Enumeration of starter cultures in fermented milks

Abstract

SummarySome media available for the isolation and enumeration of starter cultures employed for the manufacture of cheese, yogurt and bio-yogurt were examined. Reddy's medium or a modification of Elliker's medium was found to be most satisfactory forLactococcusspp., while trypticase phytone yeast (TPY) agar with a mixture of antibiotics proved suitable for the discrete enumeration ofBifidobacteriumspp. The inclusion of Prussian blue (PB) in reinforced clostridial medium or tryptone proteose peptone yeast extract (TPPY) agar gave excellent differential counts for the starter bacteria in yogurt even when the culture was imbalanced, while TPPY (PB) agar allowed the visible separation of all four of the organisms that might be found in a typical bio-yogurt, namelyLactobacillus delbrueckiisubsp.bulgaricus, Streptococcus thermophilus, a,Bifidobacteriumsp. andLb. acidophilus. It was noted that variation among different strains of any given species could change the expected reactions, so for quality control purposes the suggested media may need to be modified to cope with the specific cultures in use.