So yeast is a living organism which uses sugar to make energy for growth. If there is no oxygen around yeast cannot extract all the energy from sugar and throws out ethanol as a waste product. To function, yeast also needs amino acids, enzymes and minerals which collectively we call nutrients. As well as throwing out ethanol as a waste product, yeast throws out another 1300 other compounds which we can call "volatiles". These volatiles fall into chemical categories;
Higher alcohols (also called Fusel oils) Esters
All fermented alcoholic drinks contain these volatiles, whether made in the home or made commercially. Indeed, it is basically the amounts and types of these volatiles that make say dark Rum taste and smell like dark Rum, or that make whisky taste and smell like whisky. Now this is important to make clean, pure ethanol in the home we don't want these volatiles. This is why activated carbon is used after distillation, to remove these volatiles. But, even the best activated carbons will not remove a large amount of volatiles so it is important to try not to make them in th first place. The choice of yeast strain and nutrients have the greatest influence on keeping volatile production to a minimum. The only control you have here is to buy a good Turbo sachet. It is the Turbo manufacturers job to select the best yeast strains for the job and use the correct nutrition. However, the temperature you use through out fermentation, and the activated carbon used all influence volatile concentration.
All about temperature
There are two types of temperature we need to talk about;
1: The air temperature 2: The liquid 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 25°C. 17 grams of sugar ferments to 1% of alcohol in 1 liter mash. Providing you keep the liquid temperature below 30°C all the way through fermentation (25°C for very high alcohol) you will not kill the yeast. This is easy with volumes up to 25 Litres 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 L water containers after about 12 hours into the fermentation. Gold Turbo 200 sachet has been designed with this problem in mind, it is "fully stackable" up to 200 L so use 1 sachet for 25 L, 2 for 50 L etc up to 8 sachets for 200 L. Above 200 L you need to introduce cooling or use fewer sachets (eg 16 sachets for 600 L). 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 L will take 3 days but at 15°C it will take nearly 2 weeks!
To keep down production of volatiles a liquid temperature of 25°C is recommended.
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