Effect Of Rice Dreg Protein Hydrolysate On The Quality Of Yogurt And Its Growth Promoting Mechanism Of Lactic Acid Bacteria

The average annual compound growth rate of yogurt product sales in China is as high as 9%,and the sales growth rate is more than 11.67%.Therefore,the research on new yogurt products is still an important development trend of the food industry.Streptococcus thermophilus and Lactobacillus bulgaricus are widely used in yoghurt starter culture because of their high acid production rate in mixed culture,but Streptococcus thermophilus has weak protein decomposition ability.Rice residue is a by-product of organic acids and starch sugars produced by rice.Although the content of dry base protein in Rice dreg protein(RDP)is more than 70%,its application in the food industry is greatly limited due to its poor solubility and emulsification.In order to broaden the application of RDP and develop high quality yogurt,this paper firstly studied the effects of Rice dreg protein hydrolysate(RDPH)on the fermentation time and quality of yogurt.Secondly,Rice dreg protein hydrolysate 24 h(RDPH24)had different growth and acid production rates on Lactobacillus bulgaricus and Streptococcus thermophilus.Finally,Transcriptomic techniques were used to investigate the differences in mRNA gene expression of RDPH24 pairs of Streptococcus thermophilus in logarithmic phase.Alkaline protease was used to hydrolyze RDP,and the optimal addition amount of hydrolysate to promote the fermentation effect of yogurt was studied,and the influence of RDPH with different hydrolysis degrees on the fermentation time and the quality of yogurt during storage was explored.The results showed that when RDPH24 was increased to 3%,the fermentation time of yogurt was shortened by 1.09 h compared with that of Control group(CK),but the fermentation time was prolonged when RDPH24 was increased.When RDPH supplemental level was 3%,the degree of hydrolysis was positively correlated with the ability of lactic acid bacteria to promote fermentation,and the longer the hydrolysis time,the more obvious the effect of promoting fermentation.After rice residue protein hydrolysis for 36 h,yogurt fermentation time can be shortened by 1.58 h.Hydrolysate improved the texture characteristics and water holding capacity of yogurt,inhibited the post acidification of yogurt,and improved the sensory score of yogurt.The addition of RDPH24 increased hardness 19.60%,viscosity 23.73%,water retention 22.22%and sensory score 5.89%higher than CK after storage for 1 day.After 21 days of storage,RDPH24 increased hardness by 18.41%,viscosity by 30.69%,water retention by 26.41%and acidity by 10.92%compared with CK.The growth promoting effect of RDPH24 on Streptococcus thermophilus and Lactobacillus bulgaricus showed that:After fermentation for 4 h,the OD600 of Streptococcus thermophilus was increased by 0.059 compared with CK,and the lactic acid yield was increased by 2.27 mmol/L.The OD600 of Lactobacillus bulgaricus was increased by 0.026 compared with CK,and the lactic acid yield was increased by 0.67 mmol/L.The number of cocci added with RDPH24 was 9.50×106 CFU/mL,while that in the control group was only 5.00×105 CFU/mL.The number of bacilli added with RDPH24 was 2.00×104 CFU/mL,while the number of bacilli in the control group was only 4.75×103 CFU/mL.At the same time,adding RDPH24 increased the number of viable bacteria of mixed bacteria by an order of magnitude compared with CK.After fermentation for 12 h,the OD600 of Streptococcus thermophilus was increased by 0.071 compared with CK,the OD600 of Lactobacillus Bulgaricus was increased by 0.035 compared with CK,and the lactic acid production of Streptococcus thermophilus and Lactobacillus Bulgaricus was increased by 1.23 mmol/L and 0.72 mmol/L compared with CK,respectively.The viable bacterial counts of Streptococcus thermophilus and Lactobacillus bulgaricus were 3.45×108 CFU/mL and 8.50×106 CFU/mL,respectively,which were higher than 3.85×107 CFU/mL and 2.70×106 CFU/mL in the control group.At the same time,adding RDPH24 made the pH of lactobacillus at stable stage significantly lower than that of CK group.Based on OD600 and viable bacteria count,RDPH24 was more effective in promoting growth of cocci than bacillus.Transcriptomic techniques were used to investigate the difference of RDPH24 gene expression in logarithmic phase of Streptococcus thermophilus.The results showed that 282 genes of Streptococcus thermophilus were up-regulated and 191 genes were down-regulated in the logarithmic stage after adding RDPH24 in the medium for 4 h.Among them,acoA,pdhB,acoL and acoC in pyruvate metabolic pathway were upregated by 1.59,1.28,1.35 and 1.16 times respectively,which better promoted oxidative decarboxylation of pyruvate and finally produced acetyl CoA,which entered the tricarboxylic acid cycle and was thoroughly oxidized to carbon dioxide and water,and released a lot of energy at the same time.Promote the growth and reproduction of Streptococcus thermophilus.In the amino acid metabolism pathway,metAA,yclM and msrC were upregulated by 1.38,2.29 and 1.89 times,respectively,indicating that the metabolism of cysteine and methionine of Streptococcus thermophilus added with RDPH24 was more vigorous.In the nucleotide metabolic pathway,purE,purB,purD and purK genes were up-regulated.It was speculated that the faster growth of Streptococcus thermophilus might be due to the generation of more hypoxanthine nucleotides.