Fermentation is the process in the course of which sap becomes wine. The yeasts are the workers of the real factory that is the fermenting must.

Yeasts mainly eat sugar and excrete alcohol (in optimal conditions, 1 kg sugar yields 0.6 L pure alcohol.

Besides ethanol, the yeasts make, in lesser amounts, a lot of other substances that will contribute to the wine bouquet: other alcohols (mainly glycerol whose presence, very desirable, will give the wine mellowness). Wines contain a little methanol [ Methanol, a very toxic alcohol, is always present, in small amount in a wine. Contrary to what is generally thought, it is not formed during the fermentation, but in the must, well before the fermentation starts, by hydrolysis of the fruits pectin by dint of an enzyme: pectin methylesterase present in the fruits. Very ripe fruits contain already some methanol (as well as ethanol). ]  as well.

In the past, the fermentation evolved spontaneously thanks to wild yeasts present on fruits skin. The process was uncertain, and bad surprises were not rare because, on the fruits skin, there are numerous yeasts and bacteria varieties, often adverse to the wine fermentation. It followed the production of disagreeable and even toxic substances called "wine illnesses".

Since some decades, the human genius succeeded in selecting various notable yeast strains:

The spontaneous metabolism is now thrown over in aid of "pitching", which presents numerous advantages. Yeast election is essential.

Besides the yeasts election, other factors can favourably act on the fermentation in order to obtain a successful wine. The yeasts have to be well fed. Besides nutritive salts, it was notices that some substances favourably act on the fermentation. Adding 20 mg/L of unsaturated fatty acids [ In a Pyrex receptacle (never aluminium!), prepare a solution of 100 mL water containing 20 g potassium hydroxide (KOH). Bring to a light boil and add bit by bit, while stirring 100 mL (90 g) soy oil. Continue the heating for half an hour while stirring. Add 15 mL alcohol, mix well and, when the mass is homogeneous, add water up to 430 mL (= 86 g acids, 0.2 g/mL).
Add 1 mL of that solution in 10 litres juice. ]
 stimulates the fermentation. Ergosterol, 5 mg/L dissolved in these fatty acids also stimulates the fermentation. All these products are quite natural (vitamines).

Since the apparition of lyophilized yeasts, the starter culture method "pied de cuve" is obsolete. The only thing to do nowadays is to let rehydrate the yeast for 15 minutes in 10 times its volume tepid (30-35 C) sweetened (5%) water, and pour it into the must. Fermentation starts rapidly.

Mean doses:

AirlockDo not forget to close the container by means of an airlock [ Water valve allowing the outflow of CO2 , but avoiding any air-intake. It is preferable to fill it with a mixture of water and glycerol. ]  filled with a mixture water/glycerol (glycerol does not evaporate).

Depending on the yeast strain, the culture conditions, the temperature, the fermentation can start between from 1-3 hours after seeding up to 2-3 days (lag phase). During this time, the yeasts spawn and consume the must oxygen (respiration).

The first fermentation phase, said "tumultuous phase" is characterized by a violent clearing of CO2 and an overheating of the must. It lasts a few days up to 15-20 days depending on the conditions. During this time, most sugar is converted into alcohol. Cold turkey, the yield of CO2 decreases and continues slowly. It is the "slow fermentation". It is short for dry wines poor in alcohol, and long lasting for sweet wines with a high alcoholic rate. This phase is sensitive to adverse conditions. The temperature must remain stable.

The yeasts and plant debris settle. An eventual sugar addition has to be made at the end of the tumultuous phase. When fermentation stops, pour in the airlock a dash of sodium metabisulfite. At the time no more CO2 escapes, shake the carboy. If the airlock remains silent for 2 days, the fermentation is finished. From now on, the wine will be good or bad, according to the care devoted to it.

Some factors positively influence the flavours yield in the wine. Beyond the choice of yeast strain, let us mention a low temperature and fermentation under pressure. This pressure can be obtained by fermenting in a pressure barrel with a valve, or using an airlock made up of two interlocked cylinders. Loading an object (coin...) on the top of the inner cylinder will create some pressure in the fermentation vessel.

A second ilk of fermentation can occur later on (sometimes several months later): malolactic fermentation.


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