Abstract:The lactic acid bacterial strains SDC4-1, MDC3-1, and WDC3-1 were isolated from milk cake and acid whey obtained from Sani area of Yunnan province. A superior strain was identified based on acid resistance and bile salt tolerance tests. The fermentation capability of the superior strain was studied in terms of acid production, post-acidification acetaldehyde production, protease activity, and β-galactosidase assay. All three strains were found to be acid tolerant; MDC3-1 showed the strongest acid resistance with a survival rate of 81%; WDC4-1 showed the weakest acid resistance, with survival rate of 52%. However, only SDC4-1 was tolerant to bile, and the survival rate was 81% after being in contact with 0.5% bile salt for 4 hours. Therefore, SDC4-1 was considered the superior strain. In the fermentation test that followed, SDC4-1 strain showed a perfect curd consistency and the fermented yoghourt was rich in flavors. Additionally, high viable count was noted during the storage period, reaching 9.79 lg CFU/g after a 15-day cold storage, which is far more than the national standard. SDC4-1 showed increased rate of acid production, where acidity reached 82.0°T after 13 hours. Meanwhile, SDC4-1 exhibited good post-acidification, good acetaldehyde-producing ability, strong proteolytic activity, and high β-galactosidase production (up to 7.7 μg/mol), while maintaining stable yoghurt quality during cold storage. Therefore, the selected strain, SDC4-1, showing good fermentability, can be considered for use in the preparation of fermented food.
