Directed Evolution of Oenococcus oeni: optimising yeast-bacteria interactions for improved malolactic fermentation

Abstract

Malolactic fermentation (MLF) is a secondary step in the vinification process and it follows alcoholic fermentation (AF) which is predominantly carried out by Saccharomyces cerevisiae. These two processes result in the degradation of metabolites to produce secondary metabolites which also contribute to the final wine flavour and quality. AF results in the production of ethanol and carbon dioxide from sugars and MLF stems from the degradation of L-malic acid (a dicarboxylic acid) to L-lactic acid (a monocarboxylic acid). The latter process results in a smoother texture as the acidity of the wine is reduced by the process, it also adds to the flavour complexity of the wine. The species responsible for this fermentation step belong to the Pediococci, Lactobacilli and Oenococci genera. Only Oenococcus oeni and Lactobacillus plantarum have been commercialised. The former is the dominant species that is often found in both spontaneous and inoculated fermentations. In spite of inoculation MLF is quite unstable and a successful fermentation is not always guaranteed. Sluggish or stuck fermentations may occur due to many physico-chemical factors. Also, the interactions between the yeast and bacteria during the vinification process play an important role in the success of MLF. Therefore, appropriate selection of strains is important, unfortunately selecting strains is time consuming and limited only to specific winemaking conditions. To overcome this, research has investigated strain improvement, however recombinant technology is controversial. The use of non-recombinant techniques such as mutagenesis, hybridisation and Directed Evolution has become popular. The aim of this study is to optimise yeast-bacteria interactions by use of Directed Evolution as a tool to improve lactic acid bacteria, in this way, try and guarantee the success of MLF. Two S. cerevisiae strains (Cross Evolution® and EC1118®) were used as selective pressures for O oeni S5 populations. The bacterial populations were exposed to synthetic wine fermentations for 30 and 50 generations after which 30 bacterial isolates were evolved using both yeast and were characterised for fermentation efficacy. The results show that the general performance of the isolates was improved in comparison to the parental strain. Only 3 isolates after 30 generations showed a specific improvement when inoculated with ‘driver’ yeast than with other yeast strains. After 50 generations all the strains showed improvement in terms of fermentation rates, but not all strains had a higher fermentation efficacy in comparison to the parent strain. This study shows the potential of Directed Evolution as a tool for strain improvement using a biotic selective pressure as opposed to physico-chemical selective pressures. It also, shows the possibility of improving yeast-bacteria interactions by having a tailor-made pair for successful AF and MLF.