Lactose-fermenting yeasts in yoghurt and their effect upon the product and the bacterial flora.

With the exception of certain sour milks such as kefir and koumiss, the presence of lactose-fermenting yeasts in dairy products is most undesirable because it results in gassy fermentation. Since the early days of microbiology, many lactosefermenting yeasts, both sporogenous and asporogenous, have been isolated. In most of the cases, however, the descriptions of these yeasts, viewed by present standards, are inadequate for a conclusive comparison with similar yeasts identified in recent years. In view of this, and because the literature on these yeasts from 1887 to 1910 has been reviewed by Hammer and Cordes (1920), no specific reference of these sources is given. In 1918, Hunter recognized a nonsporulating lactosefermenting yeast responsible for "foamy" cream. In 1920, Hammer and Cordes isolated from "gassy" cream two types of lactose-fermenting yeasts which they described in detail and named Torula cremoris and Torula sphaerica. In England, Allen and Thornley (1929) reported the isolation from "gassy" cheese of two lactosefermenting yeasts, one sporogenous and the other asporogenous, referred to as B and C, respectively. In 1942, Weiser isolated from split-rind Swiss cheese a lactose-fermenting yeast which he identified as T. cremoris previously described by Hammer and Cordes (1920). The author, examining samples of yeast-contaminated yoghurt, isolated lactose-fermenting yeasts in four instances (Soulides, 1955). This investigation deals with the study of these yeasts in connection with their effect on the product and its bacterial flora.