Bacteria, malolactic fermentation

Davis, C.R. 1985. Taxonomy and ecology of lactic acid bacteria. Malolactic Fermentation. T.H. Lee (ed). Adelaide The Australian Wine Research Institute, pp. 3-17. 

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. 

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). 

Malolactic fermentation (MLF) is an important secondary fermentation that occurs in many wines generally about 2-3 weeks after completion of the alcoholic fermentation. Lactic acid bacteria, principally Oenococcus oeni (formerly Leuconostoc oenos) are responsible for this fermentation. 

Together with proteins and peptides, amino acids constitute the main components of the nitrogenous fraction of musts and wines. They are also the most studied and best known nitrogenated components in wines. Free amino acids in musts are of paramount importance. 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 (Kosir and Kidric, 2001). In certain cases, some amino acids... 

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). 

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. 

As well as fruity and buttery aromas, MLF has also been associated with other characteristic aromas such as floral, roasted, vanilla, sweet, woody, smoked, bitter, honey, etc. (Flenick-Kling 1993 Sauvageot and Vivier 1997). However, further studies are required to be able to relate the wine characteristics that are modified during malolactic fermentation with the production and/or degradation of a specific chemical compound by wine lactic acid bacteria. With this information, the winemaker can choose the best strain of lactic acid bacteria to obtain wine with a specific aroma or flavour. 

Control of bacteria development is essential for the production of wines with consistent compositional and sensory characteristics. Although malolactic fermentation (MLF) is frequently carried out in the winery due to its beneficial effects on wine aroma and palate, for certain grape varieties and wine styles MLF is considered detrimental. Moreover, even in the cases where MLF is desirable, the wine might... 

Commercial 0. oeni strains are selected for their oenological parameters, including the absence of amino acid decarboxylases. According to the in vitro studies done by Moreno-Arribas et al. (2003), none of the four commercial malolactic starter cultures tested could produce histamine, tyramine or putrescine. Martln-Alvarez et al. (2006) also compared inoculation with spontaneous malolactic fermentation in 224 samples of Spanish red wine. They found that inoculation with a commercial starter culture of lactic acid bacteria could reduce the incidence of biogenic amines compared to spontaneous malolactic fermentation in wines. Starter cultures could eliminate indigenous bacteria, or could possibly degrade the biogenic amines produced by the undesirable strains. 

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). 

Davis, C.R., Wibowo, D.J., Lee, T.H. Eleet, G.H. (1986). Growth and metabolism of lactic acid bacteria during and after malolactic fermentation of wines at different pH. Appl. Environ. Microbiol, 51, 539-545. 

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). 

Asenstorfer et al. 2003). Similarly, malolactic fermentation can also affect the production of the pigment since lactic acid bacteria have the capacity of using pyruvic acid (Asenstorfer et al. 2003). No synthesis and/or losses of the pigment were found at low SO2 concentration and occurrence of malolactic fermentation, while a maximum production was achieved under the opposite conditions. 

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). 

In grapes or grape juices, the tartaric esters may be hydrolysed by enzymes from contaminant fungi or from commercial pectolytic preparations, both with cin-namoyl decarboxilase activity, releasing free hydroxycinnamic acid forms (Dugelay et al. 1993 Gerbaux et al. 2002). However, the tartaric esters are mostly hydrolysed after malolactic fermentation (Hernandez et al. 2006, 2007), it being hypothesised that the hydrolytic activity of lactic acid bacteria follows the completion of malic conversion to lactic acid (Cabrita et al. 2007) (see Table 11.4). 

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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