The earlier studies have also supported the stationery mode for lactic acid production. Gandhi et al. (2000) used stationary conditions for the lactic acid production using different lactobacilli cultures (L. delbrueckii subsp. bulgaricus, L. acidophilus, L. casei etc.). However, stationary conditions for growth and agitation mode for fermentation have also been reported in some earlier studies (Roy et al., 1986).
Since, no difference was observed for lactic acid production with agitation, stationary mode was selected in further investigations.
Effect of incubation period
To find out the optimal incubation time for the maximal lactose utilization and lactic acid production, the whey medium inoculated with bacterial culture was incubated for 48 h under the above optimized conditions. The samples were drawn at specified time intervals and the results obtained are presented in Fig. 7. As evident from the results, an increase in lactose utilization and subsequent lactic acid production was found up to 36 h and thereafter no improvement in both the functions was observed. This could be attributed to the growth of the culture reached to the stationary phase and as a consequence of metabolism, micro-organisms continuously change the characteristics of the medium and the environment. A maximum lactic acid production of 33.73 g/L with lactose utilization of 95.62% (w/v) was observed after 36 h of incubation. The reduction in fermentation period is additionally advantageous to improve the economics of the process. Therefore, an incubation time of 36 h was considered optimal for maximum lactose conversion to lactic acid.