Fact #1 – High temperatures at the start of a red wine fermentation can lead to lower yeast viability towards the end of fermentation.
Practical advice – Maintain temperatures between 20 – 25°C for the first third of fermentation during the exponential growth phase of the yeast. Avoid adding a high dosage of DAP that can cause a sudden heat peak. Only add DAP or a complete yeast nutrient towards the end of the first third of fermentation.
Fact #2 – The optimum temperature range for most Saccharomyces cerevisiae wine yeasts is 25 – 30°C. Ethanol toxicity for yeasts increases with increasing temperatures.
Practical advice – Never exceed 30°C during a red wine fermentation. Towards the end of fermentation, after 10% alcohol is reached, lower the fermentation temperature to approximately 25°C.
Fact #3 – The cap temperature can be much higher than the temperature of the liquid (must) – up to 12°C warmer. There can also be up to an 8°C difference between the centre and the sides of the cap.
Practical advice – Always measure a red wine fermentation’s temperature after a pump-over. Do regular pump-overs or punch downs to avoid an excessive temperature gradient between the cap and the liquid.