The effect of container type on the growth of yeast and lactic acid bacteria during production of Sethemi, South African spontaneously fermented milk

Sethemi, a South African traditional fermented milk, was prepared in the laboratory by spontaneously fermenting raw milk in four different types of containers; namely, a clay pot, gourd, nickel jar and plastic bowl. The development of bacteria and yeasts in the different containers was followed by taking samples at 24 h intervals for 7 days. The predominating microorganisms in all cases were the aerobic mesophilic bacteria, lactobacilli–leuconostocs (LAB on MRS) and lactococci (LAB on M17). No significant (p > 0.05) differences were found in the number of microorganisms between the different containers. However, there were significant (p < 0.05) differences with respect to the types of dominant yeast species, and the diversity of yeast species. The least time to attain maximum LAB counts was recorded for the nickel container. In milk fermented in the clay pot and gourd the most dominant yeast species were Debaryomyces hansenii and Saccharomyces cerevisiae, respectively, while Cryptococcus curvatus predominated in both the nickel and plastic containers. The highest yeast diversity was obtained from milk samples fermented in the clay pot represented by 15 species. Of the 23 species isolated from all the containers and raw milk, only D. hansenii, Cryptococcus humicola and Kluyveromyces marxianus were isolated from both raw and naturally fermented milks of all containers. The use of different types of containers therefore influenced the diversity of yeasts in the fermented milk and may influence the characteristics of the fermented milk product. The clay pot offered the greatest yeast diversity suggesting that it could be the best suited for preparing traditional fermented milk.