Sugar in, alcohol out
Yeast is a single-celled fungus. Given sugar and no air, it splits each molecule of glucose into two of ethanol and two of carbon dioxide, and keeps a little energy for itself: C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂. On paper a gram of sugar gives about half a gram of alcohol; in practice a distillery gets roughly nine tenths of that, because yeast also grows and makes glycerol and other by-products.
The rest is not lost on the senses. Alongside ethanol, yeast makes small amounts of hundreds of other compounds: fruity esters, flowery and oily higher alcohols, sharp aldehydes. These congeners are what the still will later sort into heads, hearts and tails.
Why a wash is weak
Alcohol is a poison to the yeast that makes it, and distillers do not even try to push it. A malt whisky wash ends at about 8%, a tequila must at 4–7%, the wine for brandy at around 9%. Higher strengths would mean a slower, stressed and off-tasting ferment, and the still will concentrate the alcohol anyway.
Heat and gas
Fermentation gives off heat: a whisky washback set at about 19 °C can climb into the low thirties. It also gives off a great deal of gas, around 400 litres of carbon dioxide for every litre of alcohol made. In a wooden washback the head of foam can rise fast enough that a rotating arm, the switcher, is used to knock it down. The gas collects above the wash and can suffocate: washback rooms are ventilated and nobody leans in.
Where the sugar comes from
Yeast can only eat simple sugars. Fruit and cane give them freely; grain and agave keep their energy locked up and must be opened first.
| Raw material | Locked as | Opened by | For |
|---|---|---|---|
| Barley | starch | malting: the sprouted grain makes enzymes that turn starch into sugar in hot water (63–67 °C) | whisky |
| Corn, rye, wheat | starch | cooking, then barley malt or added enzymes | bourbon, vodka, grain whisky |
| Rice, sweet potato, sorghum | starch | a mould (koji in Japan, qu in China); starch is converted while yeast ferments | shochu, baijiu |
| Agave | inulin, a chain of fructose | cooking for hours (autoclave) to days (oven or pit) | tequila, mezcal |
| Grapes, other fruit | free sugar | nothing: crush and ferment | brandy, cognac |
| Sugar cane | free sugar | nothing: fresh juice, or molasses diluted with water | rum |
American distillers call their wash beer, and it nearly is one, only without hops. Brandy makers distil a wine. Either way the distiller wants a clean, sound ferment: a fault in the wash comes through the still, and the still can only sort, not repair.