Developments in Malo-Lactic Fermentation of Australian Red Table Wines

Control of malo-lactic fermentation in red table wines is one of the most important technical problems in Australian winemaking. These wines are usually higher in pH (3.4-4.3; mean 3.85) than wines of the same type from other countries (reasons are postulated), and malo-lactic fermentation raises pH still further (0.05-0.35 increase), frequently resulting in lower quality, and in serious cases, bacterial spoilage.

The most important and frequently the only contribution of malo-lactic fermentation is bacteriological stability of the wine, providing that the fermentation goes to completion. Quality may or may not be impaired, depending on the wine and the climatic area in which the grapes are grown (malolactic fermentation is considered to be desirable in very cool areas), but the fruit and varietal character is frequently lost.

The malic acid content of wines which have not undergone malo-lactic fermentation ranges from about 1-1.5 (the minimum level has not been established precisely) to 4 grams per liter. Residual malic acid after malo-lactic fermentation is usually less than 0.1 gram per liter, though some wines contain between 0.1 and 1.0 gram per liter. This indicates that either malo-lactic fermentation has commenced and not gone to completion (as has recently been shown to occur) or that wines with and without malic acid have been blended. Since malo-lactic fermentation can recommence, these wines are still bacteriologically unstable and have to be handled accordingly. Precise enzymatic analysis of malic acid to detect these low levels is now being used in winery laboratories to supplement semi-quantitative paper chromatography.

Acidification is practiced, either by acid addition or hydrogen-ion exchange, to adjust pH to within the range 3.5-3.8. If malo-lactic fermentation occurs, the resultant increase in pH is usually corrected.

Induction of malo-lactic fermentation by bacterial inoculation with Leuconostoc oenos has only been partly successful and is little used at present.

Since malo-lactic fermentation occurs readily in warm areas in wines with high pH values, prevention is difficult. Nevertheless, routine prevention is now being achieved by pH reduction (preferably before the yeast alcoholic fermentation), early clarification and sterile filtration, sulfur dioxide addition, cool storage, and sterile filtration into the bottle. Fumaric acid addition shows promise as a bacterial growth inhibitor, though more winery data are needed. Most winemakers would prefer malo-lactic fermentation not to take place if they can prevent it.