Yeast interactions and wine flavour.

The winemaking process includes two main steps: lactic acid bacteria are responsible for the malolactic fermentation which follows the alcoholic fermentation by yeasts. Both types of microorganisms are present on grapes and on cellar equipment. Yeasts are better adapted to growth in grape must than lactic acid bacteria, so the alcoholic fermentation starts quickly. In must, up to ten lactic acid bacteria species can be identified. They belong to the Lactobacillus, Pediococcus, Leuconostoc and Oenococcus genera. Throughout alcoholic fermentation, a natural selection occurs and finally the dominant species is O. oeni, due to interactions between yeasts and bacteria and between bacteria themselves. After bacterial growth, when the population is over 106CFU/ml, malolactic transformation is the obvious change in wine composition. However, many other substrates can be metabolized. Some like remaining sugars and citric acid are always assimilated by lactic acid bacteria, thus providing them with energy and carbon. Other substrates such as some amino acids may be used following pathways restricted to strains carrying the adequate enzymes. Some strains can also produce exopolysaccharides. All these transformations greatly influence the sensory and hygienic quality of wine. Malic acid transformation is encouraged because it induces deacidification. Diacetyl produced from citric acid is also helpful to some extent. Sensory analyses show that many other reactions change the aromas and make malolactic fermentation beneficial, but they are as yet unknown. On the contrary, an excess of acetic acid, the synthesis of glucane, biogenic amines and precursors of ethylcarbamate are undesirable. Fortunately, lactic acid bacteria normally multiply in dry wines; moreover some of these activities are not widespread. Moreover, the most striking trait of wine lactic acid bacteria is their capacity to adapt to a hostile environment. The mechanisms for this are not yet completely elucidated. Molecular biology has provided some explanations for the behaviour and the metabolism of bacteria in wine. New tools are now available to detect the presence of desirable and undesirable strains. Even if much remains unknown, winemakers and oenologists can nowadays better control the process. By acting upon the diverse microflora and grape musts, they are more able to produce healthy and pleasant wines.

Lactic acid bacteria in the quality improvement and depreciation of wine

The winemaking process includes multiple stages at which microbial spoilage can occur, altering the quality and hygienic status of the wine and rendering it unacceptable The major spoilage organisms include species and strains of the yeast genera Brettanomyces, Candida, Hanseniaspora, Pichia, Zygosaccharomyces etc, the lactic acid bacterial genera Lactobacillus, Leuconostoc, Pediococcus, etc and the acetic acid bacterial genera Acetobacter and Gluconobacter The faults caused include bitterness and offflavours (mousiness, ester taint, phenolic, vinegary, buttery, geranium tone), and cosmetic problems such as turbidity, viscosity, sediment and film formation These spoilage organisms can also affect the wholesomeness of wine by producing biogenic amines and precursors of ethyl carbamate The judicious use of chemical preservatives such as sulphur dioxide (S02) during the winemaking process decreases the risk of microbial spoilage, but strains vary considerably in their S02 sensitivity There is, moreover, mounting consumer bias against chemical preservatives, and this review focuses on the possible use of biopreservatives in complying with the consumers' demand for "clean and green" products

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Microbial Spoilage and Preservation of Wine: Using Weapons from Nature's Own Arsenal -A Review

Saccharomyces cerevisiae-Oenococcus oeni interactions in wine: current knowledge and perspectives.

Malolactic fermentation (MLF) is a secondary wine fermentation conducted by lactic acid bacteria (LAB). This fermentation is important in winemaking as it deacidifies the wine, it contributes to microbial stability and lastly it contributes to wine aroma through the production of metabolites. Oenococcus oeni is the main species used in commercially available starter culture currently, but research has indicated that different Lactobacillus species also partake in MLF and this has shifted the focus in the MLF field to evaluate the potential of lactobacilli as starter cultures for the future. There are 17 different species of Lactobacillus associated with winemaking either being associated with the grapes/beginning of alcoholic fermentation or the MLF and wine. Lactobacillus associated with wine is mainly facultative or obligatory heterofermentative and can withstand the harsh wine conditions such as high ethanol levels, low pH and temperatures and sulphur dioxide. Wine lactobacilli contain the malolactic enzyme encoding gene, but sequence homology shows that it clusters separate from O. oeni. Lactobacillus also possesses more enzyme encoding genes compared to O. oeni, important for the production of wine aroma compounds such as glycosidase, protease, esterase, phenolic acid decarboxylase and citrate lyase. Another characteristic associated with wine lactobacilli is the production of bacteriocins, especially plantaricins which would enable them to combat spoilage LAB. All these characteristics, together with their ability to conduct MLF just as efficiently as O. oeni, make them suitable for a new generation of MLF starter cultures.

Lactobacillus : the Next Generation of Malolactic Fermentation Starter Cultures—an Overview

In addition to ethanol, the fermentation of grape must by yeasts produces fatty acids as secondary metabolites which inhibit growth of lactic acid bacteria. The degree of inhibition depends upon the fatty acid concentration and chain length and on the composition of the medium.

Inhibition of malolactic fermentation of wines by products of yeast metabolism

L'addition au vin d'ecorces de levure (0,2 g par litre) permet une stimulation de la fermentation malolactique. Leur action s'exerce en augmentant la population bacterienne et surtout en retardant et attenuant la phase de declin. Le traitement du vin par des polysaccharides (alginates, polysaccharides extraits de vin) ou par l'extrait de levure accelere aussi le processus de degradation de l'acide malique. Ces adjuvants agissent probablement en limitant l'action inhibitrice de certains metabolites levuriens tels les acides gras.+++Addition of yeast ghosts to wine (0,2 g per liter) stimulates the malolactic fermentation. The bacterial population is increased furthermore the declin phase is delayed. Addition of polysaccharides to wine (alginate, polysaccharides extracted from wine) or yeast extract also accelerate the malic acid degradation. These additions probably limit the inhibition by some products of the yeast metabolism like fatty acids.

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Stimulation de la fermentation malolactique par l'addition au vin d'enveloppes cellulaires de levure et différents adjuvants de nature polysaccharidique et azotée

Les lipides extraits des membranes de bacteries lactiques du vin sont separes par chromatographie sur couche mince en phospholipides, glycolipides et lipides neutres. Le pourcentage de repartition de chaque classe varie, non seulement en fonction des souches, mais aussi selon leurs conditions de culture.+++Lactic and bacteria of wines are inhibited in fermenting must. Ethanol, fatty acids and other unknow yeast metabolites are responsible for the loss of viability and malolactic activity. Prior results suggest that they induce an alteration of the plasma membrane. Owing to their nature, we assume the lipid constituants to be the target of the inhibitors. So, we have undertaken the study of the membrane lipids of two strains, L. plantarum and L. oenos.After extraction, the samples are analysed by HPTLC. Comparing the strains, the patterns reveal small differences in the distribution of phospholipids, glycolipids and neutral lipids.Some culture factors were investigated for their influence. Within the cell cycle, the distribution of the various classes changes according to the period of incubation. If ethanol is added in the culture medium, neutral lipids decrease by 25 to 50 per cent and glycolipids increase by 30 per cent depending on the age of the culture. Phospholipids remain quite constant.Further researchs are in progress to test the influence of others factors. These results might explain the loss of viability of the bacteria varies when they are added to wine, as well as the variable response to yeast antagonism according to the strains.

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Catherine Desens

Papers

Inhibition of malolactic fermentation of wines by products of yeast metabolism

Stimulation de la fermentation malolactique par l'addition au vin d'enveloppes cellulaires de levure et différents adjuvants de nature polysaccharidique et azotée

Etude de la constitution lipidique des membranes de bactéries lactiques utilisées en vinification