Studies On The Effect Of Milk Compositions On The Probiotics Soy Yogurt

Soymilks prepared from soybean with milk supplements were fermented at 42°C to produce soy yogurt with the combined probiotics of Lactobacillus helveticus B02 and Lactobacillus casei-01. The physicochemical, rheological characteristics, microstructure, proteinmetabolism and storage stability of the fermented products were subsequently analyzed.The effects of different compositions including lactose, casein and whey protein on the characteristics of fermented soy yogurt were evaluated. Results showed that the three different compositions have different effect on the flavour of soy yogurt. Lactose stimulated growth of Lactobacillus helveticus B02 and Lactobacillus casei 01. Lactose obviously accelerated the production of acid and the acidity reached 84.10 0T after 6h fermentation with the addition of 1% lactose. Casein was able to increase the hardness and water holding capacity of soy yogurt. Whey protein concentrate (WPC) accelerated the production of acid slightly, decreased the hardness, fracturability and gumminess of the soy yogurt. The soy milk supplements with 1% lactose, 0.5% or 1% WPC had the better flavour.The effects of fermentation substrate on the characteristics of yogurt and soy yogurt were assayed. SDS-PAGE profile of soymilk demonstrated that Lipoxygenase gradually slightly degraded. Results of HPLC demonstrated that, the total amount of free amino acids of soy yogurts with 1%L, 0.5%WPC or 1%WPC decreased from 474, 574 to 295, 334μg/mL respectively. The viscoelasticity of soy yogurts with 1%L, 0.5%WPC or 1%WPC had improved significantly. Shear thinning of soy yogurts with Lactose and WPC became thin down. The soy yogurts supplements with Lactose and WPC had more pores and the pore size were much smaller. Results demonstrated that the soy yogurts with 1%L, 0.5%WPC or 1%WPC had the better flavour.The physicochemical, rheological and storage properties of the fermentation yogurts were analyzed. After 21 days cold storage at 4°C, the soy yogurts with 1%L, 0.5%WPC or 1%WPC remained high storage stability, while viable LAB numbers were still over 107 CFU/mL, and no obvious changes were observed in water holding capacities, chromaticity during the entire cold storage period. The water holding capacity of pure soy yogurt decreased slightly, while viable LAB numbers was only 106 CFU/mL. After 21 days cold storage at 4°C, the yogurt had seriously whey sepersted and postacidification. Moreover, soy yogurts were possessed of higher storage (G') and loss modulus (G") in the linear viscoelastic region after the entire cold storage period. The pure soy yogurts was possessed of higher storage (G') and loss modulus (G"), which reached above 3000Pa,750Pa. Shear sweep test demonstrated that the flow behavior index (n) of fermentation yogurts slightly rose, which were much closer to 1. The area of hysteresis loop were in the following order: SY with 1%L, 1%WPC>SY with 1%L, 0.5%WPC>SY>RSMY. The apparent viscosity of SY with 1%L, 0.5%WPC or 1%WPC rose from 0.259 mPa·s,0.291 mPa·s to 0.345,0.358 mPa·s(22.8s^-1) respectively.