Microflora of Austrian Natural-Set Yogurt

Randomly selected Austrian natural-set yogurts were examined for storage-induced changes in the populations of viable yogurt starter organisms and microbial contaminants. Typically, numbers of Streptococcus thermophilus and Lactobacillus bulgaricus remained above 10⁸ cfu/g in yogurt stored at 10°C until the sell-by-date (15-20 d after manufacture). Both elevated storage temperature and storage periods markedly reduced the survival of yogurt starter bacteria. Depending on the sample origin, the yeast populations increased from less than 10/g to above 10⁶/g when yogurts were stored at 10°C until the sell-by-date. The isolated yeasts were identified as Candida inconspicua , C. intermedia , C. parapsilosis , C. rugosa , C. tropicalis , C. zeylanoides , Debaryomyces hansenii , Metschnikowia reukauffii , Pichia guilliermondii , Rhodotorula mucilaginosa , Torulaspora delbrueckii , Trichosporon beigelii , and Yarrowia lipolytica . While a few out of 233 isolates were found to assimilate lactose, none were able to ferment lactose.