Malic acid fermetation: influence and applications in winemaking

Abstract

Wine is one of the oldest products where microbiological processes contribute significantly to the overall quality of the product. Lactic acid bacteria (LAB) – mainly Oenococcus oeni – induce malolactic fermentation (MLF) and L-malic acid decarboxylation into L-lactic acid, resulting in a wine with a softer mouthfeel. Besides, this process provides biological stability and improves the final aroma of wines by modifying fruit-derived aromas and producing aroma-active compounds. Glycosidic compounds constitute a reserve of powerful odor-active compounds largely present in grapes as β-D-glucosides, being that aglycone moiety is often dominated by monoterpenes, C13-norisoprenoids and benzene derivatives. O. oeni strains well-adapted to perform MLF can represent a source of glycosidase enzymes capable of operating under oenologycal conditions. In Argentine wines, O. oeni was the predominant bacteria, resulting in 68% of the total LAB isolated, which in high proportions showed detectable levels of β-glucosidase activity; in whole cells at the end of the exponential growth, in MRS medium adjusted to pH 4.8. Studies on the effect of winemaking-related factors such as L-malic, citric acids and sulfur dioxide on the growth and β-glucosidase activity of six O. oeni strains from Argentinean wines at pH levels of 4.8 and 3.8 showed that all of them grew under the examined conditions. An analysis of sugar and organic acid utilization profiles revealed interesting characteristics of these strains: 1) Glucose and organic acids are cometabolized regardless of medium composition and initial pH values; and 2) high L-lactic acid levels are recovered from L-malic acid decarboxylation, confirming their good malolactic potential. In addition, all the strains exhibited β-glucosidase activities at initial pH values of 4.8 and 3.8, even though they were >4.8. In general, the L-malic and citric acids stimulated β-glucosidase activity, thus partially reverting the inhibition caused by acid stress. Using different cell fractions, the βglucosidase activity of the test O. oeni strains was associated with the cell surface. This fact results from the great interest in their potential technological application as sources of an enzyme that could improve wine quality and aromatic complexity under vinification conditions. When the effect of grape glycosides was investigated on growth, MLF and glycosidases activities of three selected O. oeni strains in a wine-like medium results obtained indicated that the FML process contributes toward flavor enhancement in winemaking. Moreover, natural glycosides positively impacted on growth parameters, L-malic acid degradation rates and glycosidase activities. These increments correlated to significant changes in the volatile profile, mainly by the formation of aroma esters and to a lower extent higher alcohols and other compounds, indicating the presence of ester-synthesizing enzymes in O. oeni strains that, in combination with the glycosidic activities could play an important role in improving wine quality. Therefore, O. oeni prevails as a candidate for MLF starter culture via growth capacity, non-biogenic amines production, malolactic potential and glycoside activities. Thus, the results obtained encourage the potential use of selected strains as an effective tool to enhance wine aromatic complexity during MLF.