Spontaneous fermentation of Munkoyo: a cereal-based beverage in Zambia

Abstract

Fermentation is the oldest technology used in the production of many traditional cereal-based beverages across Africa. Munkoyo, a spontaneously fermented cereal-based beverage from Zambia and Democratic Republic of Congo is one of those beverages and the focal product studied in this thesis. It is consumed by large parts of the population, as an energy drink during travel or work on the field and is also consumed at social gatherings like wedding festivals and funerals. The processing of Munkoyo has to date not been standardized and greatly varies between different regions and processors. This may affect the quality and the safety of the beverage. Optimization of the processing methods and microbial composition of Munkoyo to attain good quality and a safe product can be a milestone leading to increased consumption of the beverage. Like all fermented foods, processing of Munkoyo relies on microbial activity transforming raw materials (cereals such as maize, millet and sorghum) into a processed product. The thesis focused on understanding the art of making Munkoyo by surveying different processors and linked this to the microbial community composition of fermenting microbes and their functionality in producing aroma compounds. Further, we investigated whether the source of the fermenting bacteria could lie in the Rhynchosia roots that are added during processing. Chapter 2 describes a survey using a semi structured questionnaire and focus group discussions in Lusaka, Chongwe, Chibombo and Mumbwa. The survey revealed that Munkoyo is consumed by the entire population, as an energy drink, during long hours of manual work, at social gatherings and is mainly processed at household level. Characterization of the bacterial communities of over 90 samples with 16S amplicon sequencing on DNA extracted from the entire bacterial community revealed six families of mainly lactic acid bacteria to dominate the bacterial communities. Chapter 3 reports that there are principally three processing methods of making Munkoyo with reference to three different agro-ecological zones of the country. Microbial community composition is affected by processing method. Chapter 4 describes a study where we used six single strain cultures of bacteria commonly found as members of the microbial communities in Munkoyo and combinations of them to determine the most influential bacteria or groups of bacteria in the production of aroma compounds. We found that spontaneously fermented Munkoyo and Munkoyo made with the combination of all six species of bacteria had a similar composition of aroma compounds compared to the regular product implying that a complete mixture of microbes is required to potentially be used as starter culture for Munkoyo production. Chapter 5 reports on a study addressing the question if the addition of Rhynchosia root during processing provides the source of lactic acid bacteria responsible for fermentation. The study showed that microbial communities present on the roots used to make Munkoyo are mainly non-lactic acid bacteria like Propionibacteriaceae. Lactic acid bacteria were present at relative abundances below ten percent. After fermentation, the resulting product was dominated by lactic acid bacteria (Lactobacillaceae, Enterobacteriaceae, Leuconostocaceae, Streptococcaceae), which is a normal microflora in most cereal-based beverages. This supports the idea that Rhynchosia root is a potential source of lactic acid bacteria in spontaneous fermentation of Munkoyo. The work presented in this thesis highlights that spontaneous fermentation of Munkoyo possesses the challenge of product inconsistence from different processing methods. Further, the consequences and risks associated with contamination by pathogenic bacteria remains to be explored. Optimizing the processing methods and standardizing the microbial composition by administering starter culture can ultimately enhance quality and safety of the product to formalize production, increase consumption and contribute to food security.