Development and preservation of a starter culture for commercial production of bongo, a traditional fermented milk beverage from Uganda

Abstract

Bongo is a popular traditional fermented milk produced spontaneously, or by backslopping. This predisposes the product to microbial contamination, slow fermentation and variations in product sensory properties. Controlled fermentation using defined starter cultures is vital to address these challenges. Three lactic acid bacteria (LAB) previously isolated based on milk acidification and coagulation, Lacticaseibacillus rhamnosus BM55, Lactococcus lactis BM01 and Leuconostoc pseudomesenteroides BM70 were used in this study to evaluate their potential as starter cultures in fermented cows’ milk. The isolates were evaluated both individually and in combination. Microbial counts, pH, titratable acidity, viscosity, flavour development, syneresis and consumer acceptability were determined using standard methods. The sensory profile of Bongo prepared using the cultures, as well as the spontaneously fermented Bongo, was described using Quantitative Descriptive Analysis (QDA). The relationship between the sensory profile and hedonic scores was determined using preference mapping. The most promising combination of isolates (BM01, BM55 and BM70) was lyophilised in 10% (w/v) skimmed milk and 10% (w/v) soluble starch prior to being stored under refrigeration (4 °C) for 6 months. Survival rate and fermentation characteristics (pH, viscosity, titratable acidity and LAB counts) of Bongo made using the starter culture were determined. The combination of all the three isolates showed the highest potential application as a starter culture with the lowest pH, optimal viscosity, minimal syneresis and enhanced flavour development comparable to that of spontaneously produced Bongo. This was further confirmed by the QDA results. Significant correlations were observed between lumpiness, smoothness and astringency underscoring the importance of the choice of microorganism on the product attributes of Bongo. Texture was the major factor responsible for the differences between the Bongo samples produced by the different starter culture isolates. There was a decline in viability and fermentation ability post lyophilisation and storage because the cryoprotectants do not fully prevent cell damage due to freezing effects and dehydration. LAB lyophilised in skimmed milk showed the highest viability (92%) when compared to LAB lyophilised in soluble starch (78%). Bongo made using LAB lyophilised in skimmed milk had a higher viscosity, lower pH, higher titratable acidity compared to that made with LAB lyophilised in soluble starch implying that skimmed milk had a better protective power. The freeze dried culture maintained acidification ability in the first four months of storage at 4 °C. The mixed starter culture containing Lactococcus lactis BM01; Lacticaseibacillus rhamnosus BM55 and Leuconostoc pseudomesenteroides BM70 lyophilised in 10% (w/v) skimmed milk could be adopted for commercial scale processing of Bongo.