Preparation And Quality Evaluation Of Soybean/Milk Dual Protein Yogurt

Yogurt is rich in active ingredients such as protein,fat,and minerals,as well as beneficial bacteria such as Lactobacillus bulgaricus and Streptococcus thermophilus,which has high nutritional value.However,China’s increasing population and increasing shortage of milk sources have led to the need for a large number of imported dairy products.We need to find new sources of protein.In recent years,"dual protein engineering"has become a research hotspot,and dual protein products can achieve complementary advantages of animal and plant proteins.The development of dual protein products using plant proteins instead of animal proteins has become a trend.Soybean protein has become an ideal substitute for milk protein due to its advantages such as good food processing performance and easy availability.In this study,the mixed systems of soybean milk and milk with the addition of 25%,50%and 75%of soybean milk were homogenized,and the double protein yoghurts were prepared with Lactobacillus bulgaricus and Streptococcus thermophilus.The effects of the ratio of soybean milk to milk on the physical and chemical properties,nutritional components and functional activities of yoghurt were studied with pure milk fermented yoghurt and pure soybean milk fermented yoghurt as controls.The purpose is to promote the utilization of milk and soybean resources,expand the development and quality control of dual protein yogurt,and expand the potential application direction of animal and plant protein.The main results of the study are as follows:(1)The protein and moisture content of soybean powder are higher than those of milk powder,while its total sugar and fat content are lower than those of milk powder.The swelling capacity,bulk density,particle density,and porosity of milk powder are higher than those of soybean powder,but no significant differences in tap density are observed.Both soybean powder and milk powder are powders with good fluidity.The rehydration property of soybean powder is not as good as that of milk powder.Soybean powder contains more high molecular weight proteins than milk powder.36 and 35 volatile compounds are detected in soybean powder and milk powder,respectively.(2)The acidity,water holding capacity,firmness and consistency of yogurt increased with the increase of fermentation time and the decrease of soybean milk addition.All yoghurts show pseudoplastic,n(0%)>n(75%)>n(25%)>n(50%)>n(100%)and K(25%)>K(0%)>K(50%)>K(75%)>K(100%).At the same shear rate,the apparent viscosity of yogurt samples decreased with the increase of soybean milk addition.The yoghurt of each group showed the characteristics of weak gel system dominated by elasticity.The storage modulus and loss modulus decreased with the increase of soybean milk addition.With the increase of soybean milk in the sample,the L*value and b*value of yogurt gradually decreased.The chromaticity values of the five yogurt groups were significantly different.SEM images showed that with the increase of soybean milk addition,the pores of fiber network structure became larger.Laser confocal scanning microscope pictures showed that with the increase of soybean milk addition,the pores formed in yogurt were larger and the protein aggregates were more dispersed.(3)The protein content and the number of Lactobacillus bulgaricus and Streptococcus thermophilus in the five groups of yogurt meet the requirements of GB 19302-2010.The content of protein,fat,total sugar,soluble solids and sensory score of yoghurt with 25%soybean milk addition were the highest,while the content of fat,total sugar,soluble solids and sensory score of yogurt fermented with pure soybean milk were the lowest.There is a higher number of viable lactic acid bacteria in yoghurt fermented with the soybean milk-milk mixed system compared to that fermented with a single raw material.A total of 89 volatile compounds were identified from five groups of yogurt.The types of total volatile substances of double protein yoghurt were lower than those of yoghurt fermented by pure milk,but higher than those of yoghurt fermented by pure soybean milk.The volatile matter content of double protein yogurt with 25%and 75%soybean milk addition was higher than that of yogurt fermented with pure milk and pure soybean milk.Compared to yogurt fermented with a single raw material,the aroma of dual protein yogurt is more intense.(4)The DPPH radical scavenging rate,ABTS~+radical scavenging rate and ferric ion reducing capacity of double protein yoghurt increased significantly with the increase of soybean milk addition in the system,which were higher than those of pure milk fermented yoghurt,and lower than those of pure soybean milk fermented yoghurt.The change trend of hydroxyl radical scavenging rate of yoghurt was opposite.After simulated oral digestion,the DPPH radical scavenging rate of dual protein yogurt was higher than that of two groups of single raw material fermented yogurt.ABTS~+radical scavenging rate and hydroxyl radical scavenging rate were higher than those of pure soybean milk fermented yoghurt,and the ferric ion reducing capacity was higher than that of pure milk fermented yogurt.After simulated gastric digestion,the DPPH radical scavenging rate and ferric ion reducing capacity of double protein yoghurt were higher than those of pure milk fermented yoghurt,and the ABTS~+radical scavenging rate and hydroxyl radical scavenging rate were higher than those of pure soybean milk fermented yoghurt.After simulated intestinal digestion,the ABTS~+radical scavenging rate of double protein yoghurt was higher than that of pure soybean milk fermented yoghurt,and the ferric ion reducing capacity and hydroxyl radical scavenging rate were higher than that of pure milk fermented yoghurt.(5)Undigested yogurt has no ACE inhibitory effect.But after different stages of in vitro simulated digestion,the ACE inhibitory activity of yogurt has been significantly improved.The ACE inhibitory activity of three groups of double protein yoghurt after intestinal simulated digestion was lower than that of pure milk fermented yoghurt,but higher than that of pure soybean milk fermented yoghurt.The above research results can provide data support for the preparation and quality control of dual protein yogurt.