Acid Bacteria and Malolactic Fermentation in Wine

Lactic Acid Bacteria and Malolactic Fermentation in Wine 

From all the species of lactobacilli and cocci identified in wine, Oenococcus oeni (known as Leuconostoc oenos until 1995, Dicks et al. 1995) dominates the system. Initially physiological and metabolic studies revealed the originality of O.oeni while recent genomic studies allow us to partly interpret it. Some unusual features of O. oeni, including the tolerance to acidity, ethanol, and other natural inhibitors of wine and some metabolic pathways, are also common to other species of the oenological niche. 

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Of all the metabolic activities that lactic acid bacteria can carry out in wine, the most important, or desirable, in winemaking is the breakdown of malic acid, but only when it is intended for this to be removed completely from the wine by malolactic fermentation. Although the breakdown of malic and citric acids has considerable consequences from a winemaking perspective, it is also evident that lactic acid bacteria metabolise other wine substrates to ensure their multiplication, including sugars, tartaric acid, glycerine and also some amino acids. We will now describe some of the metabolic transformations that have received most attention in the literature, or which have important repercussions in winemaking. 

The sugars in fruits such as grapes are feimented by yeasts to produce wines. In winemaking, lactic acid bacteria convert malic acid into lactic acid in malolactic fermentation in fruits with high acidity. Acetobacter and Gluconobacter oxidise ethanol in wine to acetic acid (vinegar). 

Free amino acids in musts are of paramount importance, since they constitute a source of nitrogen for yeasts in alcoholic fermentation, for lactic acid bacteria in malolactic fermentation and can also be a source of aromatic compounds. In certain cases, some amino acids can produce undesirable compounds in wines, such as ethyl carbamate, biogenic amines, ochratoxin A (from 2-phenylalanine) and 3-carbolines (from tryptophane) (Herraiz and Ough 1993 Herraiz et al. 1993). 

Table 4.3 lists the lactic acid bacteria most often encountered in grape must and wine. Oenococcus oeni is known for ensuring malolactic fermentation in the great majority of cases. So far, the strictly homofermentative lactobacilli of Group I have not been isolated in must or wine. The species are therefore divided into facultative and strict heterofermenters for lactobacilli and into homofermenters (Pediococcus) and heterofermenters (Leuconostoc) for cocci. It is likely that this classification will be modified—on one hand due to anticipated progress in the identification of new species in wine, and on the other hand dne to eventual reclassifications of lactobacilli in the gronps described above. 

In addition to alcohoHc fermentation, a malolactic fermentation by certain desirable strains of lactic acid bacteria needs to be considered. Occasionally, wild strains produce off-flavors. Malolactic fermentation is desirable in many red table wines for increased stabiUty, more complex flavor, and sometimes for decreased acidity. Selected strains are often added toward the end of alcohoHc fermentation. AH the malic acid present is converted into lactic acid, with the resultant decrease of acidity and Hberation of carbon dioxide. Obviously this has more effect on the acidity the more malic acid is present, and this is the case in wine from underripe, too-tart grapes. Once malolactic fermentation has occurred, it does not recur unless another susceptible wine is blended. 

Malolactic fermentation (MLF) in wine is by definition the enzymatic conversion of L-malic acid to L-lactic acid, a secondary process which usually follows primary (alcoholic) fermentation of wine but may also occur concurrently. This reduction of malic acid to lactic acid is not a true fermentation, but rather an enzymatic reaction performed by lactic acid bacteria (LAB) after their exponential growth phase. MLF is mainly performed by Oenococcus oeni, a species that can withstand the low pFi (<3.5), high ethanol (>10 vol.%) and high SO2 levels (50 mg/L) found in wine. More resistant strains of Lactobacillus, Leuconostoc and Pediococcus can also grow in wine and contribute to MLF especially if the wine pH exceeds 3.5 (Davis et al. 1986 Wibowo et al. 1985). The most important benefits of MLF are the deacidification of high acid wines mainly produced in cool climates, LAB contribute to wine flavour and aroma complexify and improve microbial sfabilify (Lonvaud-Funel 1999 Moreno-Arribas and Polo 2005). 

Moreno-Arribas and Lonvaud-Funel (1999). Moreno-Arribas et al. (2000) isolated and identified a number of tyramine-producing lactic acid bacteria in wine that had undergone malolactic fermentation all belonging to the lactobacilli. Tyrosine decarboxylase was then purified (Moreno-Arribas and Lonvaud-Funel 2001) and the corresponding gene was purified and sequenced (Lucas and Lonvaud-Funel 2002 Lucas et al. 2003). As far as the literature suggests, no tyramine-producing 0. oeni strain has yet been reported, with the exception of one strain (O. oeni DSM 2025) that was shown to be able to produce tyramine in a laboratory medium (Choudhury etal. 1990). 

The interaction between aroma compounds and other wine micro-organisms (e.g. lactic acid bacteria) or with metabolites produced during malolactic fermentation has been studied to a limited extent. Interactions between polysaccharides produced by the most common wine lactic bacteria (Oenoccocus oeni) during malolactic fermentation have been shown to be responsible for the reduced volatility of some aroma compounds in wines (Boido et al. 2002). The possibility of direct interactions between the surface of the bacteria cells and aroma compounds should also be considered since this type of interaction has been found for other food lactic bacteria (Ly et al. 2008). 

Cantos et al. 2003 Gambuti et al. 2004). It is also influenced by yeast enzymatic activities, in particular those of isomerase and glucosidase (Jeandet et al. 1994). Equally, activities of lactic acid bacteria, which are responsible for malolactic fermentation (Hernandez et al. 2007), can also affect stilbene content in wine (Poussier et al. 2003). Aging of wine appears to have no important influence on the concentration of stilbenes (Jeandet et al. 1995). 

Malolactic fermentation (MLF) is an important process, nowadays also conducted on an industrial scale, aimed at improving organoleptic characteristics and conferring microbiological stability to quality wines (Davis et al., 1985). The main transformation of the wine occurring in this process operated by lactic bacteria, is decarboxylation of L(—)-malic acid with formation of L(+)-lactic acid (Figure 1.5). 

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