Interactions between Lactobacillus acidophilus strains and the starter cultures, Lactobacillus bulgaricus and Streptococcus thermophilus during fermentation of goats’ milk

Abstract

It has been observed that Lactobacillus acidophilus has poor survivability in fermented goats’ milk. In this study, interactions between L. acidophilus and starter cultures during goats’ milk fermentation were investigated using three strains of L. acidophilus: ATCC-11975, LA-5 and NCFM, and the starter cultures (Lactobacillus delbrueckii subsp. bulgaricus (LB350) and Streptococcus thermophilus (ST350), isolated from a commercial yogurt starter. Selective enumeration methods were validated; de Mann, Rogosa and Sharpe (MRS) agar with 0.2% bile and anaerobic incubation at 37°C (72 h) was found to be suitable for all L. acidophilus strains; MRS agar with pH adjusted to 5.2 and anaerobic incubation at 37°C (24 h) was used for strain LB350 and M17 agar with 0.5% lactose and aerobic incubation at 37°C (24 h) for strain ST350. Addition of LB350 and/or ST350 into the goats’ milk inoculated with L. acidophilus strains accelerated pH decrease compared to L. acidophilus strains used alone. Antagonism between each of the L. acidophilus strains and LB350 occurred, most noticeably with the LA-5 culture. However, it varied widely between the acidophilus strains indicating that antagonism is likely to be strain specific.