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Project

Optimisation of enset fermentation: physicochemical and microbial dynamics and development of starter cultures

Enset (Ensete ventricosum (Welw.) Cheesman, Musaceae) is a multipurpose crop widely cultivated in the southern, south-western and central parts of Ethiopia. Its persistence to recurrent droughts and high yield potential per unit area of land assures its vital role in ensuring food security. The plant does not produce edible fruits, but its swollen pseudostem and corm are processed and fermented in soil pit or bamboo basket (or erosa) into a starch-rich food product, kocho. However, the fermentation process is mainly based on the knowledge of the local people and it has been practiced for generations without scientific inputs. This dissertation aimed to provide fundamental knowledge on the physicochemical and microbial dynamics during enset fermentation, taking into account enset variety and fermentation system. Moreover, lactic acid bacteria strains were screened for the development of starter cultures specific for enset fermentation.

To this end, at the start of the research a detailed survey was made to obtain information on the current traditional enset fermentation practices. It delivered the first comprehensive assessment of the current enset fermentation practices in the Gamo highlands and identified the major problems associated with the current fermentation systems. The survey pointed out that enset fermentation in the Gamo highlands is mainly based on the empirical knowledge of the local people and greatly varies among villages. The fermentation process takes more than two months and there is no standardized starter culture or other traditional starter material being used in the highlands. This causes variability in the sensory qualities and market prices of kocho.

Next, physicochemical and microbial dynamics during enset fermentation were assessed, taking into account a selected number of factors that possibly influence the dynamics, such as enset variety and fermentation system. In all cases, moisture content and the pH decreased, whereas the titratable acidity increased during fermentation. The fermenting enset is dominated by a considerable number of lactic acid bacteria, such as Leuconostoc, Lactobacillus and Weissella, making it a lactic acid bacteria-rich microbial ecosystem. Moreover, as revealed by Illumina MiSeq sequencing of 16S ribosomal RNA genes, the bacterial community composition of fermenting enset generally becomes more diverse as the fermentation proceeds to the end. However, the slow start of the fermentation favours the growth of undesirable microorganisms such as Enterobacteriaceae, Clostridium and molds. The traditional pit and erosa fermentation systems, having their practical problems such as not always being perfectly anaerobic and being contaminated easily by the environment, can be replaced by fermentation in a jar, as demonstrated in this PhD. This improves food safety of the fermented enset or kocho.

In the third place, the use of traditional starter materials was characterized in terms of preparation and microbial composition. In most enset growing regions of Ethiopia, traditional starters are prepared in local households in an artisanal way and added to the scraped enset to aid the fermentation. To this end, traditional starter materials were collected in eighteen districts (covering seven zones) in southern Ethiopia. The main ingredients used to prepare the starters were generally the same within a zone but varied between zones, which reflect the fact the no or little information is exchanged between the producers of different localities. The collected starter materials had a pH ranging from 3.68 to 5.62. Lactic acid bacteria were present in high numbers in all samples (up to 9.07 log cfu/g). Sequencing of the V4 region of the 16S rRNA gene also indicated that Lactobacillus, Lactococcus and Leuconostoc were the dominant LAB genera identified in the starters. These isolates were expected to be good candidates for a standardized starter culture for enset fermentation and they were included in the further research. The universal presence of Clostridium spores in all starter materials rises questions on safety risks and involve a good argument for the introduction and commercialization of (a) standardized starter culture(s).

Lastly, the isolates obtained from fermented enset and from traditional starter materials as mentioned before were investigated for their biochemical and physiological characteristics. Generally, three isolates, namely Lactobacillus plantarum (FX 15 and SAM 6) and Leuconostoc mesenteroides (HM 53), were identified as potential commercial starters for enset fermentation. Enset fermented with each of the three starters showed a larger pH reduction and a faster colonization of lactic acid bacteria than the uninoculated mass and the strains had a positive effect on taste. This PhD dissertation hence delivered new possibilities to optimize the traditional enset  fermentation processing in two ways: (1) by introducing a new fermentation system, the sauerkraut jar, as an improved, safer and more practical alternative for the most commonly used traditional fermentation systems, the pit and erosa, (2) by obtaining three isolates to be used as a (single) starter culture strain to ferment enset. Fermenting enset with each of these isolates enhanced the sensory characteristics of both unbaked and baked kocho, which is promising for the kocho market value and for commercialization of enset-based foods to the cities and outside Ethiopia.

Date:20 Jan 2017 →  9 Dec 2019
Keywords:Enset, Fermentation, Microbial dynamics
Disciplines:Microbiology, Systems biology, Laboratory medicine
Project type:PhD project