Species diversity, community dynamics and metabolite target analysis or Romanian vegetable and Belgian leek fermentations

Student thesis: Doctoral Thesis

Abstract

Worldwide, many different vegetables and fruits are subjected to fermentation, as
fermentation not only prolongs their shelf-lives, but also delivers end-products that are
widely appreciated for their typical sensory properties. Furthermore, the development of new
fermented vegetables and fruits could contribute to product diversification and innovation,
which may help to strengthen their market positions. The majority of all vegetable and fruit
fermentation processes still relies on spontaneous fermentation. However, the application of
starter cultures is getting increased interest as to improve the control of these processes and
the quality of the end-products, as well as to introduce nutritional, sensorial, technological,
and/or health advantages. Detailed insights into the fermentation microbiology are available
for sauerkraut, kimchi, pickles, and table olives, which are all produced on industrial scale,
whereas only few studies focus on the diversity of fermented vegetables and fruits produced
on small scale or at household level.
In the first place, this study set out to determine whether the same microorganisms
prevail during different vegetable fermentations or if specific microorganisms can be
associated with certain vegetables. Therefore, a thorough study of traditional Romanian
vegetable fermentations and Belgian leek fermentations was carried out. The fermentation of
leek was of particular interest, as leek is the second most important field crop in Belgium. Its
fermentation was investigated as an innovative approach to valorize leek, in particular its
green leaves that are mostly considered as waste materials, owing to their restricted
applications in comparison with the white leek shafts. The multiphasic analyses,
encompassing microbiological and metabolite target analyses, of these fermentations revealed
that the lactic acid bacteria (LAB) species Lactobacillus (para)brevis, Lactobacillus
plantarum, Lactobacillus sakei, and Leuconostoc mesenteroides were the most frequently
occurring microorganisms, independent of the raw materials used. Furthermore, these
vegetable fermentations showed a three-step fermentation pattern, of which the initial phase
was characterized by the presence of Enterobacteriaceae, Pseudomonadaceae, yeasts, and a
wide LAB species diversity, followed by a second phase with a more restricted LAB species
diversity, and then a final phase often prevailed by Lb. (para)brevis and Lb. plantarum.
The second part of this study dealt with the development of a starter culture to
perform controlled leek fermentations. As the prevailing LAB species during various
vegetable and fruit fermentations, Lb. plantarum, Lb. sakei, and L. mesenteroides strains were
promising candidate starter cultures. Out of several strains belonging to these three species
and representing subclusters within the (GTG)5-PCR banding patterns of genomic DNA of all
isolates available, one strain of each of these species was selected on the basis of growth rate,
acidification capacity, carbohydrate consumption, and end-metabolite and aroma production
via laboratory fermentations in a newly developed leek juice medium. Additional laboratory
fermentations with the three selected strains (the diacetyl-producing Lb. plantarum IMDO
788 and the fast-growing and deep-acidifying Lb. sakei IMDO 1358 and L. mesenteroides
IMDO 1347), both during mono- and co-culture fermentations, provided a meticulous
characterization of their kinetic behavior and mutual interactions. Subsequent application of
Lb. plantarum IMDO 788 solely, Lb. sakei IMDO 1358 solely, and the combination of Lb.
plantarum IMDO 788 and L. mesenteroides IMDO 1347 as starter cultures for actual leek
fermentations revealed that the mixed starter culture was the most successful, as with this
starter culture fermented leek of good microbiological and sensorial quality was obtained. In
addition, Lb. plantarum IMDO 788 was used as a starter culture for traditional Romanian
vegetable fermentations, which resulted in the prevalence of the added starter culture and an
acceleration of the fermentation process. However, to avoid secondary fermentation by
yeasts, owing to high residual carbohydrate concentrations, the addition of a
heterofermentative LAB species, such as Lb. brevis, to the starter culture formulations could
be helpful.
The third part of this study focused on the sensory aspects and the consumers'
acceptance of fermented leek as a new fermented vegetable product to evaluate its possible
market introduction. In general, the fermented white leek parts were chosen over the
fermented green leek parts, mainly owing to the toughness and dryness of the latter. Yet, the
fermentation of both leek parts together, whether or not using a starter culture, delivered
acceptable fermented leek, at the same time still valorizing a portion of the green leek parts.
Date of Award18 Jan 2013
Original languageEnglish
SupervisorLuc De Vuyst (Promotor)

Keywords

  • fermentation
  • leek

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