Growth characteristics of Candida kefyr and two strains of Lactococcus lactis subsp. lactis isolated from Zimbabwean naturally fermented milk

Abstract

Lactic acid bacteria (LAB) and yeasts constitute part of the microflora in Zimbabwean traditional fermented cows' milk, amasi. The present study was carried out to investigate the growth characteristics of Candida kefyr 23, Lactococcus lactis subsp. lactis biovar. diacetylactis C1 and L. lactis subsp. lactis Lc261, previously isolated from amasi, in ultrahigh temperature (UHT)-treated cows' milk. The strains were inoculated into the UHT milk as both single and yeast–LAB mixed cultures, and incubated at 25°C. The changes in volatile organic compounds, organic acids, carbon dioxide, pH, and viable cell numbers were recorded during the 48-h fermentation and after storage of the fermented milk at 4°C for 5 days. The LAB reached maximum and stationary phase numbers of about 9 log 10 cfu ml −1 after 18 h, while the yeast attained maximum populations of 7 log 10 cfu ml −1 after 24 h. L. lactis subsp. lactis biovar. diacetylactis C1 remained viable during storage when it was co-cultured with the yeast, while Lc261 decreased during storage in both single and co-cultures. Both Lactococcus strains accumulated galactose during active growth while utilising about 1.0 g 100 g −1 lactose after 48 h. Acetaldehyde was produced at a faster rate in co-culture and the levels continued to increase during storage. Methyl aldehydes were also produced continuously by the C1/23 co-culture during fermentation and storage. The study showed that a stable fermented milk product could be produced using a mixed culture of C1 and C. kefyr 23, and that a fermentation period of 24 h at 25°C would be sufficient.