Characterisation of yeasts isolated from traditional opaque beer beverages brewed in Zimbabwean households

Variability exists in raw materials and processing methods used to produce household traditional opaque beers in Zimbabwe, resulting in beers of variable quality, depending on fermenting micro-organisms involved. Yeasts are important in determining the alcohol content, nutrition and organoleptic properties of the beers. This study aimed at determining the diversity and characteristics of the predominant yeasts isolated from a variety of beers collected from rural households in different geographical localizations. Predominant yeasts from 13 beer samples were characterized using morphological, biochemical and physiological tests. A total of 14 morphologically different yeasts were isolated. Yeast counts in the beer samples ranged from 7.87 to 9.56 log colony forming units/ml. From the 14 yeast isolates, a total of 11 yeasts were identified to species level. Saccharomyces cerevisiae was the predominant species identified in the beers. Other yeast species identified in the beers were Issatchenkia occidentalis, Kluyveromyces marxianus, Candida glabrata and Sporobolomyces holsaticus. Two yeast isolates were identified as belonging to the genus Rhodotorula. Ten of the isolates were able to ferment at least one of the fermentation substrates D-glucose, D-galactose, maltose, sucrose and raffinose, while three isolates were incapable of fermenting any of the fermentation substrates used. None of the isolates were able to ferment lactose. Five of the S. cerevisiae isolates were able to grow at 40°C while K. marxianus was the only isolate capable of growing at 45°C.