SOME ROLES OF MALIC-ACID IN THE MALOLACTIC FERMENTATION IN WINE-MAKING

The control of the malolactic bacteria, the lactic acid bacteria that carry out the malolactic fermentation, is an important part of the technology of modern commercial wine production. The operative reaction of these bacteria, the decarboxylation of malic acid to lactic acid, can be of small or of large importance, depending upon the climatic environment for the growth of the grapes used as the starting material for the wine and upon the style of wine to be made. In this review, recent information is surveyed on the application of malolactic bacteria in wine production, on the control of these organisms, and on the intermediary metabolism involved in the seemingly simplistic decarboxylation of malic acid to lactic acid of the malolactic fermentation. Central to this survey is the awareness of the special importance of malic acid in the microbiology of the malolactic bacteria and in the production of wine undergoing the malolactic fermentation. In some of the newer winegrowing districts of the world, great strides have been made in the acceleration of this so-called secondary fermentation by use of bacterial starter cultures, often added to the grape must at the same time as the yeast starter cultures are added for the alcoholic fermentation. The early completion of the malolactic fermentation allows early application of the cellar operations for storage and aging of the wine, which is needed for the protection against further microbial attack. This new technology of addition of bacterial starter is the result of, and in turn has brough about, a surge in the commercial availability of malolactic bacteria in various dried, frozen or liquid forms. There has been a corresponding increase in research interest in these bacteria. Characterizations of the purified malolactic enzymes from several strains of bacteria from several research laboratories are presented here as well as information concerning genetic control of the malolactic activity, including some information about plasmids. Also presented is the new awareness of the discovery of bacteriophages in wine, which has led to research on the susceptibility of the malolactic bacteria to virus attack and to the associated commercially potential dangers. The continued research interest on some of the environmental influences (pH, nutritional additives, time of removal of the gross yeast lees and temperature) on the malolactic fermentation is discussed, as is the isolation of various new strains of bacteria. From a teleological point of view, it is becoming clear what advantage the malolactic capability has for these bacteria, even though the thermodynamics of the conversion of a molecule of malic acid to lactic acid and carbon dioxide provide for hardly any free energy. This is discussed in terms of the decarboxylation reaction, which has been shown to be not absolutely complete. Instead, the reaction seems to provide for a small net production of hydrogen acceptors. This is evidenced by a decrease in the lag period at the beginning of the fermentations in presence of malic acid; that is, a stimulation of the initial growth rate of the bacteria, which under the harsh conditions of practical wine production is sometimes very welcome to the winemaker. In addition to this, the deacidification itself, by increasing the intracellular and extracellular pH, through the conversion of malic acid to lactic acid, is recognized to be advantageous to the cell. Results are also presented, which, in addition to the formation of extra hydrogen acceptors, show capture of small amounts of free energy of the reaction for the net formation of high energy storage compounds, again to the cell's advantage.