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Fermented dairy products are an important part of the human diet because they provide nutrients and extend the shelf life of raw milk, which is perishable. In Africa, there are several traditionally fermented milk products (TFMPs) mostly, produced at household level for domestic consumption such as mabisi from Zambia. Mabisi is made by spontaneous fermentation of raw milk at ambient temperature in a calabash, plastic or metal container for two days. The main aim of this study was to characterise the microbial community composition of mabisi and optimise its production process. Thus, a survey was carried out to determine the different production practices/methods of mabisi, key production parameters, uses and microbial community composition. Then, a laboratory experiment was carried out to determine the effect of fermentation temperature on the quality of mabisi and its microbial community composition. Furthermore, a field experiment was also conducted to determine the microbial dynamics of three types of mabisi. We found seven different production methods of mabisi, which include tonga, barotse, backslopping, illa, creamy, cooked and thick-tonga mabisi. Tonga mabisi was found to be the most popular and widely produced mabisi variant throughout the country by all ethnic groups. The key production parameters identified were: type of fermentation container, fermentation temperature, backslopping, alternate removal of whey in combination with addition of raw milk, heating and cooling, fermentation time and finally, agitation (churning). We further found that mabisi is a versatile product with various uses. The microbial community composition of mabisi from across the country was unravelled and showed that the top 10 most abundant genera were Lactococcus, Lactobacillus, Streptococcus, Enterobacter, Klebsiella, Kluyvera, Aeromonas, Acinetobacter, Buttiauxella and Citrobacter, which belong to two phyla, Firmicutes and Proteobacteria. The microbial community composition was affected by geographical location, production method, type of producer, fermentation container, pH and fermentation time. Furthermore, the production of mabisi in the country was divided into two main regions, (i) the traditional mabisi production region (TMPR) and (ii) the non-TMPR. The products found in TMPR were dominated by lactic acid bacteria (LAB) while the products from non-TMPR were characterised by a diverse community of non-LAB but with Lactococcus lactis driving the fermentation process in these products. Tonga mabisi had a more complex and diverse microbial community composition than the rest and was widely produced in non-TMPRs.
The fermentation temperature, which was one of the key production parameters showed that tonga mabisi fermented at tap water temperature (20°C), room temperature (22°C), constant temperatures of 25° and 30°C retained a complex and diverse microbial community composition with Lactococcus as the main LAB driving the fermentation. In addition, the use of single or multiple batches of raw milk for mabisi production as well as the use of a new or previously used container, also did not have an effect on the microbial community composition of tonga mabisi. However, barotse mabisi incubated at 25°C was dominated by Lactococcus and retained moderate pH of 4.2 whereas the one incubated at 30°C was dominated by Lactobacillus and had a much lower pH of 3.2. High incubation temperature (30°C) led to faster fermentation, high levels of syneresis and a low consistency, which is not desirable. The producer and production method of mabisi had an effect on microbial community composition of mabisi. Three types of mabisi produced by four different producers over ten production cycles showed that tonga mabisi retained a more complex and diverse composition than illa and backslopping mabisi after ten production cycles. The 6th and 10th cycles of illa and backslopping mabisi on the other hand were both dominated by Lactococcus and Lactobacillus. In conclusion, this study established the different production methods of mabisi and its uses, the microbial community composition and the factors that affect them and finally, the process conditions required to produce good quality mabisi. This knowledge can be used to develop starter cultures for mabisi in order to standardise production. Alternatively, spontaneous fermentation with standardised process conditions can be employed to produce standardised products.
|Qualification||Doctor of Philosophy|
|Award date||15 Oct 2019|
|Place of Publication||Wageningen|
|Publication status||Published - 2019|