Studies On Methods Of Selective Enumeration Of Lactobacillus Delbrueckii Subsp. Bulgaricus In Yoghurt

Scientific studies have demonstrated that yoghurt is good for healthof people, which has been widely accepted by man. Streptococcus thermophilic and Lactobacillus delbrueckii subsp. bulgaricus are the most common starter cultures in yoghurt.The ratio of the living Streptococcus thermophilic and Lactobacillus bulgaricus has important infl-uences in the flavors and tastings in yoghurt. The living lactic acid bacteria in yogurt must keep a certain number (every g or mL not less than 106 cfu inproduct) in the storage period, Now the most dairy industries and inspection departments mainly adopt the plate count method. However, the selectiving enumerations to the numbers of the living Lact-obacillus bulgaricus has disadvantages, for example, complicated compositions of MRS solid media, strict culturing conditions (higher temperature, anaerobic), longer culturing time, and so on.Four strains of Streptococcus thermophilic and four strains of Lactobacillus bulgaricus are studied in the test, the compositions of the soli-d media and inhibitive substance and culturing conditions are changed. To explore these conditions will have effects on the numbers of living Streptococcus thermophilic and Lactobacillus bulga-ricus, and to gain the selectiving solid media of the living Lactobacillus bulgaricus in yogurt. Main results are as follows:The concentration of sodium acetate is 3.00%(m/m) in MRS solid media, at 37℃ with culturing 48h; the pH of AST solid media is 5.50, at 37℃ with culturing 48h; the concentration of sodium acetate is 4.00%in AST, at 45℃ with culturing 48h; the conce-ntration of sodium acetateis 2.50% in the optimizing AST solid media, at 37℃ with cult-uring 48h; the pH of the optimizing AST is 5.70, at 37℃ with culturing 48h. The selecti-ving enumeration of Lactobacillus bulgaricus is achived in yoghurt by these solid media and, culturing conditions. However, it is failed to selectively enumerate the numbers of living Lactobacillus bulgaricus in yoghurt by adding a certain concentration of antibac-terial substance(penicillin G) in MRS solid media.Finally, it is concluded that the concentration of sodium acetate is 2.50% in the opti-mizing AST solid media, at 37℃ with culturing 48h. It turned out that the compositions of solid media of selectiving enumeration are simple, the culturing conditions are achie- ved easily, culturing time is short. the result of the numbers of living Lactobacillus bulgaricus is more accurate. Therefore, the new media can provide a reference to reduce the cost of the laboratory operations and improve the efficiency of work.