Interaction between wine yeast and malolactic bacteria and the impact on wine aroma and flavour

ENGLISH ABSTRACT: Wine is a product of the fermentation of grape juice. Alcoholic fermentation is mainly conducted by the yeast Saccharomyces cerevisiae which metabolises grape sugars to mainly ethanol, CO2 and glycerol. Aside from these primary fermentation compounds, the yeast also produces many secondary metabolic by-products that are important to wine quality and style. Malolactic fermentation (MLF) is a secondary fermentation that normally occurs after alcoholic fermentation. Lactic acid bacteria (LAB) are responsible for the conversion of malic acid to lactic acid and CO2 during MLF, which is important for wine deacidification and also contributes to microbial stability. Malolactic fermentation and LAB strains can also influence the aroma profile of wines. The main genera associated with this process are Oenococcus, Lactobacillus, Pediococcus and Leuconostoc. Oenococcus oeni is the main species associated with MLF because it is able to survive the harsh physiochemical environment of winemaking. Recently L. plantarum has also been introduced as a commercial MLF starter culture. Research has started to focus on the potential of wine yeast and LAB interactions or combinations to alter the wine aroma profile via the production and/or degradation of aroma compounds. The overriding goal of this study is to unravel the interactions between wine yeast and different LAB strains and their impact on wine aroma and flavour. The first aim was to assess LAB growth during co- and sequential inoculation strategies, the ability to complete MLF and the impact on the production of aroma compounds in combination with two different yeast strains in a medium containing full complement of nitrogen supplementation. Malolactic fermentation was successful in the different inoculation strategies and the bacterial combination (L. plantarum and O. oeni) completed MLF in the shortest time. The impact of the bacterial strains on the modification of aroma compounds was bigger in co- than sequential inoculation. A general increase in total esters (contributing to the fruity character of wines) especially ethyl lactate and ethyl acetate was observed. The production of esters, volatile fatty acids and higher alcohols proved to be dependent on either the yeast strain used and/or the LAB strains used. The second aim of the research was to assess the effect of NH4Cl (ammonium) and amino acids supplementation on yeast and LAB strains (both in co- and sequential inoculation strategies) and the impact on the aroma profile of the fermented must. Fermentations supplemented with ammonia as sole nitrogen source showed the highest total bacterial growth in terms of cell numbers. Malolactic fermentation was completed in the shortest time with O. oeni and the bacterial combination inoculums. The co-inoculated strategies in combination with amino acids supplementation showed the biggest impact on the aroma compound profiles of the different fermentation strategies and bacterial treatments. A general increase in total esters was observed for NH4Cl additions with ethyl lactate and ethyl acetate showing the highest concentrations. The concentration of esters, volatile fatty acids and higher alcohols were strongly influenced by the yeast and the single LAB strains used. The results generated from this study showed that the chemical composition of the fermentation medium and the selection of yeast and LAB strains are important because these factors have an influence on the aroma and flavour profiles of wines.