So what makes great wine so special? One could argue that the answer simply lies in its complexity. Few beverages if any, can match the number and diversity of characters that meld to give the pleasurable sensory experience of outstanding wine. Some of these characters emanate from the grape itself, but the grape can contribute only so much. Fortunately winemakers are armoured with a variety of options that can be used to build complexity into the relatively bland canvas that is a newly fermented wine. One well-known option to achieve added complexity is to ferment and/or age the wine in oak cooperage. The other less known but equally important option is that of malolactic fermentation.

Malolactic fermentation is commonly referred to as "MLF", or (in winemaker's speak as) "malo" (pronounced may-low). So if MLF is a type of fermentation, what ferments, what does the fermenting, and most importantly, what sort of changes does MLF make to the final sensory quality of the wine? MLF usually occurs shortly after the end of the primary fermentation (when the grape sugar is converted to alcohol by yeast). It is undertaken by the family of lactic acid bacteria (LAB); Oenococcus oeni, and various species of Lactobacillus and Pediococcus. 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 conversion is accompanied by the production of carbon dioxide (so hence the term, fermentation). Lactic acid tastes markedly less sour than malic acid. In addition lactic acid has a mouthfeel "softness" about it in comparison to the oft described "hard" and "metallic edged" malic acid. In short, MLF results in a natural de- acidification and softening of the wine's palate. Grapes produced in cool regions tend to be high in acidity much of which comes from the contribution of malic acid. For wines produced from such grapes, de-acidification via MLF is particularly useful as it results in a more balanced and palatable wine.

Although acid reduction is the most obvious result of the growth of lactic acid bacteria in wine, their action can also significantly modify the wine's aroma, flavour and mouthfeel. These changes may be either good, bad or positively ugly depending to a large extent on which of the lactic acid bacteria dominates the MLF. Some of the Lactobacillus species have been implicated in the production of fetid milk, sauerkraut and sweaty characters. Whilst many high quality Old World wines are characterised and complexed by lactic nuances such as these, when dominant they are rather unpleasant. Some forms of Lactobacillus are also responsible for the production of "mousy taint" which is arguably the most unpleasant of all wine faults. Oenococcus oeni on the other hand is a far more desirable LAB as it typically produces substances that have pleasant and wine sympathetic aromas and flavours. Diacetyl is the most important of these substances, as it provides the most recognisable and characteristic of all MLF characters; butteryness. However, when in excess, diacetyl imparts strong caramel and rancid butter like characters, which can easily dominate the wine. Luckily, the more oenologically desirable Oenococcus oeni generally dominates the MLF as it has a greater tolerance to the high acid and high alcohol environ- ment of wine than the other lactic acid bacteria.

MLF is also thought to generally enhance the body and flavour persistence of wine, producing wines of greater palate softness and roundness. Many winemakers also feel that better integration of fruit and oak character can be achieved if MLF occurs during the time the wine is in barrel.

Wines that typically undergo, and are improved by MLF, are the full-bodied dry whites and medium to full bodied dry reds. But it must be stressed that not all wines benefit from MLF. Rieslings are a classic case in point. As a general rule, the quality of lighter bodied fruit driven wines that require crisp acidity are reduced by the action of MLF. The growth of all LAB are inhibited by cool temperatures and the anti-microbial agent, sulfur dioxide (SO₂). Winemakers are therefore able to arrest the onset of MLF when making these styles by maintaining both low temperatures and reasonable SO₂ levels during winemaking and subsequent bottling.

There is also a major practical reason why MLF is encouraged during the making of many wines, and in particular reds wines that have previously undergone malo in tank or barrel are far less likely to go through malo when in bottle. The onset of MLF in the bottle is disastrous as the wine will appear to the consumer to still be fermenting (as a result of CO₂ being produced). The wine may also lose its fruit integrity and take on the unpleasant lactic aroma of cured meats.

So next time you open that bottle of Chardonnay, spare a thought for those marvellous critters that helped create that complex aroma and that round out and soften its palate. Cheers to these little creators of diversity.