction temperature 45 ?, time 30 min, peptide to ferrous sulfate ratio 1: 2. Under these conditions,the chelating rate was 56.67%. At the same concentration, the rank of antioxidant capacity was: VC> Fe-chelating peptides >soybean peptide.