Influence of Yeast Strain and Malo-Lactic Fermentation on Composition and Quality of Table Wines

Abstract

The micro-organisms which can grow in grape juice and wine are limited to yeasts, lactic and acetic acid bacteria, and in rare cases, possibly sporing bacilli. Pure yeast cultures are becoming widely used in technologically advanced winemaking countries. The yeast must conduct the alcoholic fermentation or at least dominate the naturally occurring microflora. The benefits found from use of selected pure yeasts are rapid onset of fermentation, even and complete fermentation leaving no residual sugar, and elimination of undesirable products of fermentation such as hydrogen sulfide. Yeasts may also be selected for their ability to decompose L-malic acid partially or completely. Mixed yeast cultures may be desirable, but their use in practical winemaking presents problems. Sherry-flor and other oxidative film yeasts bring about changes in the composition and quality of wines. More chemical studies of products formed by such yeasts are desirable. Yeast clouding of bottled wine is a recurring problem, particularly with sweet wines of low alcoholic strength. Malo-lactic fermentation is the bacterial decomposition of L-malic acid to lactic acid and carbon dioxide. It is brought about by lactic acid bacteria and is widespread in red table wines in most winegrowing areas. It renders the wine stable to subsequent breakdown of malic acid, and may improve the flavor of the wine. Diacetyl and other secondary products may be formed as an indirect result of malo-lactic fermentation. Dry red wines with pH values above about 3.8 do not benefit from malo-lactic fermentation. If sugar is present in such wines, bacterial growth may cause spoilage. Factors which encourage malo-lactic fermentation are low alcohol, high pH, low sulfur dioxide, late racking, and warm temperature. The opposite factors discourage malo-lactic fermentation. Considerable progress has been made toward induction of malo-lactic fermentation by addition of selected bacteria, but further work is needed.