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

The survival of undesirable micro-organismsYersinia enterocolitica,Escherichia coli,Rhodotorula mucilaginosa,Kluyveromyces marxianusin fruit yoghurts after inoculation at two different levels (102–103cfu/ml and 104–106cfu/ml) was investigated during storage at 4 and 8°C. When stored at 4°C, the survival ofY. enterocoliticawas not significant (3 days) except when the size of the initial inoculum was larger than 105cfu/ml: in this case, viable cells of the pathogen were still recovered after 17 days of storage. Cells ofE. coliwere detected in the yoghurt samples after 24–48 days at 4°C, regardless of the inoculum size. The growth of yeasts easily occurred at 4°C;R. mucilaginosashowed an increase of 10–100 fold andK. marxianus) was able to grow from 102to 105–106cfu/ml during the 30 days of storage. Similar trends were noted with 8°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 106cfu/mlR. mucilaginosaand 108cfu/ml (K. marxianusin the case of the large inoculum. Throughout the whole experiment, the counts for lactic streptococci were around 108–109cfu/ml whereas lactobacilli levels were 106cfu/ml, and viability declined after 15 days of storage. The competitionin vitrobetween bothLactobacillusandStreptococcusstrains isolated from yoghurt andE. coliwas also investigated. WithLactobacillus, the inhibition ofE. coliwas very effective; no viableE. colicells were detected after 24 h incubation. TheStreptococcusstrain was also able to inhibitE. colibut not so rapidly; slight growth ofE. coliwas observed during the first 3 h, followed by a marked decrease in the number of cells.