The growth and interaction of yeasts and lactic acid bacteria isolated from Zimbabwean naturally fermented milk in UHT milk

Nine yeast and four lactic acid bacterial strains, previously isolated from Zimbabwean traditionally fermented milk, were inoculated into ultra-high temperature treated (UHT) milk in both single and yeast–lactic acid bacteria co-culture. The lactic acid bacteria (LAB) strains consisted of Lactococcus lactis subsp. lactis biovar. diacetylactis C1, L. lactis subsp. lactis Lc39, L. lactis subsp. lactis Lc261 and Lactobacillus paracasei subsp. paracasei Lb11. The yeast strains used were Candida kefyr 23, C. lipolytica 57, C. lusitaniae 63, C. lusitaniae 68, C. tropicalis 78, Saccharomyces cerevisiae 71, S. dairenensis 32, C. colliculosa 41 and Dekkera bruxellensis 43. After 48-h fermentation at 25°C, the samples were analysed for pH, viable yeast and bacterial counts, organic acids, volatile organic compounds (VOC) and carbon dioxide. The Lactococcus strains reduced the pH from about 6.6 to between 4.0 and 4.2, while Lb. paracasei subsp. paracasei Lb11 reduced the pH to about 5.4. Most of the yeasts, however, did not affect the final pH of the milk except for C. kefyr 23, which reduced the pH from 6.6 to 5.8. All the Lactococcus strains grew two log cycles during the 48-h fermentation period, while Lb. paracasei subsp. paracasei Lb11 grew about one log cycle. S. cerevisiae 71, C. colliculosa 41 and D. bruxellensis 43 showed poor growth in the milk in both single and co-culture. The other species of yeast grew about two log cycles. Candida colliculosa 41, S. dairenensis 32 and D. bruxellensis 43 showed reduced viability when in co-culture with Lb. paracasei subsp. paracasei Lb11. The samples in which C. kefyr 23 was used were distinct and characterised by large amounts of acetaldehyde, carbon dioxide and ethanol. However, in the samples where S. dairenensis, C. colliculosa, D. bruxellensis, C. lusitaniae, C. tropicalis, C. lipolytica and S. cerevisiae were used in co-culture, the final pH and metabolite content were mainly determined by the corresponding LAB strain. From the observations, it was concluded that C. kefyr 23 grows well in UHT milk and produces VOC that may be important to the flavour profile of the fermented milk. Enhancement of production of some flavour compounds such as acetaldehyde and malty compounds in some yeast–LAB co-cultures was presumed to be indicative of interaction between the microorganisms.