Malo-lactic bacteria

Fixed Acid. The total acid (as tartaric) less the volatile acidity (as tartaric) is the fixed acidity. It is useful to make this calculation when one suspects activity of acid-reducing bacteria, as in the malo-lactic fermentation. 

There is no good agreement as to the origin of malo-lactic bacteria in wineries (cf. Ref. 4). Apparently microflora of definite individuality can be established in a winery, presumably in used cooperage. We have discussed (4) several workers suggestions for the origin of these microbes... 

Fumaric Aero Inhibition. Another means of preventing malo-lactic fermentation is to add fumaric acid after alcoholic fermentation is complete (45, 46, 47,48). The inhibition is relative and its extent is dependent on the amount added. The susceptibility to fumaric acid is also dependent on the strain of malo-lactic bacteria tested (49). However, we know of no case where fumaric acid addition at the levels suggested by Cofran and Meyer (45) (about 0.05%) did not delay malo-lactic fermentation under normal winemaking conditions. This includes several experiments from our pilot winery (50). Nevertheless, we have not been hasty to recommend the use of fumaric acid as an inhibitor because 1) of the difficulty in solubilizing the acid in wine 2) we do not know the mechanism of action of its inhibition [Pilone (47, 48) has shown that the bacteria metabolize low levels of fumaric acid to lactic acid but, at inhibitory levels at wine pH, the acid is bactericidal] and 3) of the desirability of minimizing the use of chemical additives. 

We have found a high degree of correlation between induction of rapid malo-lactic fermentation and bacteria addition in starter cultures when the wines were treated as suggested above to encourage the fermentation. Peynaud and Domercq (51) obtained malo-lactic fermentation by inoculating commercial wines of a locality which had not had a natural fermentation for many years. 

Malo-lactic strains of bacteria are commercially available from Equilait (38, avenue de la Republique, 15-Aurillac, France). The cul-... 

Detection of Malo-Lactic Fermentation. It is imperative that the winemaker, to control malo-lactic fermentation, has a satisfactory method for its detection. Disappearance of malic acid is the indication of the fermentation, but the formation of lactic acid is not sufficient evidence since it might also be formed by yeast and by bacteria from other carbohydrate sources. The rate of conversion of malic acid is expected to reflect bacterial metabolism and growth. In New York State wines, Rice and Mattick (41) showed bacterial growth (as measured by viable count) to be more or less exponential to 106-107 cells/ml, preceding disappearance of malic acid. The rate of loss of malic acid is probably also exponential. Malic acid seems to disappear so slowly that its loss is not detected until a bacterial population of about 106-107 cells/ml is reached then it seems to disappear so rapidly that its complete loss is detected within a few days (41). Rice and Mattick (41) also showed a slight increase in bacterial population for a few days following this. 

The literature concerning malo—lactic fermentation—bacterial conversion of L-malic acid to L-lactic acid and carbon dioxide in wine—is reviewed, and the current concept of its mechanism is presented. The previously accepted mechanism of this reaction was proposed from work performed a number of years ago subsequently, several workers have presented data which tend to discount it. Currently, it is believed that during malo-lactic fermentation, the major portion of malic acid is directly decarboxylated to lactic acid while a small amount of pyruvic acid (and reduced coenzyme) is formed as an end product, rather than as an intermediate. It is suspected that this small amount of pyruvic acid has extremely important consequences on the intermediary metabolism of the bacteria. 

In discussing the studies of Brechot et al. (24) and Peynaud et al. (25), Kunkee (I) found it odd that bacteria which ordinarily produce d or DL-lactic acid from glucose produce L-lactic acid in wine as a result of malo-lactic fermentation. Peynaud et al. (26) reported that organisms which produced only D-lactic acid from glucose produced only L-lactic acid from L-malic acid. He postulated further that the malo-lactic fermentation pathway has no free pyruvic acid as an intermediate because the optical nature of L-malic acid would be lost when it was converted to pyruvic acid since pyruvic acid has no asymmetric carbon atom. Therefore, if pyruvic acid were the intermediate, one would expect d, l, or DL-lactic acid as the end product whereas L-lactic acid is always obtained. These results lend considerable support to the hypothesis that free pyruvic... 

This system appears to be somewhat novel, and it is expected that malic acid utilization in malo-lactic bacteria other than Leuconostoc oenos may occur through this mechanism. 

Acid by Malo-Lactic Bacteria, Symposium on Lactic Acid Bacteria in Beverages and Foods, Long Ashton Research Institute, University of Bristol, September 10-21, 1973. 

Malo-lactic Bacteria. Promptly separating the new wine from the thick layer of yeast and pulp particles is most important. Many wines are spoiled by procrastination. The yeast cells, if left in a thick layer, will begin to digest themselves and produce bad-smelling materials. Some breakdown products from this action have odors reminiscent of rotten eggs—the odor of hydrogen sulfide. Once the wine has acquired a rotten egg odor it is very difficult to remove it. 

The growth of malo-lactic bacteria in wines is favored by moderate temperatures, low acidity, very low levels of S02, and the presence of small amounts of sugar undergoing fermentation by yeast. It is frequently possible to inoculate a wine with a pure culture of a desirable strain of bacteria and obtain the malo-lactic fermentation under controlled conditions. The pure-culture multiplication of the selected strain of bacteria is difficult, however. It is also difficult to control the time of the malo-lactic fermentation—sometimes it occurs when not wanted, and at other times will not go when very much desired. For the home winemaker it is probably most satisfactory to accept the malo-lactic fermentation if it occurs immediately following the alcoholic fermentation. The wines should then be siphoned away from deposits, stored in completely filled containers at cool temperatures, and have added to them about 50 ppm S02. If the malo-lactic fermentation does not take place spontaneously and the wine is reasonably tart, the above described regime of preservation will likely prevent its occurrence. When the malo-lactic transformation takes place in wines in bottles, the results are nearly always bad. The wine becomes slightly carbonated, and the spoiled sauerkraut flavors are emphasized. 

Malo-Lactic Fermentation. The bacterial conversion of malic acid to lactic acid usually does not occur in North Coast white wines because of low pH, high S02, and cool storage temperatures, factors that inhibit lactic acid bacteria activity. Some work has been done in the North Coast with malo-lactic fermentation in Chardonnay, following the traditional practices in Burgundy. Factors used to encourage experimental malo-lactic fermentation in white wines have been bacterial inoculation, warmer (18°-21° C) fermentation temperatures, prolonged lees contact, and low free-S02 levels until fermentation is complete. 

Sweet wines, with potential for yeast refermentation, or wines with potential for malo-lactic fermentation, go through a membrane filtration prior to bottling. The membrane filters come in different porosities 0.65-p, pore size is used most commonly when 100 percent yeast removal is desired, and 0.45-p, pore size is used for malo-lactic bacteria removal. Proper sterilization of bottling equipment downstream of the membrane filter is essential to maintain the yeast- or bacteria-free nature of the wine after filtration. 

Prior to 1960, few made any attempts to control the malo-lactic fermentation in red wines. Ingraham et al. (19) showed that there were at least five types of lactic acid bacteria capable of decomposing malic acid in California wines. Webb and Ingraham (20) showed that the malo-lactic fermentation could be induced in the winery. Prior to that time, most malo-lactic fermentations occurred spontaneously from the inoculum normally present in the vats. 

Several provisos should be mentioned here. The lactic bacteria are extremely sensitive to free S02 and are inhibited by its presence even in low concentrations. The bacteria become inactive at temperatures much below 18.3° C (65° F). The reduction of the total acidity of the juice is not directly proportional to the amount of water amelioration. On occasion, the natural increase of the bacterial population does not reach the critical point at which the malo-lactic reaction takes off. 

As already stated, the ameliorative effect of malo-lactic fermentation is equally important in the preparation of red wines—probably more so since a degree of tartrate not desirable in a red wine is often wanted in a white wine. It is also necessary to achieve bacterial stability. So long as malic acid remains in the wine and there is any presence of lactic bacteria, there is a risk, however remote, of delayed fermentation in the bottle accompanied by hazes and gassiness. In former times, French Burgundies often were subject to this accident. Finally, the slight increase of volatile acidity that accompanies malo-lactic fermentation may help to heighten and improve aroma and flavor. 

Chalfan, Y., Goldberg, I., and Mateles, R.I. 1977. Isolation and characterization of malo-lactic bacteria from Israeli red wines. J. Food Sci. 42, 939-943. 

Kunkee, R.E., Ough, C.S., and Amerine, M.A. 1964. Induction of malo-lactic fermentation by inoculation of must and wine with bacteria. Am. J. Enol. Vitic. 15, 178-183. 

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