Page 14 - MALOLACTIC FERMENTATION – IMPORTANCE OF WINE LACTIC ACID BACTERIA IN WINEMAKING

MALOLACTIC FERMENTATION IN WINE
hexose sugars by the phosphoketolase pathway. In this pathway, equimolar concentrations of lactic acid, CO2 and acetic acid or ethanol can be produced from one mole of glucose, with a concomitant energy gain of one mole of ATP. The oxidation/reduction potential (redox) of the system also affects the ratio of ethanol/acetic acid produced, with aerobic conditions favouring the formation of acetic acid, and anaerobic conditions favouring the production of ethanol (Kandler 1983, and Condon 1987). Depending on the species or the genus of LAB involved, the isomers of lactic acid produced from the fermentation of carbohydrates can be either L(+), D(-) or a combination of both the L(+) and D(-) forms (Kandler 1983, Boulton et al. and Condon 1998). For example, Leuconostocs, including Oenococcus oeni, produce the D(-)-lactic acid isomer from the fermentation of hexose sugars. In contrast, however, the decarboxylation of L(-)-malic acid in the malolactic fermentation yields only the L(+)-lactic acid isomer (figure 1).
Overall, the LAB group can utilize a wide range of carbohydrates, including the hexoses (glucose, fructose, mannose and galactose), as well as other pentoses, polyols and oligosac- charides. This capability is dependent on the species and strains involved, as well as the pH of the medium. Moreover, since malic acid cannot be used by wine LAB as a sole carbohydrate source (see below), the availability and utilization of fermentable carbohydrates in wine by LAB is essential to enable the onset of bacterial growth and the occurrence of MLF. Further- more, recent studies have clearly demonstrated that grape-derived phenolic glycosides also significantly stimulate the growth of O. oeni in a synthetic wine medium.
MALOLACTIC CONVERSION – THE DEACIDIFICATION REACTION
Overall, three main pathways have been proposed for the degradation of L-malic acid to L-lactic acid by LAB during MLF. The first involves the activity of three separate enzymes – malate dehydrogenase, oxaloacetate decarboxylase and L-lactate dehydrogenase – and proceeds via the intermediates, oxaloacetic acid and pyruvic acid. A second mechanism pro- ceeds via pyruvic acid and utilizes a combination of malic enzyme and lactate dehydrogenase. It was not until the 1970s that the enzymatic basis for this reaction was more fully elucidated in wine malolactic bacteria, specifically Leuconostoc oenos (O. oeni) ML34, by Kunkee 1975 and Morenzoni 1974. This work revealed that a single enzyme, commonly known as the “malolactic enzyme,” exhibits two separate enzyme activities that act simultaneously on L-ma- lic acid. The predominant “malolactic activity” of this enzyme (malate: NAD+ carboxylyase) catalyzes the direct conversion (decarboxylation) of the dicarboxylic acid L-malic acid to the monocarboxylic acid L-lactic acid, and requires NAD and Mn+2 as co-factors.
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