Biodiversity, population dynamics and metabolite analysis of lactic acid bacteria involved in the fermentation of vegetables (IWT450)

Fermentation techniques such as spontaneous fermentation and backslopping have been used since ancient history to extend the shelf-life and to improve the organoleptic properties and safety of foodstuff. In several industries (e. g. the dairy and meat industry) the use of bulk starters has already been introduced. By using bulk starters, the fermentation process can be controlled and predicted in a better way. In the vegetable industry however, the use of bulk starters is still in an experimental stage. In contrast with the industrial starter cultures, the naturally occurring wild type strains that dominate the traditional fermentations, are more competitive and may have extensive metabolic capacities to produce more aroma components. In order to develop a starter culture for industrial use, starting from these wild type strains, an extensive knowledge of the biodiversity of the ecosystem, of the population dynamics of the fermentation process and of the substrate consumption and metabolite production in relation to the organoleptic properties and the microbial interactions is indispensable. Nonetheless, this kind of study is rather rare for vegetable fermentation ecosystems. Therefore the first aim of this project is to determine the microbial diversity and population dynamics of fermented vegetables, particularly fermented sweet pepper and fermented leek. Secondly the method to analyze the metabolome of fermented foodstuff will be optimized to study the metabolome of fermented vegetables. Finally selected (autochthonous) lactic acid bacteria will be applied as a functional starter culture to perform controlled vegetable fermentations.
In order to study the microbial diversity and the population dynamics culture-dependent (platings on selective culture media followed by rep-PCR-fingerprinting) and culture-independent methods (PCR followed by gel electrophoresis with a denaturing gradient) will be applied. The analysis of the metabolome will mainly be performed through an optimized method for gas chromatography-mass spectrometry (GC-MS method). Sampling of the different metabolites will be carried out by means of static headspace (SH) or solid phase micro extraction (SPME). In the case that some interesting compounds cannot or can hardly be detected with the GC-MS method, additional analysis techniques such as high performance liquid chromatography and liquid chromatography-MS will be applied. These methodologies will be used to look at the presence or absence of specific bacterial metabolites that are important for the quality aspects of fermented vegetables. This will be examined in both the Romanian fermented vegetable samples and the bacterial cultures, obtained from the isolates of the fermented vegetable samples. Subsequently the kinetics of the potential starter cultures will be studied by means of laboratory fermentations (10 l scale) in a vegetable simulation medium. Furthermore small scale fermentations will be performed in vegetable extracts to verify how much the vegetable simulation medium resembles reality.
This PhD research will be performed in collaboration with the Institute of Biology in Bucharest (IBB, Dr. Medena Zamfir), with which a bilateral project concerning the microbial biodiversity of Romanian dairy products has already been started (BWS02/04, 2002-2005). Further will be co-operated with the 'Instituut voor Landbouw- en Visserijonderzoek' (ILVO, Marc De Loose) in Ghent. Here the necessary infrastructure to perform controlled vegetable fermentations is present. For the taxonomic part of this study will be co-operated with the BCCM yeast and bacteria collections.

Keywords:Applied Biology

Disciplines:Biological sciences, Chemical sciences