Lactase activity in yoghurt and lactic acid bacteria

Individual starter culture were inoculated into liquid medium and incubated at for 16 hours. Whole cell were obtained and evaluated for activity using orthonitrophenyl- (ONPG) as substrate. S. thermophilus had more activity than other Lactobacilli did. To study the effect of storage temprature on enzyme activity of yoghurt, some samples of cultured yoghurt were stored under refrigeration , and the others under room temperature . At , yoghurt had activity and many viable bacteria in 1 month. After 20 days, yoghurt had maximum activity. At , yoghurt had activity by 5 days. As this experiment shown activity was ascribed to viable bacteria, especially S. thermophillus. Commercial yoghurt had lower activity. There were considerable variations with regard to the lactose hydrolyzing capabilities of commercial yoghurt samples.