Ziziphus mauritiana (masau) fruits fermentation in Zimbabwe: from black-box to starter culture development

This thesis reports on studies of microbiological and biochemical properties of masau (Ziziphus mauritiana) fruit fermentation and the development of starter cultures for the production of masau beverages. A survey to document the traditional processing techniques was conducted using a questionnaire and focus group discussions in each of the three districts, i.e., Mudzi, Mt Darwin and Muzarabani in Zimbabwe. The survey results showed that the masau fruit is usually gathered by women and children, and eaten raw or processed into products such as porridge, traditional cakes, mahewu (non-alcoholic fermented beverage), jam, which are sold at local markets. It is also naturally fermented under uncontrolled conditions and distilled into kachasu. The nutritional composition of the masau fruit was analysed. The fruits are good sources of nutrients such as carbohydrates, protein, and essential micronutrients such as calcium, potassium, phosphorus, copper, iron, zinc and vitamin C. In order to enable the selection of starter cultures for the production of masau wine and distillate, yeasts, yeast-like fungi, and lactic acid bacteria present on the unripe, ripe and dried fruits, and in the fermented masau fruits were isolated and identified using physiological and molecular methods. The predominant species were identified as Saccharomyces cerevisiae, Pichia kudriavzevii, P. fabianii, Aureobasidium pullulans, Lactobacillus agilisand L. plantarum. The yeast species were then characterised with respect to ethanol and flavour compounds production. Significant differences in the production of ethanol and other volatile compounds were observed during fermentation of masau juice among and within the tested Saccharomyces, Pichia and Saccharomycopsis species. Alcohols and esters were the major volatiles detected in the fermented juice. Ethyl hexanoate and ethyl octanoate were produced in highest amounts as compared to the other flavour compounds. Two traditional low-tech methods for preserving starter cultures, i.e., stabilisation of yeast cultures in dried plant fibre strands, and in rice cakes, were compared with standard lyophilisation. Viable cell counts made during six months storage at 4 °C and 25 °C of lyophilised yeasts, and yeast cultures preserved in dry rice cakes and dry plant fibre strands showed that the rice cake method performed significantly better than lyophilisation. The developed library of fermentation characteristics of yeasts can help in the design of mixtures of strains to obtain a specific melange of masau product functionalities. The defined starter cultures could be preserved using the traditional approaches, which are suitable for small-scale, low-tech applications.