Exploration of wild yeasts isolated from marula fruit wine as suitable candidates for industrial craft beer brewing

ABSTRACT Exploration of the natural microbial diversity of indigenous fruits can lead to the discovery of promising candidate yeasts that bring diversity and unique characteristics to commercial brews compared with commercial strains. These properties have been domesticated through thousands of generations of adaptive selection. This study investigated the potential of yeasts isolated from marula fruit wine in South Africa in beer production and the attributes these yeasts contribute during the brewing process. This was achieved by fermenting wort with three yeast isolates in separate processes. Different parameters, such as microbial enumeration, sensory evaluation, yeast attenuation and chemical profiling, were analysed. The performance of the Saccharomyces NY23 strain was higher than that of the commercial yeast strain AEC1 and Saccharomyces OY15 strain in terms of ethanol production, which was 57.13 g/l despite its lower attenuation. The non- Saccharomyces yeast isolates Meyerozyma AlC and Meyerozyma 15M5 produced beers with lower ethanol concentrations of 29 and 30 g/l, respectively. The beer that was fermented with Meyerozyma AlC yeast contained the highest acidity levels of above 0.30 g/l and the lowest pH level below 3.9, whereas the beers that were fermented with other yeast strains contained the recommended beer acidity levels between 0.5 and 0.25 g/l and pH level of between 4.6 and 4.9. The tasting panellists described the beers as pleasant, including those from the non- Saccharomyces strain, which had a low alcohol concentration and a higher sugar content of above 40 g/l. The results indicated that marula fruit wine yeasts have the potential for applicability in beer production. Further studies on co-inoculations of both Saccharomyces and non- Saccharomyces for flavour diversity and low alcohol content according to consumer preferences should be envisaged in the future. Significance of the study What is already known on this subject? Beer production is traditionally dominated by Saccharomyces cerevisiae . Non- Saccharomyces yeasts generally have lower fermentation efficiency; they coexist in fermented beverages and are known to enhance flavours and aroma. What are the new findings? Wild yeasts isolated from marula fruit demonstrated potential for beer production, particularly the non- Saccharomyces yeasts, which demonstrated the ability to produce low-alcohol beers with unique sensory characteristics such as fruity and floral aroma. What is the expected impact on horticulture? The findings highlight marula fruit as a valuable source of novel fermentation yeasts. This may increase the value of utilization of indigenous fruits and promote the development of value-added horticultural products.