Survival of Streptococcus thermophilus and Lactobacillus bulgaricus in Commercial and Experimental Yogurts.

Elliker's Lactic agar and Rogosa SL Agar, an agar medium selective for Lactobacillus bulgaricus , were used to determine numbers of Streptococcus thermophilus and L. bulgaricus in commercial and labortatory-manufactured yogurt. Typically, the population of viable yogurt organisms increased initially after manufacture of yogurt, and then decreased during refrigerated storage of the product. Numbers of S. thermophilus and L. bulgaricus remained above 100 million/g in plain and strawberry University of Wisconsin (UW) yogurts stored at 5°C for 60 d. Numbers of the bacteria decreased more when these yogurts were stored at 10°C, although they remained above 10 million/g, except for S. thermophilus in plain UW yogurt, which decreased to less than 1 million/g after 47 d of storage. Commercial custard-style plain and blueberry yogurts had S. thermophilus populations above 300 million/g during the 60-d storage period at both 5 and 10°C. Numbers of L. bulgaricus decreased from 300 million/g at 15 d after manufacture to less than 1 million/g after 42 to 56 d. Numbers of S. thermophilus in commercial stirred plain and raspberry yogurts remained above 1 million/g when the products were stored at 5°C for 60 d, although they decreased to less than 1 million/g after 48 to 58 d when yogurt was stored at 10°C. The L. bulgaricus populations decreased to less than 1 million/g after 25 to 32 d for both varieties of stirred yogurt at both storage temperatures. Presence of various fruits in commercial yogurts had little effect on survival of the yogurt bacteria. With laboratory-manufactured yogurt, different incubation temperatures and milk mixes used to make the yogurt had little effect on survival of the yogurt organisms during subsequent refrigerated storage of the product.