Chapter 7

Malolactic Fermentation in Wine

Effects of Malolactic Fermentation

The primary and most significant process that occurs during malolactic fermentation is biological deacidification, consists in decarboxylation of L-malic acid to L-lactic acid. Another role, apart from biological deacidification, of malolactic fermentation is to modify the flavor, aroma, and texture of produced wine. Malolactic fermentation also leads to enhanced microbiological stability.

Acidity

Growth of malolactic fermentation is generally encouraged where malolactic fermentation is required to reduce the acidity of the wine. The reduction of acidity is beneficial to the quality of wine made in cool winegrowing regions, because the grapes naturally contain high levels of organic acids. Worldwide consumer preferences currently desire fruit-driven wines with moderate acidity, which means acid reduction has become an important issue for the production of wines from cooler climates.

Flavor Modification

In addition to deacidification, flavor modification of the wine is an important aspect to be considered when choosing malolactic fermentation. Wine bouquet is intensified and the character and firmness of the wine are improved, as long as the lactic notes are not excessive. The taste of the wine is also considerably improved. Texturally, wines undergoing malolactic fermentation can be described as having more body and roundness, greater length on the palate, and silky tannins.

Oak Barrels

Malolactic fermentation is typically carried out in oak barrels (Figure 7.2). Although malolactic fermentation in stainless-steel tanks may simplify control of the process, using of oak barrels during malolactic fermentation affects the chemical and sensory attributes of wines.

Diacetyl

Perhaps one of the most identifiable volatile compounds produced by the growth of wine lactic acid bacteria is the compound known as diacetyl. Readily identifiable because of its very low aroma threshold and its distinctive aroma, it is the compound responsible for a buttery aroma. In some instances, diacetyl is a desirable attribute and in other cases it is undesirable. In low concentrations, it can be responsible for toasty and nutty aromas, while at high levels it imparts a buttery aroma.

Wine Spoilage by Lactic Acid Bacteria

Malolactic fermentation is not always beneficial and can be responsible for undesirable changes to the sensory properties of wine. As mentioned above, several species of lactic acid bacteria may conduct malolactic fermentation. When malolactic fermentation occurs at a pH below 3.5, it is generally induced by Oenococcus oeni and is less likely to generate off-odors, although some indigenous strains may have the capacity to do so. The undesirable odors such as buttery, cheesy, milky, metallic, and earthy, as well as excessive amounts of acetic acid, are usually associated with malolactic fermentations that occur above pH 3.5 and have been conducted by species of Pediococcus and Lactobacillus.

Microbial Stability

Another major benefit of malolactic fermentation is increased microbial stability. A wine that has fermented dry and completed malolactic fermentation will have very little sugar, malic acid, citric acid, or micronutrients for spoilage bacteria to feed on. Wines that have not undergone malolactic fermentation before bottling/ packaging, risk the possibility that malolactic fermentation will spontaneously. If this happens, the wine becomes gassy and turbid, and is considered spoiled.

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