LEE’S AGAR

Description

Yoghurt is a fermented milk product in which Streptococcus thermophilus and Lactobacillus bulgaricus are the essential microbial species that are active in a symbiotic relationship. To obtain optimum consistency, flavour and odour, the two species should be present in about equal numbers in the culture. Dominance by either species can cause defects. Lees Agar, described by Lee et al is used for the differential enumeration of yoghurt starter bacteria. This medium is also recommended by APHA for the same purpose. Lees Agar contains sucrose, which most L. bulgaricus strains will not ferment, but S. thermophilus will, and lactose, which both species utilize. With a suitable combination of sucrose and lactose, the rate of acid production by S. thermophilus is enhanced and that of L. bulgaricus restricted. Therefore, Streptococci grow first and produce a creamy, buttery aroma from diacetyl and similar metabolites. The redox potential is also thus, lowered by Streptococci, which enables Lactobacilli to grow, thereby growth stimulatory products for Streptococci are synthesized by Lactobacilli. Hence the typical sharp acetaldehyde flavour of mature yoghurt is formed.