| By the method of long-term continuous multiplication, a strain of Streptococcus thermophilus named ST-7P was obtained, which has the stability of propagation, acidification, viscosity production and exopolysaccharides production. In order to improve the production of EPS, a new type of protein enzymatic hydrolyzate medium was developed subsequently. At the same time, the stable strain ST-7P was used in fermented milk to research its rheological properties, the results were as follows:1. Comparing the viscosity and EPS concentrate of fermented medium by 5 strains of Streptococcus thermophilic, the 3 strains of ST-5P, ST-7P and ST-134 P were defined as ropy strains; the strains of ST-1P and ST-9P were defined as non-ropy strains. 3 ropy strains were fermented to multiplication for 30 periods, the results showed that the CV(coefficient of variation) of exopolysaccharides production by ST-5P, ST-7P and ST-134 P were 22.94%, 12.00% and 18.54% respectively; the CV of viscosity production by ST-5P, ST-7P and ST-134 P were 25.75%, 6.30% and 16.15% respectively. It showed ST-7P was significantly more stable than that of ST-5P and ST-134 P. Therefore, ST-7P with comprehensive stability was the experimental strain in subsequent research.2. By the optimization of protein enzymatic hydrolyzate, the results showed that the EPS concentration of were 124.01mg·L-1, 104.95mg·L-1 and 71.58mg·L-1 in the cultures with WPC, SPI and CS enzymatic hydrolyzate respectively, so the culture with WPC enzymatic hydrolyzate was the best medium for ST-7P. The results of carbon source and additional nitrogen source optimization showed that when adding 3.00% lactose and 1.00% urea, EPS concentrate got the highest of 206.07mg·L-1. After the optimization of carbon source and complemented nitrogen source, EPS concentrate increased by 66.17% in total. Ratios of the optimized medium were: WPC enzymatic hydrolyzate 94.78%, lactose 3.00%, urea 1.00%, Mg SO4 0.02%, K2HPO4 0.20%, 100.00% up with deionized water.3. By single factor experiments, the results showed that the conditions of 48 h, inoculum size 1×106CFU·m L-1, initial p H 6.50 and 42 ℃ were the best single factor condition respectively. Determining 48 h as the cultivation time, inoculum size, initial p H value and temperature were chosen to the response surface experiments of 3 factors and 3 levels, the results showed that inoculum size 1×106CFU·m L-1, initial p H 6.49 and 40.64℃ are the optimal conditions. Under these conditions, the maximal EPS concentrate was 499.54 mg·L-1, increased by 142.41% than optimization before.4. The ropy strain ST-7P and non-ropy strain ST-1P were applied in fermented milk respectively. The water-holding capacity of M10Y-7P was 31.69% higher than that of M10Y-1P; Frequency scanning and shear scanning showed that M10Y-7P was greater than M10Y-1P in aspect of hardness, viscosity and texture stability. Different content of protein yogurts fermented by ST-7P were compared with two kinds of common commercial yogurt, the results showed that: With the increase of protein content, viscosity and texture stability of yogurts were also increased; Two kinds of common commercial yogurt(protein content 2.8% and 2.7% respectively) and M8Y-7P(protein content 2.4%) performed similar rheological properties, but much worse than M10Y-7P(protein content 3.0%) Sensory evaluation analysis included the taste, smell and so on demonstrated the contribution of ropy strain ST-7P to the quality of yogurt. |
PDF Full Text Request