Science of Fermentation, Part I
Turbo Yeast - Page 2 of 4
The Science of Fermentation
You don't need to understand the science of fermentation to make good spirits and liqueurs in the home unless you want to experiment with the fermentation system, i.e., fermenting larger volumes or higher alcohol levels.
If you intend to stick to the instructions to the letter, (see later) then skip this section. Seeing fermentation from the yeast's perspective helps in understanding the science.
Yeast is a living organism very similar to the individual cells in our own bodies. It is easy to think of dried yeast as "just another ingredient" like nutrients or sugar. Nothing could be further from the truth.
Yeast's sole aim in life is to reproduce. It does this by " budding" to produce a daughter cell identical to the parent.
Given a plentiful supply of oxygen, sugar, minerals, enzymes and amino acids, it will reproduce itself every 30 minutes and one thus ends up with a bucket full of yeast! Take away the oxygen and you get much less growth and a bucket full of alcohol.
As far as the yeast is concerned, sugar (a sugar molecule) is a source of energy the yeast cell imports (eats).
Glucose has 6 carbon atoms joined together by chemical bonds. It breaks these bonds one by one, each time liberating energy which is then used for growth.
Without oxygen, it can only break one bond and so liberates only a little energy (also little growth). What's left is thrown out of the cell as a waste product: ethanol. So, if you want to make alcohol, keep the oxygen out!
To grow, yeast also needs amino acids, enzymes and minerals as well as the energy it extracts from sugar.These are needed to build new proteins (by creating bonds between amino acids) and carry out the many enzymatic reactions within the cell. A good Turbo sachet will contain all of these essential growth ingredients collectively we call these "yeast nutrients". If you have ever tried to ferment pure sugar with just yeast, you will know that you get very little alcohol, this is because yeast needs these other nutrients as well as sugar.
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)
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, for example, dark Rum taste and smell like dark Rum, or that make whisky taste and smell like whisky.
It is important to make clean, pure ethanol in the home. We don't want these volatiles. 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 the first place.
The choices of yeast strain and nutrients have the greatest influence on keeping volatile production to a minimum. The only control you have 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 situation 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 (e.g. 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.