Abstract
The winemaking process involves the alcoholic fermentation of must, often followed by malolactic fermentation (MLF). The latter, mainly carried out by the lactic acid bacterium Oenococcus oeni, is used to improve wine quality when acidity reduction is required. Moreover, it prevents microbial spoilage and improves the wine’s organoleptic profile. Prior observations showed that O. oeni is able to resist several months in harsh wine conditions when adhered on oak barrels. Since biofilm is a prevailing microbial lifestyle in natural environments, the capacity of O. oeni to form biofilms was investigated on winemaking material such as stainless steel and oak chips. Scanning Electron Microscopy and Confocal Laser Scanning Microscopy showed that O. oeni was able to adhere to these surfaces and form spatially organized microcolonies embedded in extracellular substances. To assess the competitive advantage of this mode of life in wine, the properties of biofilm and planktonic cells were compared after inoculation in a fermented must (pH 3.5 or 3.2 and 12% ethanol) The results indicated that the biofilm culture of O. oeni conferred (i) increased tolerance to wine stress, and (ii) functional performance with effective malolactic activities. Relative gene expression focusing on stress genes and genes involved in EPS synthesis was investigated in a mature biofilm and emphasized the role of the matrix in increased biofilm resistance. As oak is commonly used in wine aging, we focused on the O. oeni biofilm on this material and its contribution to the development of wine color and the release of aromatic compounds. Analytical chromatography was used to target the main oak aging compounds such as vanillin, gaiacol, eugenol, whisky-lactones, and furfural. The results reveal that O. oeni biofilm developed on oak can modulate the wood-wine transfer of volatile aromatic compounds during MLF and aging by decreasing furfural, gaiacol, and eugenol in particular. This work showed that O. oeni forms biofilms consisting of stress-tolerant cells capable of efficient MLF under winemaking conditions. Therefore surface-associated behaviors should be considered in the development of improved strategies for the control of MLF in wine.
Highlights
The winemaking process involves the alcoholic fermentation (AF) of must performed by yeast, often followed by malolactic fermentation (MLF) performed by lactic acid bacteria (LAB)
Stainless steel tanks and oak barrels are used in winemaking, the development of O. oeni was characterized on both surfaces
In order to investigate biofilm tolerance mechanisms, we studied the relative expression of a set of genes encoding for stress proteins (Figure 4A) and a set of genes involved in exopolysaccharide production (Figure 4B), during the biofilm development (2-week old biofilm) and the planktonic growth with or without stress (30 min in wine at pH 3.5 and ethanol 12%)
Summary
The winemaking process involves the alcoholic fermentation (AF) of must performed by yeast, often followed by malolactic fermentation (MLF) performed by lactic acid bacteria (LAB). Several LAB genera including Lactobacillus, Leuconostoc, Pediococcus, and Oenococcus are able to decarboxylate L-malate Of the latter Oenococcus oeni appears best able to maintain its metabolism in an environment with low pH It has been shown that MLF does not necessarily require cell growth: non-proliferating cells of O. oeni at 106 to 107 CFU/ml can decarboxylate malic acid (LafonLafourcade, 1970) These results suggest that, as described in previous works for other alcoholic fermented beverages, surfaceassociated cells could be used to perform MLF
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