The primary reaction is the transformation of malic acid into lactic acid. This leads to a change in the pH which in turn modifies the colour. The colour intensity decreases and the red colour is reduced due to a partial decolouration of the anthocyanins.
The increase in pH also improves the wine's taste profile. Lactic acid is "gentler" on the palate than malic acid. The wine appears rounder and softer.
There is also a partial transformation of the citric acid into acetic acid, which causes the volatile acidity to increase. Diacetyl also forms. At low doses, this contributes to the complexity of the aromatic bouquet, but it can lead to a buttery smell that tends to mask the wine's fruitiness. This molecule can be broken down into acetoin and butanediol which are much less aromatic.
There is a risk of the formation of biogenic amines. For example, the formation of histamine from histidine via decarboxylation. This is a toxic substance for humans that is only produced by certain strains of bacteria. A maximum legal threshold exists in some countries such as Japan, Switzerland and Canada where its content must be lower than 10 mg/L.
During MLF in barrels, the oakiness is generally integrated more quickly. Some reactions allow interesting results to be obtained. Vanillin, for example, is transformed into vanillic alcohol by bacterial action. This alcohol has a higher perception threshold than vanillin. It is therefore more difficult to detect and the vanilla note is perceived as being better integrated. There is also a significant decrease in furfural (almond, toasted and caramel notes).
The resulting improved microbiological stability and taste profile make malolactic fermentation an essential stage in wine production.