Abstract Description

Autochthonous yeasts are central to the spontaneous fermentation of traditional Marula wine. However, little is known about their physiological behaviour, nutrient requirements, or how they respond to the typical fermentation stresses. The present study aimed to better understand autochthonous strains by characterising their growth patterns, sugar utilisation, and tolerance to environmental stressors such as temperature, ethanol concentration, and acidity. Baseline growth of the selected yeast, including saccharomyces and non-saccharomyces strains inoculated in yeast extract-glucose broth was determined by measuring absorbance at OD₆₀₀ and viable cell counts. The yeast strains were grown in the presence of ethanol at concentrations ranging between 4% and 10% (v/v) and at pH between 3.0 and 4.0 for ethanol and acid tolerance respectively. Adaptability to wine fermentation temperature was investigated at 15 °C and 25 °C. Sugar utilisation was investigated by growing the yeasts in the presence of glucose, fructose, and sucrose individually as well as in the presence of a sugar mixture representative of Marula juice. Saccharomyces type yeasts consistently showed strong growth, tolerance to temperature changes, moderate ethanol tolerance and robust acid tolerance. Despite the slow growth observed for non-saccharomyces yeast strain 15M5 under all conditions, it tolerated acidic conditions well and showed relatively better growth on fructose and sucrose compared to glucose. None of the isolates survived in the presence of ethanol levels from 8 - 10%, indicating a clear threshold of tolerance. These findings offer valuable insights into the functional diversity and resilience of Marula-associated yeasts, supporting their potential for domestication and for developing a yeast consortium for controlled production of marula wine and other traditional fermented beverages.