Ensuring stability in malolactic fermentation


(based on Emile Peynaud’s Connaissance et Travail du Vin)


In modern oenology, the basic principle is that a red wine (and white where applicable) is not complete and stable until both alcoholic and malolactic fermentations have finished.

- the sugars need to have been fermented by the yeasts and the malic acid by the bacteria, without allowing the bacteria to attack the sugars or other constituents of the wine along the way.

- only when sugars and malic acid have gone can a wine be considered biologically stable. This is why it's useful to rack, to use (judicious amounts of) sulphur, to filter and fine, and even in certain circumstances to flash heat.

- it is always preferable that sugars and malic acid disappear as quickly as possible, and therefore to reduce the potential moments of dangerous exposure to either yeasts or bacteria, or both, that can alter the wine. The risks are even higher in the absence of free SO2 (and often sulphur is not added until after malo, except perhaps for small amounts when the grapes are first brought in to the cellars).

- nb Denis Dubourdieu and most of the professors at the faculty of oenology do not believe in co-innoculation, as they feel yeasts and then bacteria should be allowed to go quickly and efficiently about their work, they feel doing them both at the same time runs the risk of drawing out both processes, and that it can dampen the aromas of a wine. Esteemed oenologists such as Eric Boissenot disagree, so it's a difficult call!. Dubourdieu etc believe malo should happen immediately after ecoulage (so when fermentation has finished and the wine is run off the vat).

Plenty of winemakers choose to wait to let malo happen naturally in spring. Dubourdieu would say to them that they are running an unnecessary risk. 'There are no opinions in oenology, only facts'!

Factors affecting smooth malo:
1) pH
2) sulphur
3) temperature
4) aeration

As soon as the grapes go into the vat, lactic bacteria begin to develop, in line with yeasts. But as alcohol forms with the alcoholic fermentation (AF), the yeasts are better adapted to survive and reproduce, and the lactic bacteria begin to recede and enter a 'latency period'. After AF, a variable quantity of live lactic bacteria remain, and malo should take place a few days to a few weeks later, unless either sulphur added, or temperature drops, in which case it could hold off for a few months.

However long it lasts, the bacteria will start multiplying again once the latent phase is over, and the fermentation of the malic acid will now begin. Lactic acid formation is very low in the first phase - needs to reach a certain density of cells (1 million per cm3) for the fermentation of the malic acid to really get underway. Things then move quickly, until all malic acid has gone, and a certain amount of bacteria remains, depending on pH, sulphur, clarification, filtration, ageing methods etc.

Influence of pH

The 'real acidity/power of acidity' is measured in the pH of a wine. The pH has a double effect on malo -
1) influences the species of bacteria that thrive
2) impacts upon which constituents of the wine are likely to be broken down by the lactic bacteria.

A low pH (high acidity) can make malo difficult, because less species of lactic bacteria are able to survive. Ideal pH for multiplying bacteria is 4.2-4.5, far above the (usual) pH of wine. Between 3-4 is good, and a pH below 2.9 is usually seen as too low for malo to take place (ironically, as these highly acidic wines could do with it). So, the higher the acidity, the less ripe the grapes, the more difficult it is to achieve malo (but the more likely that malo will be pure). Conversely, it's easier to achieve when acidity is lower , but there is more likelihood that bacterial spoilage will occur.

Bacterial spoilage is more likely when acidity is low. Often the 'better' wines, smooth and round, are sometimes subject to bacterial accidents. This is one of the challenges of successful ageing of wines. The most approachable are often fragile at the same time. On oenology there is often a balance to be considered between techniques that will make the best quality and those that will assure maximum security for the wines.

So if deacidfying a wine to raise pH, do it only on a small part and think of it only to set the ball rolling on the natural deacidification of malo. 

Influence of Temperature


Well known that warmer temperatures make malo quicker and easier. But not as high as alcoholic fermentation. Best 20-25 degrees, slower but possible at 15 degrees. Almost impossible above 30 degrees. Once it has got started, it's possible to continue very slowly at 10 degrees or more (why wine can bubble through a mild winter naturally). Best not to have temperature too high (18-20 is good), as side products such as acetic acid are more likely to be formed at higher temperatures.

Influence of aeration

A small aeration is often favourable to the development of the lactic bacteria. A huge amount of oxygen will slow it down, but not stop it entirely = oxygen is not a key determining factor (and different species of bacteria have different requirements)

Nutrition requirements

These are higher for bacteria than yeasts - specifically they need amino acids (vitamin B) - usually plenty in the wine, but sometimes an addition can activate them. They need 4 vitamins and 18 amino acids! Plus minerals - magnesium, manganese and potassium.

Influence of alcoholic degree

Wine bacteria are necessarily resistant to alcohol, but higher alcohol makes things more difficult. Often find malo starts first in the lowest alcohol vats in the winery. Coques are more sensitive to alcohol than bacilles - so sometimes in high alcohol sweet wine the lactobacillus survive, but bring about piqure lactique.

Influence of sulphur

The antiseptic qualities of sulphur strongly inhibit lactic bacteria (more than wine yeasts). And lower pH it has more affect (so in hot southern regions where pH is naturally highg, it can withstand a lot more sulphur without stopping the malo).

But, as soon as malo has successfully finished (has to be tested to check levels of malic acid), it is recommended to add sulphur to block any further bacterial activity.