All about temperature
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There are 3 types of temperature we need to talk about;
1: The air temperature
2: The liquid temperature
3: The killing temperature
Because yeast generates heat during fermentation, the liquid temperature will be 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 no alcohol yeast dies at 40°C, but as the alcohol increases this "killing temperature" decreases. At 14% alcohol (which is what you get using 6 kg sugar in a 25 L volume), the killing temperature drops to 33°C and at 20% alcohol down to 24°C. 17 grams of sugar ferments to 1% of alcohol in 1 liter mash. The sugar must be completely dissolved to be able to ferment to alcohol.

Providing you keep the liquid temperature below 30°C all the way through fermentation (24°C for very high alcohol), 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 keep the liquid temperature below 30°C when fermenting larger volumes. You either need to keep the heat generation down or cool the liquid by, say, introducing frozen 5 liter water containers after about 12 hours into the fermentation. Black Label yeast and Prestige Batch yeast sachet has been designed with this situation in mind, it is "fully stackable" up to 200 liters so use 1 sachet for 25 liters, 2 for 50 liters etc up to 8 sachets for 200 liters. Above 200 liters you need to introduce cooling or use fewer sachets (e.g. 15 sachets for 600 liters).
You should now understand why it is important to keep the liquid temperature below 30°C. There is another reason 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.

But there is a huge difference in fermentation time. At 25°C fermentation of 6 kg / 25 liter will take 3 days, but at 15°C it will take nearly 2 weeks! So we go for 25°C.
To keep down production of volatiles a liquid temperature of 25°C is recommended.

Killing temperatures in the fermenting liquid
The yeast will die at different temperatures depending on yeast strain and alcohol strength.
Prestige Batch and Black Label Turbo. Those 6-kg Turbos aim at 14% alcohol, but are capable to producing more.
Do not calculate with those killing temperatures for other brands of Turbos, as our Turbos are extremely temperature tolerant.

11% 35°C
12% 34,5°C
13% 34°C
14% 33,5°C
15% 33°C
16% 29°C
16,5% 22°C

Prestige Alcohol Yeast, Distillers Yeast and Pot Distillers yeast.
Those 8-kg Turbos aim at 18% alcohol, but are capable to producing more.
Do not calculate with those killing temperatures for other brands of Turbos, as our Turbos are extremely temperature tolerant.

11% 35°C
12% 34,5°C
13% 34°C
14% 33,5°C
15% 33°C
16% 32°C
17% 31°C
18% 30°C
19% 27°C
20% 24°C
21% 18°C

Use the Hydrometer to check when fermentation is completed
Prestige Batch and Black Label Turbos:
Start gravity 1090 (if you have the Oechslescale this is +90)
Final Specific gravity is 984 – 987 (if you have the Oechslescale this is -13 to -16)
You will have approx. 14% alcohol in the mash.

Prestige 8 kg Turbo, Distillers Yeast and Pot Distillers Yeast
Start gravity 1120 (if you have the Oechslescale this is +120)
Final Specific gravity is 982 (if you have the Oechslescale this is -18)
You will have approx. 18% alcohol in the mash at 982, never less.
To aim for 20% alcohol you have to start with 8.5-kg sugar (8-_ kg) and final specific gravity will be 980 (if you have the Oechslescale this is -20). This take longer time and the liquid temperature must not go over 24°C.

Different ways to keep down the liquid temperature
In countries where the air temperature is 30°C and above, it is best not to use the 8-kg Turbos, but only the 6-kg. The same is for fermenting more then 25 liters of 18% - 20% mash. Otherwise the fermentation have to be cooled down by introducing frozen containers or a jacketed tank for cooling or cooling tubes in the mash. This is complicated but always works.

Another way is to reduce the heat from fermentation is to slow down the fermentation. To add 1/3 of the sugar in the start and 2/3 after 5-7 days is one way. To use half dosage of the Turbo (but never less) is another way (Pot Distillers Yeast is the best one for this purpose). The fermentation time will double or triple and it will not always work. The temperature has to be checked.