Spanish researchers from Argentina and Spain evaluated the effect of cold and hot temperature shocks on fermenting yeasts. These temperature shocks are usually referred to as thermal shocks in scientific literature and the “how to use guidelines” of commercial yeast companies. The researchers also looked at the effect that DAP addition, before the thermal shocks, had on the recovery of the yeast after such a shock.
What they did
- The researchers exposed two different yeasts to cold shocks and three different yeasts to heat shocks in small scale fermentations conducted in synthetic grape juice. The cold shocks comprised abruptly lowering fermentation temperature to either 9°C or 1.5°C for 16 hours; two, six, 10 or 14 days after the start of fermentation. A daily cold shock variable was also included where the temperature was lowered to 1.5°C every night and allowed to return to room temperature (26°C) during the day.
- Three yeasts were exposed to three heat shock treatments of 32°C, 36°C and 40°C (from 26°C) for 16 hours on day three of fermentation. These fermentations were carried out with or without 20 g/hl DAP additions three hours before the heat shock.
What they found
- They found that cold shocks did not ultimately alter the yeast’s fermentation performance. It did slow down the fermentation momentarily, but when the temperature returned to normal, the yeast viability (to stay alive) and vitality (to function at a certain speed) was unchanged.
- However, shock temperatures of 36°C and 40°C applied in the early stages of fermentation lead to sluggish fermentations in all three yeasts tested. Yeast viability and vitality were severely affected. The three yeasts differed in how severely they were affected by the heat shocks indicating some yeasts are more resilient than others. Having said this, it is important to remember that yeast is exposed to various stresses during fermentation such as low pH, high sugar (osmotic) stress, low nutrient status and rising alcohol levels. All these factors will have an influence on yeast viability and vitality.
- The researchers did find that the addition of DAP before the heat shock allowed for some level of recovery after the heat shock compared to the control treatments, with the emphasis on “some.” Fermentations still took very long to complete and in some cases up to 40 days, which is a completely impractical situation in a harvest cellar.
Lessons to learn from this research:
- It is advised to follow suppliers’ instructions on fermentation temperatures. Their advice is based on years of solid research obtaining similar findings as this current article. Moreover, their advice is based on practical experience of dealing with the sluggish fermentations of their clients.
- Constantly measure (especially) red wine fermentation temperatures. Measure the tank as well as the cap temperature. Cap temperature is always warmer than must temperature. If you do a pump-over over the cap, your yeasts get exposed to those higher temperatures, negatively affecting their viability. More regular pump-overs and lower must temperatures (≤25°C) can help to regulate cap temperatures.
- If you want to extract more colour and tannin (favoured by high fermentation temperatures), use red wine extraction enzymes.
A fact to remember:
Grape must temperature increases 1.3°C for every 100 g of sugar consumed by the yeasts during fermentation as a result of the heat generated by the yeasts. If not properly controlled by an effective cooling system, fermentations will very rapidly self-destruct since yeasts start dying above 32°C in the presence of alcohol.
Reference:
A.S. Vargas-Trinidad, M.C. Lerena, J. Alonso-del-Real, B. Esteve-Zarzoso, L.A. Mercado, A. Mas, A. Querol, M. Combina (2020). Effect of transient thermal shocks on alcoholic fermentation performance. International Journal of Food Microbiology, Vol. 312, 108362, https://doi.org/10.1016/j.ijfoodmicro.2019.108362.
