Simultaneous Enumeration of the Characteristic Microorganisms in Yogurt Using the Hydrophobic Grid Membrane Filter System.

Utilization of the hydrophobic grid membrane system (HGMF) in conjunction with erioglaucine-supplemented tryptone-phytone-yeast extract (TPYE) agar permitted the differential enumeration of Streptococcus thermophilus and Lactobacillus bulgaricus in yogurt. An anaerobic incubation of 42°C resulted in conditions under which streptococcal colonies were light blue and the lactobacillus colonies were a distinct dark blue. Excellent correlation was achieved when the above method was compared with the International Dairy Federation Standard 117:1983 by coincident analysis of 12 commercial yogurts. The method allowed a reliable single plate determination of cocci to rod ratios from 20:1 to 1:5.