Survival of undesirable micro-organisms in fruit yoghurts during storage at different temperatures

The survival of undesirable micro-organisms Yersinia enterocolitica, Escherichia coli, Rhodotorula mucilaginosa, Kluyveromyces marxianus in fruit yoghurts after inoculation at two different levels (10(2)-10(3) cfu/ml and 10(4)-10(6) cfu/ml) was investigated during storage at 4 and 8 degrees C. When stored at 4 degrees C, the survival of Y. enterocolitica was not significant (3 days) except when the size of the initial inoculum was larger than 10(5) cfu/ml: in this case, viable cells of the pathogen were still recovered after 17 days of storage. Cells of E. coli were defected in the yoghurt samples after 24-48 days at 4 degrees C, regardless of the inoculum size. The growth of yeasts easily occurred at 4 degrees C; R. mucilaginosa showed an increase of 10-100 fold and K. marxianus) was able to grow from 10(2) to 10(5)-10(6) cfu/ml during the 30 days of storage. Similar trends were noted with 8 degrees C storage. The survival of both contaminants was longer than 8-10 days and yeasts were able to grow remarkably well, with microbial values of 10(6) cfu/ml R. mucilaginosa and 10(8) cfu/ml (K. marxianus in the case of the large inoculum. Throughout the whole experiment, the counts for lactic streptococci were around 10(8)-10(9) cfu/ml whereas lactobacilli levels were 10(6) cfu/ml, and viability declined after 15 days of storage. The competition in vitro between both Lactobacillus and Streptococcus strains isolated from yoghurt and E. coli was also investigated. With Lactobacillus, the inhibition of E. coli was very effective; no viable E. coli cells were detected after 24 h incubation. The Streptococcus strain was also able to inhibit E. coli but not so rapidly; slight growth of E. coli was observed during the first 3 h, followed by a marked decrease in the number of cells. (C) 1998 Academic Press Limited.