Malolactic fermentation & lactic bacteria
(based on Emile Peynaud’s Connaissance et Travail du Vin)
Also a useful scientific paper available here
What is malolactic fermentation?
After draining off the wine from the vat in which alcoholic fermentation takes place, a second 'finishing' phase takes place, a slower fermentation 'd'achevement' 'de finition'. Carried out correctly, malolactic fermentation softens and rounds out the wine, and increases microbiological stability. If it is not carried out (or if more harmful bacteria go to work on the wine), the wine can become hard, tough and with overly high Total Acidity.
It's a fermentation of malic acid by lactic bacteria rather than yeasts. Think of it also as a chemical de-acidification that breaks down the malic acid. Characteristics of 'mouelleux'/round mouthfeel are associated with lower acidity. For fine wines, malolactic fermentation can be seen as the first step of qualitative ageing and stability.
The presence of lactic acids in wine was first observed over a century ago, but it's only been 50 years since the recognition that the fermentation of malic acid could be both useful and controlled (because often it happened spontaneously at the end of alcoholic fermentation before ecoulage if the cuvaison is prolonged and so went unnoticed). This was discovered by Peynaud in 1956
Bacteria usually become visible very early on during alcoholic fermentation (after a few days).This is not the beginning of an infection, as was previously thought, but these are malolactic bacteria which are beginning to develop. In some years (or in certain winemaking estates who make this choice) malo takes place slowly over winter if it's mild enough to allow the wine to remain fizzy (they seem to be fermenting all winter even though they no longer contain any sugar). In other cases it happens as temperatures warm up in spring, even summer. The rise in temperature allows the bacteria to be 'activated' if there is malic acid in the wine still.
In northern regions particularly (septentrionales in french) grapes retain high levels of malic acid even after fermentation and can be almost undrinkable without this malolactic fermentation. They can regularly keep 8-10g of fixed acidity without it!
Usually during alcoholic fermentation totaly acidity drops by between 1.5 and 3 grams per litrqe. So maybe 6.5g/l (sulfuric acid) to 3.8. This is not due simply to a tartric precipitation (as used to be believed), but the disappearance of malic acid. At the same time the quantity of lactic acid increases.
Global reaction = malic acid (1g) becomes lactic acid (0.67g) + carbonic acid (0.33g or 165cm3)
The carbonic gas produced escapes and the wine becomes fizzy. This is the direct reason for the lessening of overall acidity (it is 'escaping' through the fizzy carbonic acid). Malic acid has two functions while lactic acid only has one. By transforming malic acid into lactic acid, about half of the total acidity is lost because it has transformed into volatile carbonic acid, which then disengages from the wine, turning into bubbles of carbonic gas, and disappears into the atmosphere.
At the same time, there is a slight increase in volatile acidity, of around 0.1-0.2g/l. This is partly due to the lactic bacteria attacking traces of sugar and citric acid (most usually by the coques bacteria) at the same time as they are attacking the malic acid. Because of this, it is not advised to add citruc acid during vinification.
The higher the levels of malic acid at the start, the greater the gustatory impact of this secondary fermentation. The improvement to the taste and mouthfeel comes because there is an overall lowering of total acisity, and because the malic acid, which has a markedly aggressive taste, is replaced by lactic acid, which is far less aggressive to your tastebuds. Remember after the transformation 1g of malic acid gives around 0,67g of lactic acid.
The colour of the wine will often become a little softer and rounder also after the secondary fermentation. The aromas will also have been modified, getting further away from the colours of the grape and closer to more nuanced colour of 'wine'. The texture will become more fleshy and rounder in red wines.
It is less easy to generalise with white wines. Not all regions find this secondary fermentation desirable or necessary.
In dry whites, even when it's possible, it does not always lead to an improvement in quality. White wines are primarily judged on their aroma and on the delicate balance between alcohol and acidity. The freshness given by acidity is essential in some dry whites.many white wines are able to take up to 4.2-4.5g total acidity by keeping their malic acid.
In other regions, or depending on vintage, it can be useful. Regularly carried out in Champagne, Burgundy, Switzerland, Maconnais. In Germany and Austria, they prefer usually to use a chemical disacidification. In regions where acidity is naturally low, malo would leave the wines flabby. It's a question of grape varieties, and personal taste. Some varieties lose their aromatic typicity if undergo a fermentation by lactic bacteria, and lactic notes can dominate the fruit. In this case, better to sulphite the must to stop any secondary fermentation taking place (or do it on only a small part). Varieties that are very rich in natural aroma, such as chardonnay, find that malo just adds another layer of aromatic complexity (as with red wines).
So red wines basically all need malo, whites and rosés only under caution. Generally, if you are looking for freshness and primary aromas only, better to avoid it. But if looking for vinosity and volume, possibly a good idea.
For sweet wines, they are often sulfited early on in the process, before malo can take place. They do sometimes happen, but they are not wanted becausethey can lead to a rise in volatile acidity, which is already a risk in sweet wines.
The Bacteria of Malolactic Fermentation
The fermentation of malic acid is prompted by the development of lactic bacteria (cells that are able to turn sugar into lactic acid in the same way that yeast is able to turn sugar into alcohol). These bacteria can take various shapes, mainly falling into coques/cocci (round or oval), bacilles/batonnets (long & thin)
Streptocoques (streptococci, think Guys and Dolls Adelaide singing about developing a cold - knew that song would come in useful throughout my life!)
Often early in fermentation, it is tough to distinguish between the 'good lactic bacteria' that help with malolactic fermentation, and those lactic bacteria that will lead potentially to wine spoilage. And certain conditions (such as residual sugar or high pH/low acidity) can also make these lactic bacteria more likely to be harmful. Generally, the higher the pH, the more starins of lactic bacteria are likely to be present -sometimes up to 8 or 9 strains at moment of harvest. But not all will be capable of developing in the specific pH of grape must, or in presence of alcohol, or in competition with the yeasts etc. Yeasts find the sugar etc present in must easier to cope with, which is why they establish themselves more quickly than lactic bacteria. It's when they start to diminish that lactic acid bacteria take hold.
Some are homo-fermentary (so do one thing - turn sugar into lactic acid) and hetero-fermenatary (so alongside lactic acid also produce alcohol acetic acid, succinic acid, glycerol, mannitol, carbonic gas etc).
In cold regions, cocci are more prevalent than bacillus and it hot regions its the opposite.
Wine lactic bacteria are either:
1) Leuconostoc (cocques heterofermentary)
2) Pediococcus (coques homofermentary)
3) Lactobacillus (bacilles can be both homo and heterofermentary) - and most usually these contain the harmful bacteria also
4) Oenococcus (only classified in 1995)
This classification is important because the malo will work differently, and give different results, depending on which strain if wine bacteria is present.
The lactic bacteria are found on ripe grapes at harvest, mised up with yeast, acetic acid bacteria and molds. Fairly irregular year to year. Probably also lactic bacteria in the buckets, pails etc used for harvesting and storing wine. Have to be v careful as cross-contamination possible at various stages of winemaking. Can also live in 'used barrels'. Equally a new cellar can make malolactic fermentation difficult to start.
A young wine at the end of fermentation almost invariably has lactic acid bacteria capable of breaking down malic acid if the wine is not sulfited (homofermentary). Just a few contain lactic bacteria that is capable of also breaking down tartaric acid, glycerol, raising volatile acidity etc (heterofermentary). The population is greater in young wine than in the grape must.
Useful and Harmful Lactic Bacteria
Once malolactic fermentation has taken place, the principles of 'bacteria are harmful' once again take precedence. Bacterial difficulties don't have to go as far 'piqure lactique' or the 'tourne' where all tartaric acid is consumed - more usually there effect is to raise the levels of acetic acid/volatile acidity and sometimes fixed acidity. The effect of this is a wine that seems dry of thin, lacking in the qualities that they should have, even if they don't seem especially 'faulty'. The harmful lactic bacteria are primarily in the lactobacillus genus.
So, there can be two types of lactic bacteria in wine:
1) Bacteria capable of decomposing malic acid primarily, and sugars secondarily, and sometimes citric acid, but not tartaric acid or glycerol. These are the normal agents of malolactic fermentation. They are not dangerous except in stuck fermentations, when they can cause alterations such as volatile acidity - if there is residual sugar and/or high pH, if in non-sulphured environment.
2) Bacteria capable of composing pentose, tartaric acid, glycerol, which are more unstable and potentially more harmful. These are the harmful bacteria.
The 'ideal' lactic bacteria would only decompose malic acid and not touch the other components of wine - this bacteria doesn't exist, but there are 'degrees of danger' according to the genus of bacteria, and the conditions of vinification, from pH to residual sugar.