| LAB is the gram positive bacteria, which can produce lactic acid from carbohydrates. LAB is classified into 23 species in Bergey’s Manual of Systematic Bacteriology, and 7 species is widely used in food and medical area. Yoghurt is one of the familiar food fermented by LAB. Closely relation was found between yoghurt quality and the choosing strains. Recent researches revealed that some strains could produce exopolysaccharide, These strains were known as ropy strains. Ropy strains could rise the viscosity, reduce the syneresis, improve the quality and lessen the use of thickener, safety and reliability, so it became the hot research in the two decades.In this research, LAB were collected, isolated and screened from different materials and four ropy strains of Streptococcus thermophilus were chosen out. Consequently, yoghurt samples with these four ropy strains were evaluated by organleptic analysis. Viscosity stability, growth curve, pH curve and viscosity curve during the fermentation of K2 strain with highest scores were studied, and it was applicated in yoghurt production. The results were as follows:1. Separation and Indentification of LAB. From different materials, forty-one strains of LAB were screening, which were identified to 10 species by microscopic observation, physiological and chemical experiment. They were Bifidobacterium longum, Leuconostoc mesenteroides, Streptococcus thermophilus, Lactobacillus bulgaricus, Lactobacillus helveticus, Lactobacillus casei, Lactobacillus plantarum, Lactobacillus rhamnosus, Lactobacillus acidophilus and Lactobacillus carvalus.2. Screen and evaluation of ropy strains. Twentity-five strains were selected out from the forty-one strains to prepare yoghurt, and the viscosity was compared with each others. The results showed viscosity values of yoghurt prepared by K2, K4, K8, K9 were in 4500mPa·s, The samples fermented with K4 strain had the highest viscosity values and it reached 4568mPa·s. Furthermore, quality evaluation was applied with organleptic analysis, viscosity, syneresis, water holding capacity were developed by using objective and subjective shares. The result showed K2 strains had the highest scores. Consequently, the strain was used in the follow experiment. 3. Analysis of fermented characters of K2 ropy strain. First, fermented characters of Streptococcus thermophilus K2 with highest scores were studied. The result revealed skimmed milk was solidified at 4h in fermentation. The viscosity reached to 4700 mPa·s, titrable acidity, pH reached to 4.3 and 96°T separatively at the same time. The effect of total solid, treatment temperature and time of milk, fermentation temperature on the viscosity were also investigated. The results found yoghurt with good quality could be made and the viscosity value could reach 4700mPa·s, if only total solid was above 10%, the heat temperature was over 60℃, the heat time was longer than 5min, fermentation temperature was between 37℃and 42℃. The effect of stirred technology on viscosity was also studied, the results showed optimum condition: stirring speed was 30r/min, stirred time was 30s, storage temperature was 2℃and storage time was 24h.4. Preliminary researches of K2 strain applied in yoghurt production were studied. Viscosity changes of mixed strains {Streptococcus thermophilus K2 and Lactobacillus bulgaricus G) were studied. The results were the viscosity values of mixed strains were higer than single strain and viscosity recovery was quicker than single strain in shelf-life. Applied K2 strain in yoghurt production, compared the viscosity with nomal yoghurt added with gelation and modified starch. The result showed that the viscosity of yoghurt with K2 strain was equal to the viscosity with 0.3%-0.4% gelation, 0.2%-0.25% No.6 thickener, 0.3%-0.4% No.7 thickener and 0.2%-0.4% No.8 thickener. |
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