Friday, January 7, 2011

Malolactic fermentation

Winter is the time when vignerons world wide kick back and reflect on their years work and what is to come for the coming season. Very important work is being done in the vineyards as last years growth is being pruned and the vines are being set up for the coming season. Also there are some fascinating developments occuring very slowly in the winery as the red wines are undergoing Malolactic fermentation.

What is Malolactic fermentation (MLF)?
MLF is the conversion of Malic acid to Lactic acid by Lactic bacteria. This conversion will occur in all wines but is stopped in most white wines as it adds undesirable flavours and alters the acid balance of wines.In red wines it is essential that all malic acid is converted to lactic acid. Otherwise this conversion could continue in bottle and spoil the wine. The only truely effective way to stop MLF is with Sulphur dioxide SO².

How is MLF initiated?
MLF will occur spontaneously as there is always a population of Lactic bacteria in wine. However the process can be helped by the addition of a bacterial culture. This can be added at the same time as an yeast culture is added. However this can cause an increase in acetic acid, which can impede primary fermentation, as the Lactic bacteria chews up the sugar in the must. The most common timing for this addition is at the copmpletion of primary fermentation. At this stage the inhibiting effects of any SO² adds have been reduced and there is only a tiny amount of sugar left that will not cause any acetic acid problems. Critically the temperature of the wine is generally quite warm at this stage which is necessary for the bacteria to work. The third method is allow the bacteria to occur naturally. This is the method that we employ at Domaine la Sarabande but it is crucial that the conditions are correct for a clean MLF to occur.

The optimum conditions for MLF?
There are three types of bacteria that are most common in MLF. Leuconostoc is the most favourable and is the main bacteria in any freeze dried cultures. Pediococcus and Lactobacillus are considered spoilage bacteria but are unfortunately quite common in winemaking. At the completion of primary ferment there is a tiny amount of unfermentable sugars (pentoses, arabinose and xylose) plus residual fermentable sugars ( glucose and fructose). Lactic bacteria can live on these tiny amounts of fermentable sugars. It is also important that the pH of the medium is suitable low (3.30-3.40) so that Leuconostoc can dominate the MLF. The other important factors that enable a clean MLF include temperature (+18°C), keeping SO² additions low at crush and by not overly clarifying or racking the medium.

What can go wrong?
Leuconostoc dominates at lower pH but is slower working than the other spoilage Lactic bacterias. This means nthat MLF can take several months. Therefore it is important to have vessels full so that Oxygen cannot oxidise or feed other spoilage bacterias. However if the wine has a high ph (3.50+) then you are in a world of spoilage bacteria pain! At best Pediococcus and Lactobacillus will form high levels of diacetyl. This is perceived as buttery but can also be like sour milk and is very unpleasant. Spoilage bacteria will also cause a condition which the French call Graisse or Ropiness in English. The wine will appear gassy and thick in the glass with ropy thick legs in the glass. Tourne is the French expression that is the fermentation of Tartaric acid by Lactic bacteria. This is serious spoilage as the acid is what keeps wine stable. Loss of tartaric acid can turn wines brown and open to numerous other spoilage. Wines can go cloudy and develop a mousy aroma. Again this happens in high pH wines especially if no SO² is added in the wines life. This can be quite a common fault in 'natural wines'.

It is the winemakers role to ensure that wines get to bottle unspoilt. Malolactic fermentation is a time when numerous things can go wrong. It is importatnt to have the correct conditions so that a clean MLF can occur. This is mostly down to having a stable pH at the end of primary fermentation. Easier said than done and dependent on to many factors to get into here! It can be a long winter until the wines are through MLF and SO² additions can be made. Patience is the key!!

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