4.1.8.b Alcoholic fermentation

What is the purpose of settling and how should it be managed?

The two fundamental tools for settling are:

• Temperature control

• Turbidity measurements using a nephelometer

Settling is a key step in white winemaking. The lees present in the must after pressing are made up of plant debris, earth, residues of phytosanitary products, etc. They can be a source of reduction, bitterness and herbaceous aromas.
Settling is all the more important when the harvest is compromised (rot), crushed or under-ripe.
The must also contains a lipid fraction (palmitic, oleic, linoleic acid, etc.) which can be assimilated by the yeasts in their membrane phospholipids. These lipids are necessary in a certain proportion.

However, a degree of turbidity must be maintained, since the lees act as a support for the yeast and provide certain desirable compounds, as previously mentioned. The lees have the capacity to absorb the fatty acids produced during alcoholic fermentation that can inhibit the yeasts' growth.
A deficiency in lees leads to acetic acid production. An excess leads to the production of methionol due to better absorption of its precursor, methionine.
The importance of precise settling quickly becomes apparent.

How do you decide when the settling process is finished?

There are several settling methods, the most traditional being a cold static settling for 12 to 48 hours. The colder the temperature, the faster the settling.
Turbidity measurement indicates if there has been sufficient settling. If the settling time is too long, it can be necessary to reincorporate the fine lees without incorporating any of the coarse lees that are stuck to the bottom of the vat.

Is it appropriate to use bentonite fining on the must?

Bentonite stabilizes proteins in the wine and thus prevents any instability regarding the wine's clarity.
In the cases discussed here, i.e. premium-quality white wines with long ageing on the lees, fining is not necessary. Ageing on the lees enables natural protein stabilization of the wine and protection against oxygen.

Should vinification be carried out in vats or barrels?

Some profiles are best aged in the barrel. During fermentation, yeasts can break down certain aromatic molecules in the wood. For example, yeast transforms vanillin into vanillic acid which is ten times less aromatic. Integration of oakiness is better when the fermentation is carried out in the barrel. This approach is very interesting for white wines where new barrels have a very significant impact on the aromatic profile.

Such cases mainly concern new barrels, as the effect is less significant with used barrels. A method that enables homogeneous fermentation is to start the alcoholic fermentation in the vat and then transfer the wine to barrels when it is a third of the way through alcoholic fermentation.
Another interesting point is that the transfer is naturally protected against oxygen when it is done during alcoholic fermentation.

However, when the goal is to preserve the maximum amount of thiol aromas, vinification in a more reductive environment, such as a stainless-steel vat, is much more appropriate. It is possible to divide the batch with a proportion of the wine in stainless-steel vats for the subsequent ageing to achieve a good balance in the finished wine. Vinification in vats of all or part of the batch is then appropriate.

What is the target temperature for fermentation?

As mentioned above, an ideal fermentation temperature for white wines is between 16 and 22°C.
At the organoleptic level, alcoholic fermentation temperature is particularly influential for white wines.
In the case of Sauvignon Blanc, whose aromatic typicity is largely due to the varietal thiols (4MMP and 3MH), the fermentation temperature has a major impact on these molecules. To obtain a maximum of these molecules, a temperature of approximately 18°C is to be preferred.
Below this temperature, the ester/acetate type aromas characteristic of wines with a fermentative profile are favoured. These aromas are rather unstable in the long term and concern mainly wines with short ageing potential. One well-known example is isoamyl acetate, responsible for a banana smell.
At high temperatures, aromatic complexity is generally lost and rather heavy wines are obtained because of higher alcohol production, sometimes with a more oxidative profile.

Is it necessary to provide oxygen during fermentation?

The risks are identical to the risks for red wine fermentation. An aerated pumping-over or micro-oxygenation should be carried out when the density has fallen approximately fifteen points. A second aeration can be considered after three days of alcoholic fermentation.

What can be done if ethanal is present?

Ethanal occurs when the wine has been over-oxygenated. The yeasts also contribute to the production of ethanal during alcoholic fermentation.
If its levels become too high, which is very rare, malolactic fermentation can break down the ethanal. However, this fermentation is not always desirable for certain white wines.
Sulphur dioxide also combines with ethanal but it is not the ideal solution because the combined SO2 has no effect on the yeast. The doses required will therefore be high. It is better to conduct a malolactic fermentation, even if it is only partial.

Is it necessary to rack when alcoholic fermentation is finished?

Racking is not really necessary, especially if the fermentation was carried out in barrels. Maturing on fine lees is often very beneficial for white wines. See the chapter on maturing white wines.
On the other hand, if fermentation is carried out with significant turbidity, racking is recommended to avoid reduction. However, the fine lees should be kept.