The malolactic fermentation also known as malolactic conversion is a wine making process in which malic acid is converted into lactic acid which reduces the tartness to softness as lactic acid tastes soft whereas malic acid tastes tart. Malic acid is naturally present in grape must. Malolactic fermentation is basically a decarboxylation process where carbon dioxide is liberated in the process. The fermentation is carried out by lactic acid bacteria family, including Lactobacillus and Oenococcus.
Malolactic fermentation enhances the flavor persistence of wine, producing softer wines. lactic acid has a mouth-feel “softness” taste in comparison to the often described “hard” and “metallic edged” taste of malic acid.
Malolactic fermentation is primarily done to decrease the acidity of wine. It reduces the tartness of alcohol. It converts diprotic malic acid to monoprotic lactic acid. Not only just it causes reduction in acidity of wine but it also increases aroma and flavour persistence of wine, producing wines of greater softness and less tartness.
Almost all red wines and some white wines like Chardonnay and Viognier undergo malolactic fermentation. Malolactic fermented wines have buttery or creamy taste. Malolactic wines are aged in oak barrels. It is a very convenient way to add texture and body to the wine without losing required flora and citrus aromas from wines.
Malolactic bacteria are very peculiar about their conditions to work at. They aren’t tolerant of high alcohol, high sulfur dioxide, low temperatures and low pH. High-acid grapes make it difficult to cultivate malolactic bacteria, in general, it will work in red wines with a pH of 3.3 or higher and in whites with a pH of 3.1 or above.
Malolactic bacteria are inoculated in the wine sample at the end of fermentation when the Brix counts are almost zero and not sugars are left in the sample or are completely fermented away, because if malolactic bacteria ferment in the presence of sugar and most of the nutrients have been gobbled up by yeast, the bacteria could degrade the sugar and create volatile acidity, and also we can see if there are any flaws in the sample before inoculation of the malolactic bacteria, for example if there are any off flavored substances are formed by yeast fermentation, so any problems that might have occurred can be rectified before adding the malolactic bacteria to the sample.
Warm temperatures between 60°F to 75°F are needed for malolactic fermentation. It gets necessary to keep checking if malic acid is getting converted to lactic acid or not, which can be done by simple test using the kits that are available.
Lactic acid bacteria
Lactic acid bacteria (LAB) is a group of Gram-positive, generally non-sporulating, non-respiring rod or cocci. A common metabolic characteristic is their ability to produce lactic acid as a major metabolic end product of carbohydrate fermentation and can grow at a lower pH range. This allows the lactic acid bacteria to outcompete other bacteria in natural fermentation, since they can withstand the increased acidity caused by the lactic acid production. All these bacteria grow anaerobically, but unlike most anaerobes, they grow in the presence of oxygen as “aerotolerant anaerobes”. Because they obtain energy from the metabolism of sugars, hence lactic acid bacteria are can only grow in the environments in which sugars are present.
Lactic acid bacteria produce lactic acid by metabolising sugars or L-Malic acid. Special of these lactic acid producing bacteria differ in the manner how they metabolise different carbohydrates i.e. glucose, fructose, pentose. Lactic acid bacteria also helps prevent the spoilage and pathogens or other unwanted and harmful bacteria by increasing acidity of the products. There can be two pathways through which the lactic acid is produced named homofermentative and heterofermentative pathways. Only one main end product is usually produced via homofermentative pathway, that is, lactate. Whereas multiple end products can be produced via heterofermentative pathway, that may include carbon dioxide, ethanol and acetate.