Reduction of herbaceous aromas by wine lactic acid bacteria mediated degradation of volatile aldehydes

Abstract

Consumers typically prefer wines with floral and fruity aromas over those presenting green-pepper, vegetal or herbaceous notes. Pyrazines have been identified as causatives for herbaceous notes in wines, especially Bordeaux reds. However, pyrazines are not universally responsible for herbaceousness, and several other wine volatile compounds are known to produce distinct vegetal/herbaceous aromas in wines. Specifically, volatile aldehydes elicit sensations of herbaceousness or grassiness and have been described in wines well above their perception thresholds. Acetaldehyde is quantitatively the most important aldehyde and formed by yeast metabolism or through the auto-oxidation of ethanol during and after fermentations. Its grassy-green aroma typically is prevented by addition of SO2 that strongly binds to acetaldehyde hence masking its aroma. Hetero- and homofermentative wine lactic acid bacteria are responsible for the secondary malolactic fermentation in most red and some white wines and can degrade acetaldehyde. During malolactic fermentation, wine lactic acid bacteria are capable of reducing acetaldehyde levels significantly (~90%). Two reaction pathways were previously described by our group, the chemical reduction of acetaldehyde to ethanol by alcohol dehydrogenase (ADH), or its oxidation to acetic acid by aldehyde dehydrogenase (Al-DH). ADH and Al-DH are known to have a broad substrate specificity. Hence, it is possible that wine lactic acid bacteria may be able to degrade other volatile aldehydes that are known to contribute to herbaceousness in wines. Hexanal, methional, 2-methylbutanal, 3-methylbutanal, 2-methylpropanal, E-2-nonenal and phenyl-acetaldehyde are aldehydes and powerful herbaceous aroma compounds with odour thresholds between 0.5 and 16 µg/l. The odour thresholds of their corresponding alcohols are 100 to 14’000 times higher. Thus, chemical reduction of these aldehydes to the corresponding alcohols by wine lactic acid bacteria may lead to a reduction of herbaceous notes. Within the scope of this investigation, highly concentrated solutions of resting cells of several heterofermentative and facultative homofermentative wine lactic acid bacteria of the genera Oenococcus and Lactobacillus were tested for their ability to degrade these volatile aldehydes. A careful incubation and sample-taking protocol was applied in order to prevent sample evaporation. The analysis of volatile aldehydes was performed by liquid-liquid micro-extraction followed by GC-MS analysis. It could be demonstrated that all bacteria were able to degrade all volatile aldehydes efficiently. Within 50 minutes, an average of 95% of the initial aldehyde concentration was degraded with minima and maxima of 63 and 100%, respectively. The results suggest that wine lactic acid bacteria may be able to degrade volatile aldehydes during malolactic fermentation thus reducing their sensory impact and increasing sensory perception of compounds with fruity character.