Malolactic fermentation (MLF) is a biochemical process typically occurring in the vinification process after completion of alcoholic fermentation (AF), consisting of the conversion of L-malic acid into L-lactic acid and CO2. It can be conducted by different species of lactic acid bacteria (LAB), among which Oenococcus oeni is the most often associated with MLF in the harsh wine environment due to its tolerance and adaptation to high acidity and alcohol contents in wine. Various stress factors in wine such as low pH and high ethanol concentrations affect the growth of LAB or their metabolic properties and consequently the timely completion of MLF.The overall objective of this work was to address the impact of partial and complete MLF on the volatile aroma composition of white wines. The first aim was to investigate the influence of the stress factors pH and ethanol on two O. oeni strains and the volatile aroma composition of white wines from the grape varieties Riesling and Chardonnay. It was demonstrated that the wine matrix as well as the pH and alcohol concentration affect MLF and the final volatile aroma profile. Results indicate that changes in the volatile aroma composition are not necessarily related to complete MLF and that even partial MLF has distinct influences on the wine aroma profile of white wines. The next step in this study was to evaluate the impact of four different inoculation strategies in Riesling wine with high acidity on MLF and the volatile aroma composition. Treatments with simultaneous inoculation showed a reduced total fermentation time (alcoholic and malolactic) compared to the sequential inoculations. It was observed that simultaneous alcoholic and malolactic fermentation had no negative impact on fermentation success and on the final wine volatile aroma composition. Compared to sequential inoculation, wines with co-inoculation tended to have higher concentrations of ethyl and acetate ester which may result in fruitier wines. This is also the first study cloning and characterising a cystathionine beta/gamma-lyase from two O. oeni oenological strains. Biochemical characterisation led to the conclusion that the enzyme is a multifunctional pyridoxal-5´-phosphate (PLP)-dependent enzyme that on the one hand degrades cystathionine by an alpha,beta- and an alpha,gamma-elimination reaction and on the other hand exhibits cystathionine gamma-synthase activity when O-succinyl-L-homoserine and L-cysteine are present. Furthermore, the ability to demethiolate methionine into methanethiol, an unfavourable volatile sulphur compound in terms of wine aroma, was observed. The present study has highlighted the role of O. oeni in the modification of wine volatile aroma compounds and the impact of pH, ethanol and wine matrix on its metabolic activity. It was shown that changes of the wine aroma profile during MLF can be affected by the bacterial strain and the MLF inoculation regime as well as the grape cultivar or chemical wine composition. A better understanding of these bacterial activities is of great interest, as it could lead to the development of new techniques for altering the aroma of wine.
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