Managing the alcoholic fermentation

Temperature Control

Temperature control is vital in the production of fine table wines as:

  • High temperatures encourage the loss of alcohol and aroma and flavour compounds due to volatilisation. If the temperature goes above 30 -35ºC, the yeast becomes sluggish and fermentation may stop.
  • Low temperatures will lead to a poor extraction of colour and tannins in red wines and can also cause sluggish fermentations and the production of high levels of ethyl acetate.

Fermentations tend to overheat as the alcoholic fermentation generates a lot of heat. In fact, if a tank was well insulated, each percent of sugar (1º Brix) in the must generates enough heat during fermentation to raise its temperature by 1.3ºC. Much of this heat is in fact lost through the expulsion of carbon dioxide and through the tank walls, but it is still very useful to have temperature control equipment in a winery, or even to be able to control the temperature in the winery itself. Often, chilling the must after pressing will be enough to cause the desired effect.

For white wines, the temperature can be adjusted to obtain a uniform Brix reduction of 1 -2º per day; roughly corresponding to a fermentation temperature of 12 - 16ºC. For red wines a reduction of 2 -4º Brix per day is achieved at temperatures between 18 - 24ºC.

Risk of carbon dioxide poisoning

Carbon dioxide, generated by the alcoholic fermentation collects in the winery, in tanks, low cellars and in poorly ventilated rooms. It is colourless and odourless, but potentially lethal, as it is poisonous and displaces the air that contains the oxygen that we need. It is important to note that CO2 can accumulate, so it is possible to go into a winery, bend down to pick up a hose, hit the carbon dioxide layer and eventually die. Good ventilation is very important; as are precautions such as never entering a closed tank without measuring for oxygen level (lighted candles are not enough!). Large wineries should have alarms to indicate low oxygen levels.


Winemakers monitor the fermentation temperature and the changes in relative density in their fermenting tanks on a daily basis in order to be able to predict unsuitable temperatures and stuck fermentations.The temperature curve normally broadly follows the yeast cell numbers. However, there are occasions when fermentation does not run smoothly:

Stuck Fermentations

These can be prevented by:

  • Aerating the must: ADY are grown in highly aerated media, so they do not need much oxygen, but if using naturally-occurring yeasts, pumping a good proportion (1/3 – ½) of the juice over onto the surface (with splashing) will help yeast numbers get high enough in the initial stages to prevent stuck ferments.
  • Adding yeast nutrient. May be useful to add DAP at 10 – 15 g/hl or thiamine at 50 mg/hl.
  • Controlling the temperature so that it does not go too high, too low or change rapidly.

If a fermentation does stop prematurely there are a number of techniques that can be employed to re-start the fermentation :

  • Add sulphur dioxide, DAP and thiamine
  • If red, drain and press
  • Adjust the temperature to around 20ºC
  • Re-inoculate with a strain of yeast adapted for difficult conditions , making sure that the yeast is introduced to the wine gradually, by starting the culture in water and then doubling its volume with wine periodically.

Finishing the fermentation

The relative density of the wine is measured daily to monitor the alcoholic fermentation. When it drops below 1, sugar levels can be measured to check if the alcoholic fermentation is complete. This can be done by a Fehling’s (Lane & Eynon) or Rebelein test, but CLINITEST strips or tablets will work with reasonable accuracy. Note that there are up to 2 g/l of unfermentable sugars in wine.

At this point, the tanks are normally filled to the brim and the wine is cooled or allowed to cool. There can be an extended lees contact stage at this point with lees stirring to increase the wine’s body and complexity. Proteins are also stabilised and the malo-lactic fermentation (MLF) occurs more easily. If stirred the yeast in the lees scavenge the oxygen that dissolves in the wine, thus negating the need for sulphiting. Stirring will also reduce the risk of the yeasts releasing hydrogen sulphide.

If extended lees contact or the MLF is not desired, the wine is racked off the lees and sulphited to 40 – 100 mg/l. From this point on, the wine must be protected by 10 – 40 mg/l of free sulphur dioxide right up until bottling.

In off-dry white wines, the sweetness may be the result of finishing a fermentation early or of back-blending unfermented grape juice (sweetening with sugar is only permitted for sparkling wines). In order to stop a fermentation before it reached dryness:

  • Chill the wine (preferably below 5ºC)
  • Rack and filter to remove the yeasts
  • Add sulphur dioxide to produce a free level of around 50 mg/l.

Alternatively, the fermentation of sweet fortified wines, such as Port and Vin Doux Naturels, can be stopped by adding alcohol so that the actual alcohol level goes above 15%.