The differential enumeration of Lactobacillus delbrueckii subspecies bulgaricus and Streptococcus salivarius subspecies thermophilus in yogurt and labneh using an improved whey medium

Bromocresol green whey agar (BGWA), an elective differential medium for yogurt bacteria, was prepared by mixing 2 parts of whey obtained by autoclaving (121°C/15 min) reconstituted non-fat dry milk (NFDM) (15% w/w; pH 5.7 by 1 N HCl) with 1 part of a sterile agar solution (115°C/15 min) containing 3% yeast extract, 1.2% K₂HPO₄, 0.004% bromocresol green and 4% agar. Lactobacillus delbrueckii subsp bulgaricus colonies in BGWA pour plates were light in colour and in the form of an irregular mass with twisted filament projections, while Streptococcus salivarius subsp thermophilus colonies were green lenticular with entire edges. BGWA performed generally better than deMan-Rogosa-Sharpe agar and M17 agar when L bulgaricus and S thermophilus respectively were enumerated in samples of commercial yogurt and labneh (yogurt concentrated by removing part of its whey) and in single cultures of yogurt bacteria in NFDM. Immediately after processing, the counts of yogurt bacteria in labneh samples were similar to those of yogurt samples, indicating a loss of these bacteria with whey during processing. The decrease in the numbers of yogurt bacteria in labneh samples at the end of the shelf-life (14 days at 7°C) was slight, indicating a relative resistance of these bacteria to acidity and refrigeration.