Effect of the Cultivation Conditions on the Autolysis of Yoghurt Strains

Abstract:

The influences of cultivated conditions on the autolysis of Lactobacillus delbeueckii subsp.bulgaricus (KLDS 1.9201) and Streptococcus salivarius subsp. thermophilus (KLDS 3.0201) were studied in this paper. The results indicated that temperature shift (from 30℃ to 55℃), pH shift (from pH 5.0 to pH 7.5) and salt concentration shift (from 0% to 5%) not only increased the autolysis rate of KLDS 1.9201 and KLDS 3.0201, but also led to increase nucleic acid and protein released by strains. Extensive autolysis of yoghurt strains occurred when incubated in 5% salt concentration at 55℃and pH 7.5, the extent of autolysis of KLDS 1.9201 and KLDS 3.0201 reached 55% and 36.3% respectively, significantly higher than extent of autolysis of KLDS 1.9201(9.96% lysis) and KLDS 3.0201(8.6% lysis) when incubated in 0% salt concentration at 30℃ and pH 5.5 (P<0.05). As observed by transmission electron microscopy, there were remarkable differences on cell lysis between the optimal and suboptimal culture conditions. The observations revealed that cell-wall degradation occurred concomitantly with cell autolysis and were well correlated to optical density measurements during bacterial cell autolysis.