Science of Fermentation
The Science of Fermentation
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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. But know how is the base for improvements. If you read this whole document, you will know even more then most home brew shop owners (except those who also have read it)!
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 alcoholâs (also called Fusel oils)
- Carbonyl compounds
- Organic acids
- Sulphur compounds.
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 makes 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 from a well-respected manufacturer. 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.