| Qula, the traditional Tibetan food, herdsmen used yak milk as raw material after degreasing, fermentation, casein precipitation, drying and other process to fabricate the dairy products, fermentation parameters had significantly affectes on product flavor, color and quality. So far, the processing of Qula is still used traditional processing, resulting serious browing, poor color and flavor quality, affecting the value of eating and economic, and brought many difficulties to downstream-processing of casein. Therefore, Researched the Qula Starter Culture was particularly important to improved the color and smell of milk, increased the income of the herdsmen. DVS in production and used had many advantages, the development of DVS preparation has been the focus of research at home and abroad. This paper studied the preparation and storage stability of Qula Starter Culture. The main findings were as follows:(1) Lactobacillus delbrueckii subsp bulgaricus(MGD1-3), Streptococcus thermophiles(MGB39-5), Lactobacillus plantarum(BM5152) were selected, and the result showed that MGD1-3 2.5%、MGB39-5 1.5%、MGB39-5 1.5%. Under these conditions of 42℃, fermented yak milk for 5.0h, the average titratable acidity was 80.5 oT of three parallel trial. The time of Qula fermentation time was 5.27 h reduced 6.63 h(p<0.05).(2) Qula DVS preparation process parameter optimization of mixed bacteria1) According central composite experimental desidn, based on MRS culture optimizated Multiplication medium, and its optimal composition was carbon source18.11 g/L, nitrogen source26.72 g/L, Tween-80 1.88 g/L and VB61.41g/L. The OD600 of strains cultured in the optimized MRS medium reached 2.173, the cell concentration in the optimized MRS medium reached 4.01×1010 CFU/m L,which was 8.29 times higher than observed in the initial medium.2) The best culture conditions were determined by L9(34) orthogonal design. The optimum culture conditions were as follows: the temperature was 34℃, p H 6.6, inoculation amount of 5%, and the number of viable bacteria was 7.85×1010 CFU/m L.3) Comprehensive experimental design was used to obtain the best centrifuge parameters, the best parameters were 8000r/min, centrifugal 15 min, the rate of the bacteria after this was up to 74.51%.4) The optimum compounding of protectant were Accorded central composite experimental desidn, The optimum compounding of protectant was skim milk concentration of 12%, trehalose concentration 8.40%, monosodium glutamate concentration 2.10%, glycerol concentration 3.20%, Demonstration test obtained compound lactic acid bacteria survived rate was 83.78%.5) The fermentation performance of freeze-dried powder was verified, and in the fermentation of 5.3h at 42℃appears curd, the number of viable bacteria was up to 1.37 ×1012 CFU/g after freeze drying. Textural hardness and chewiness were significantly increased, gumminess decreased significantly(p<0.05), indicated that the Qula DVS starter had better coagulation effect.6) 3 strains of lactic acid bacteria powder of lactate dehydrogenase(LDH) and beta galactosidase was measured, found that adding the freeze-drying protective agent to extracellular two enzyme content decreased, the content of intracellular enzymes has increased, indicating that the protective agent of lactic acid bacteria cell membrane and intracellular enzyme has a significant protective effect.(3) Qula DVS fermentation agent under different conditions of temperature and packaging during 90 days storage, the viable count-20℃ for storage of bacteria powder during storage was significantly greater than-4℃ and room temperature storage of bacteria powder(p<0.01); coagulation time and p H with storage time extended upward trend, titratable acidity with prolonged storage time of decline. The rate change of non vacuum and stored under room temperature was higher than that of other packages. |
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