Malolactic fermentation

Malolactic fermentation (also known as malolactic conversion or MLF) is a process in winemaking in which tart-tasting malic acid, naturally present in grape must, is converted to softer-tasting lactic acid. Malolactic fermentation is most often performed as a secondary fermentation shortly after the end of the primary fermentation, but can sometimes run concurrently with it. The process is standard for most red wine production and common for some white grape varieties such as Chardonnay, where it can impart a 'buttery' flavor from diacetyl, a byproduct of the reaction. Malolactic fermentation (also known as malolactic conversion or MLF) is a process in winemaking in which tart-tasting malic acid, naturally present in grape must, is converted to softer-tasting lactic acid. Malolactic fermentation is most often performed as a secondary fermentation shortly after the end of the primary fermentation, but can sometimes run concurrently with it. The process is standard for most red wine production and common for some white grape varieties such as Chardonnay, where it can impart a 'buttery' flavor from diacetyl, a byproduct of the reaction. The fermentation reaction is undertaken by the family of lactic acid bacteria (LAB); Oenococcus oeni, and various species of Lactobacillus and Pediococcus. Chemically, malolactic fermentation is a decarboxylation, which means carbon dioxide is liberated in the process. The primary function of all these bacteria is to convert one of the two major grape acids found in wine called L-malic acid, to another type of acid, L+ lactic acid. This can occur naturally. However, in commercial winemaking, malolactic conversion typically is initiated by an inoculation of desirable bacteria, usually O. oeni. This prevents undesirable bacterial strains from producing 'off' flavors. Conversely, commercial winemakers actively prevent malolactic conversion when it is not desired, such as with fruity and floral white grape varieties such as Riesling and Gewürztraminer, to maintain a more tart or acidic profile in the finished wine. Malolactic fermentation tends to create a rounder, fuller mouthfeel. Malic acid is typically associated with the taste of green apples, while lactic acid is richer and more buttery tasting. Grapes produced in cool regions tend to be high in acidity, much of which comes from the contribution of malic acid. Malolactic fermentation generally enhances the body and flavor persistence of wine, producing wines of greater palate softness. Many winemakers also feel that better integration of fruit and oak character can be achieved if malolactic fermentation occurs during the time the wine is in barrel. A wine undergoing malolactic conversion will be cloudy because of the presence of bacteria, and may have the smell of buttered popcorn, the result of the production of diacetyl. The onset of malolactic fermentation in the bottle is usually considered a wine fault, as the wine will appear to the consumer to still be fermenting (as a result of CO2 being produced). However, for early Vinho Verde production, this slight effervesce was considered a distinguishing trait, though Portuguese wine producers had to market the wine in opaque bottles because of the increase in turbidity and sediment that the 'in-bottle MLF' produced. Today, most Vinho Verde producers no longer follow this practice and instead complete malolactic fermentation prior to bottling with the slight sparkle being added by artificial carbonation. Malolactic fermentation is possibly as old as the history of wine, but scientific understanding of the positive benefits of MLF and control of the process is a relatively recent development. For many centuries, winemakers noticed an 'activity' that would happen in their wines stored in barrel during the warm spring months following harvest. Like primary alcoholic fermentation, this phenomenon would release carbon dioxide gas and seem to have a profound change on the wine that was not always welcomed. It was described as a 'second fermentation' in 1837 by the German enologist Freiherr von Babo and the cause for increased turbidity in the wine. Von Babo encouraged winemakers to quickly respond at the first sight of this activity by racking the wine into a new barrel, adding sulfur dioxide, and then following up with another set of racking and sulfuring to stabilize the wine. In 1866, Louis Pasteur, one of the pioneers of modern microbiology, isolated the first bacteria from wine and determined that all bacteria in wine were a cause for wine spoilage. While Pasteur did notice an acid reduction in wine with the lactic bacteria, he did not link that process to a consumption of malic acid by the bacteria, but rather assumed it was just tartrate precipitation. In 1891, the Swiss enologist Hermann Müller theorized that bacteria may be the cause of this reduction. With the aid of peers, Müller explained his theory of 'biological deacidication' in 1913 to be caused by wine bacterium Bacterium gracile. In the 1930s, the French enologist Jean Ribéreau-Gayon published papers stating the benefits of this bacterial transformation in wine. During the 1950s, advances in enzymatic analysis allowed enologists to better understand the chemical processes behind malolactic fermentation. Émile Peynaud furthered enology understanding of the process and soon cultured stock of beneficial lactic acid bacteria was available for winemakers to use. The primary role of malolactic fermentation is to deacidify wine. It can also affect the sensory aspects of a wine, making the mouthfeel seem smoother and adding potential complexity in the flavor and aroma of the wine. For these other reasons, most red wines throughout the world (as well as many sparkling wines and nearly 20% of the world's white wines) today go through malolactic fermentation.