It's All About Temperature
There are 3 types of temperature:
1. The air temperature
2. The liquid temperature
3. The killing temperature
Because yeast generates heat during fermentation, the liquid temperature will be much higher than the air temperature.
The difference between the two will increase as the volume you are fermenting increases. High temperatures will kill yeast. Where there is 11% alcohol, Temperature Tolerant Turbo Yeast dies at 42-43°C, but as the alcohol increases, this "killing temperature" decreases. At 14% alcohol, the killing temperature for Temperature Tolerant Turbo drops to 40-42°C and at 17% alcohol, down to 38.5°C. For normal yeast, lower the killing temperatures by 5-8°C.
Provided you keep the liquid temperature controlled all the way through fermentation, you will not kill the yeast. This is easy with volumes up to 25 liters because the difference between air and liquid temperatures is only a few degrees.
But it is not so easy to control the liquid temperature when fermenting larger volumes in a distillery. You either need to keep the heat generation down or cool the liquid.
There is another reason (for potable alcohol) to keep the liquid temperature below 30°C - to keep volatile production down to a minimum.
In fact, the lower the fermenting liquid temperature, the lower the volatiles. So you could say, "the cooler the better." However, in practice, the amount of volatiles produced at a very cool temperature like 15°C is not much less than at, say, 25°C.
However, there is a huge difference in fermentation time. At 25°C fermentation of 14% it will take 3 days, but at 15°C it will take nearly 2 weeks!
To keep down production of volatiles when fermenting alco base for soda pop, we recommend a liquid temperature of 25°C.
Killing temperatures in the fermenting liquid
The yeast will die at different temperatures depending on yeast strain and alcohol strength.
Temperature Tolerant Turbo aims at 14% alcohol, but is capable of producing more: 17%. It is no problem to achieve 16-17% in practice. In a laboratory the yeast ferments to 19.5% and sometimes higher.
Do not calculate with those killing temperatures for other brands of yeast, as our Turbos are extremely temperature tolerant and tolerate 5-8°C more heat then most other yeast.
Temperature control must be perfect. It is very hard to cool down fermentation mid-stream.