The Study On The Starter Culture Of Fresh Acid-curd Cheese And Fermentation Technology

Because of it is high valuable, cheese will become developing direction of fermented dairy product industry. It's very important to research and develop high activity concentrated yoghourt starter culture for promoting the industrial process of lactic acid bacteria starter and development of fermented dairy industry. In this paper, it has isolated strains and optimization collocation for fine lactic acid bacteria, studied the primary factors which affect manufacturing technology of fresh acid-curd cheese. The cheap enrichment medium suited for industrial processing and freeze-drying protectant were screened. The technology of enrichment culture for Streptococcus cremoris, cell concentration, lyophilization were studied. The fermentative activity of the freeze-drying starter in milk were examined.Inoculation, fermented temperature were studied on the effect of fermentation. The growth and fermentation of four Streptococcus cremoris and five Streptococcus thermophilus strains were studied. Two special strains of LC1 and ST-SY were obtained. In this paper, yield, whey pH and whey OD were studied to screen suitable starters agent combination, addition amount of rennet and the final heating temperature. The result show that the optimum mixed starters was 8∶1, addition amount of rennet was 1% of activity unit and the optimal final heating temperature was 55℃.Jerusalem artichoke juice medium, tomato juice medium, carrot juice medium and wort medium which are apt to gain and separate are obtained by cell groeth experimented. The result showed that tomato juice was the best base medium for Streptococcus cremoris. The effect of adding 12 kinds of different nutrient factors in tomato juice medium. The result showed that soya peptone, yeast extract, glucose, corn liquor and K₂HPO₄ could promote the growth of Streptococcus cremoris. K₂HPO₄ could not only promote cell growth, but also protect lactobacillus from being damaged by low pH of medium. The optimum enrichment medium obtained by orthogonality test L₉(3⁴) consist of adding 1% soya peptone, 0.5% yeast extract, 0.7% corn liquor and 0.2% K₂HPO₄ in tomato juice medium.In the optimum enrichment medium, the living-cell number of Streptococcus cremoris were 1.42×10¹⁰cfu/mL cultivated for 14h at 30℃, which were increased by 95 times compared with tomato juice base medium and the same with the living number in the MRS used in lab. The effects of centrifuging force, time on the loss rate, survival rate and harvest rate of lactobacillus was studied. The results showed that the optimum conditions for centrifugation of Streptococcus cremoris was 5738g (7500r) for 5min and the highest harvesting rate of living cells was 98.07%.A complex protectant was screened, which could protect cell from damaging during freeze-dry. After freeze-drying, the living-cell number of starter, whether single strain or mixed strains were over 10¹¹cfu/g. The cheese fermented by protectant. The result were no differental significance compared with traditional culture which show cheese yield was 31.56%, whey OD was 0.515 and whey pH was 4.6.The quality of products was analyzed and evaluated initially. The result indicated that sensory and health items were up to the national quality standard of NY478-2002. The product was of a well- distributed white color, dairy flavor, no bitter, texture exquisite, soft and grainy.