Utilization Cold Sensitive Str Mutants of lactic acid bacteria for prolongation of fermentedmilk Shelf Life

Background and Objectives: The objective of this study was selection of streptomycin resistant cold sensitive (CS) mutants of Lactobacillus acidophilus MDC 9626 as starters for fermentation dairy foods with longer shelf life.Materials and Methods: The strain of Lactobacillus acidophilus MDC 9626 obtained from the Microbial Depository Center of NAS RA. Streptomycin-resistant mutants were obtained by treating with 300μ/ml NG and plating on MRS agar containing 100 μg/ml of streptomycin. CS mutants screened by replica plating technique of Str colonies and cultivation of replica plates at minimal growth temperature (Tmin). The mutants are not growing at minimal temperature considered as cold sensitive.Results: Five CS Str mutants of Lactobacillus acidophilus MDC 9626 were selected and characterized. The magnitude of CS mutants minimal growth temperature changes was range of 4-9°C. Except of cold sensitiveness, statistically significant up and down changes were revealed in rates of growth and milk fermentation when CS Str mutants grown at optimal temperature. Yoghurts fermented by CS starters retain high viable cells count, low titratable acidity, goodtexture and flavor at least for four weeks of cold storage without any unfavorable changes in color, odor, appearance and microbial contamination, compression with two weeks shelf life of the yogurt fermented by parental strain.Conclusions: The cold sensitive starters could serve as an alternative solution for expanding shelf life of dairy products at fridge temperatures. The conditionally lethal str mutations except of expanding shelf life can be used for improving technological characteristics e.g. promotion growth and milk fermentation rates of dairy starters.

Keywords:

Lactobacillus acidophilus , milk fermentation , cold sensitive starter , streptomycin resistant mutants , shelf life