Isolation, characterisation and evaluation of selected yeast cultures for tonto production

Abstract

Tonto, a traditional Ugandan banana-based alcoholic beverage, is produced through spontaneous fermentation resulting in variable quality and safety. This study investigated the microbial ecology, physicochemical and volatile profiles of traditional Tonto. It also evaluated the effect of selected yeast strains and fermentation strategies on Tonto quality. Spontaneously fermented samples (n = 42) were collected from Ntungamo and Kalungu districts and analysed for their physicochemical composition, volatile compounds, microbial diversity and sensory attributes. Physicochemical analysis revealed significant (p < 0.05) reductions in pH (4.9 ± 0.23 – 3.58 ± 0.32) and total soluble solids (15.01±1.76 – 5.15±0.74 °Brix), an increase in titratable acidity (0.21 ± 0.07 – 0.77± 0.64%) and ethanol content (0.0 – 4.73±1.05%) during fermentation. A total of 38 volatile compounds were identified which were dominated by esters (isoamyl acetate, ethyl butyrate, ethyl caprylate) and higher alcohols. These significantly (p < 0.05) contributed to fruity, floral, and sweet aroma notes. Samples from Ntungamo exhibited significantly higher (p < 0.05) levels of these key volatiles and correspondingly higher consumer acceptability scores compared to those from Kalungu. The most prevalent yeast species isolated from Tonto, were Saccharomyces cerevisiae, Pichia kudriavzevii, Wickerhamomyces anomalus and Hanseniaspora opuntiae, with P. kudriavzevii dominating at later stages of fermentation. Monoculture fermentations of S. cerevisiae y1.7 and P. kudriavzevii y2.4 showed significantly higher (p < 0.05) ethanol yields (4.11 ± 0.12% and 4.30 ± 0.15%, respectively) and ester production compared to other isolates. Principal Component Analysis (PCA) revealed a strong association between yeast and desirable aroma attributes and the rich volatile profile of spontaneously fermented samples. Furthermore, inoculation approach influenced Tonto quality. Sequential co-fermentation of S. cerevisiae y1.7 and P. kudriavzevii y2.4 produced Tonto with significantly higher (p < 0.05), ester concentrations and superior fruity–floral sensory scores while simultaneous co-fermentations yielded higher levels of higher alcohols, phenols, and fatty acids associated with undesirable flavours. Overall, sequential co-fermentation was most suitable for enhancing flavour balance and consumer acceptability. This study demonstrated that S. cerevisiae y1.7 and P. kudriavzevii y2.4 are promising starter culture candidates for improving Tonto quality. The findings provide a foundation for production of starter cultures that can be used in commercial production of Tonto and related traditional beverages