Study on H+-ATPase deficient mutant of Lactobacillus delbrueckii subsp. bulgaricus to post-pone postacidification of yogurt and property of fermentation.

The post-acid level of Lactobacillus delbrueckii subsp. bulgaricus named L15-4 was examin-ed,which was selected by neomycin,and the components of the optimal high density fermentation medium for the strains were studied by orthogonal test. The yogurt fermented by three Lactobacillus delbrueckii subsp. bulgaricus on 42 ℃ examined the change of the pH and titratable acidity,when the yogurt were stored at 4 and 20 ℃. And the yogurt fermented by 3 Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus the-rmophilus on 42 ℃ examined the change of the pH and titratable acidity,when the yogurt were stored at 4 and 20 ℃. MRS medium was the basic medium for the slected stains,potato juice,skim milk and yeast extract were screened for the selected strains. The result showed that the post-acid level of L15-4 was weaker than that of L15 and LW-1;and most optimum enrichment mediums was 3.0% skim milk,5.0% potato juice,and 0.5% yeast extract.