Malolactic transformation

In the case of non-proliferating cells in a laboratory medium and during winemaking, lactic acid bacteria of wine transform L-malic acid exclusively into L-lactic acid. Seifert (1901) established the reaction of the malolactic transformation according to the following equation ... 

This reaction therefore involves a decarboxylation without an intermediary product capable of following another metabolic pathway. Several authors have reported that certain bacterial strains form other molecules from malic acid, suggesting in this manner the existence of other reactions. Even if their existence cannot be ruled out, malolactic transformation is the only reaction that exists in the lactic acid bacteria involved in winemaking. 

Since oxaloacetate is easily decarboxylated into pyruvate and CO2, these two reactions lead to the formation of pyruvate from L-malate. Since the final product of the malolactic transformation in wine is L-lactic acid, MDH or the malic enzyme would be associated to an LDH catalyzing the reduction of pyruvate into L-lactate in this metabolic pathway. At least for wine bacteria, this concept is not acceptable since these bacteria only possess a d-LDH. Malic acid would only lead to the formation of D-lactic acid. 

In the fermentative process, the first step is due to yeasts which transform sugars to alcohol (alcoholic fermentation). This is followed by a second fermentation step (malolactic fermentation), which corresponds to the transformation of L-malic acid to L-lactic acid. 

Usually, after alcoholic fermentation, the wine undergoes malolactic fermentation, induced primarily by Oenococcus oeni. Not only can this lactic acid bacterium convert L-malic acid into L-lactic acid but also it is involved in many other transformations fundamental to Amarone quality. 

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. 

This is the main reaction of MLR Chemically it consists of a simple decarboxylation of the L-malic acid in wine into L-lactic acid. Biochemically, it is the result of activity of the malolactic enzyme, characteristic of lactic acid bacteria. This transformation has a dual effect. On the one hand, it deacidifies the wine, in other words, it raises the pH, an effect that is greater at higher initial quantities of malic acid. It also gives the wine a smoother taste, replacing the acidic and astringent flavour of the malic acid, by the smoother flavour of the lactic acid. 

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). 

Figure 1.5 Transformation of L(—)-malic acid into L(+)-lactic acid occurring in malolactic fermentation (MLF)...

Therefore, the hypothesis of the existence of an enzyme catalyzing the direct decarboxylation of L-malic acid into L-lactic acid was made. The enzyme, called the malolactic enzyme, was isolated for the first time in Lactobacillus plantarum (Lonvaud, 1975 Schiitz and Radler, 1974). From acellular bacterial extracts and thanks to successive purification stages, the authors obtained purified fractions responding to the fnnctional criteria of the malolactic enzyme. L-Malic acid is transformed stoichiometrically into L-lactic acid. These fractions do not have an LDH activity. 

The pH is therefore very important and comes into play at several levels in the selection of the best adapted strains in the growth rate and yield in the malolactic activity and even in the nature of the substrates transformed. 

The malolactic fermentation phase begins during the growth phase, as soon as the total population exceeds lO UFC/ml. It continues and is completed during the stationary phase, or sometimes at the beginning of the death phase. In very favorable conditions with a limited concentration of malic acid, malolactic fermentations are often completed even before the end of the growth phase. The optimum population in these cases exceeds 10 UFC/ml. As soon as a sufficient biomass is formed, malic acid is degraded. The malolactic acid bacterial activity is always present but depends on various conditions, especially the temperature. The transformation of 2 g of malic acid per liter can take more time than 4 g/1 if the population level attained is lower. 

The only bacterial intervention truly sought after in winemaking is the transformation of malic acid into lactic acid (Section 12.7.2). It is the source of the most manifest organoleptic change, resulting from malolactic fermentation the deacidification and the softening of wine. Malic acid, a dicarboxylic acid, is transformed molecule for molecule into lactic acid, monocarboxylic. The loss of an acid function per molecule is intensified by the replacement of an acid with... 

Bacteria do not transform all of the malic acid contained in the grape. From the start, during alcoholic fermentation, yeasts metabolize a maximum of 30% of the malic acid. The product, pyruvate, then enters one of many yeast metabolic pathways—notably leading to the formation of ethanol. This malo-alcoholic fermentation is catalyzed at the first stage by the malic enzyme. The bacteria must develop a sufficient population before malolactic fermentation can truly start. The production of L-lactic acid is coupled with the decrease in malic acid (Figure 6.3). 

In any case, citric acid is always degraded during malolactic fermentation, since 0. oeni species have all of the necessary enzyme equipment. Its transformation is nevertheless slower than malic... 

As soon as the malolactic fermentation is completed, the same bacteria can rapidly become detrimental and certain precautions are necessary to avoid this unwanted evolution. The bacteria are apt to decompose pentoses, glycerol, tartaric acid, etc. These transformations cause common... 

Table 12.15 shows the main chemical transformations in wine during malolactic fermentation. In this case, it is incomplete, as the wine stiU contains 0.5 g/1 malic acid that has not been degraded. 

The chemical transformations of wine by malolactic fermentation are much more complex in reality. Malolactic fermentation also produces ethyl lactate, the formation of which conffibutes to the sensation of body in wine (Henick-Kling, 1992). Additionally, other secondary products have been identified, the most important being diacetyl, produced by bacteria (a few milligrams per liter), that belongs to a complex pool of production and degradation mechanisms. At moderate concentrations, this secondary product contributes to aromatic complexity, but above 4 mg/1 the characteristic butter aroma of this substance dominates. 

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