Sclerocarya birrea (Morula tree) is one of the indigenous trees bearing wild fruits with various applications in the African communities. Wine is a globally known beverage usually made from grapes; however, recently, other fruits, including wild fruits with a considerable amount of sugars, can be used for making wines. The marula fruit wine is also important in many communities for cultural activities and can be enjoyed by people of varying age groups depending on the age of the product. In recent years, there has been growing interest in shifting from traditional marula winemaking to developing technologies for the marula winemaking process and commercialisation. The process of marula winemaking is similar to the production of grape wines, which entails collection, selection and washing of the fruits; extraction of the juice and mashing; formation and removal of the scum; and ultimately spontaneous fermentation of the resulting juice. The new process in marula winemaking would take into consideration the use of starter cultures as either monoculture or mixed cultures developed from the native marula fruit microbiota and the pasteurisation of the juice. The main challenge or difficulty with marula is the extraction of sugar and other soluble solids from the pulp more than it is for the grapes. The other challenge confronting the sustainability of marula wine is the seasonality of the fruit and poor juice yield. It is therefore imperative to develop strategies to increase the juice yield without affecting the quality, to preserve the marula fruits to ensure the year-round presence of marula fruit wine in the markets and, consequently, to improve the income generation capacity of the households dependent on the product. In addition to achieving a high juice yield, it is imperative to ensure consistent quality wine products. This review gives an overview of the S. birrea subsp. caffra and the biochemical components of the fruits or juice. It also highlights the use of marula fruits for wine production in African communities. The potential economic sustainability of the marula fruit wine is explored, particularly in southern Africa, where the marula tree (Morula) is abundant and the marula fruit wine is popularly produced. The review also examines the opportunities, challenges and future prospects of the marula fruit wine.

Marula wine is produced from ripe fruits of the Sclerocarya birrea subspecies caffra tree through spontaneous fermentation. A few culture-based studies have shown that the fermentation is largely driven by yeasts, although, in the early stages, some lactic acid bacteria (LAB) and acetic acid bacteria may be detected. Some of the microbes may produce undesirable metabolites that lead to the spoilage and short shelf life of the wine. However, there is generally limited information on the microbial composition and its contribution to the chemical characteristics of the resultant marula wine. The aim of this study was to characterise the microbial population of marula wine from different localities in the Limpopo province, South Africa. MALDI-TOF and amplicon sequencing technique were used to identify microbial strains and to determine their diversity and changes in the different stages of fermentation. The phylogenetic relationships of LAB and S. cerevisiae were analysed using multilocus sequence typing. Bacterial species that were common in the different marula wines included Gluconobacter oxydans, Lactiplantibacillus plantarum, Levilactobacillus brevis, Lacitilactobacillus nagelii, Lentilactobacillus kefiri and Lentilactobacillus parabuchneri, and the yeasts were Hanseniaspora guiliermondii, Saccharomyces cerevisiae, Rhodotorula mucilaginosa and Pichia kudriavzevii. The MLST data indicated common microbiota from different marula wines with low intraspecific diversity, suggesting that the LAB and S. cerevisiae strains that are mainly responsible for the spontaneous fermentation of marula wine are similar irrespective of the geographical differences and production preferences.

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.