Cooking science

Alcoholic fermentation

Yeast eating sugar produces carbon dioxide and ethanol in roughly equal measure. Bread keeps the gas and loses most of the alcohol; beer and wine keep the alcohol and let most of the gas go. It is the same reaction, harvested differently.

One reaction, two products

Yeast breaks sugar down in the absence of enough oxygen, and the products are carbon dioxide and ethanol. Both are made every time, in roughly equal molecular quantities.

What differs is which one you want. In bread the gas inflates the dough and is the point; the alcohol is a by-product, most of which evaporates during baking. In beer, wine and cider the alcohol is the point, and the gas is either allowed to escape or deliberately retained for fizz.

The third product is flavour. Fermentation generates a large number of aromatic compounds — esters, higher alcohols, acids — and those are much of why a long-fermented loaf tastes better than a fast one.

Where it appears in a kitchen

Bread

Gas for rise, and the flavour compounds from a long ferment.

Sourdough

Yeast alongside lactic acid bacteria, which is why it is sour as well as risen.

Beer, wine, cider, mead

The alcohol is the product.

Kombucha and kefir

Yeast and bacteria together. Both contain a trace of alcohol.

Vinegar

The next step — bacteria oxidise the alcohol into acetic acid.

Some ferments unintentionally

An over-fermented kraut or a very ripe fruit will smell faintly alcoholic. Usually normal.

Vanilla and chocolate

Both involve fermentation stages that develop their characteristic flavours.

Does the alcohol cook off?

A question with a more interesting answer than most people expect.

• Not entirely, and not quickly. Alcohol and water form a mixture that does not simply boil the alcohol away first.
• A brief flambé removes comparatively little. The dramatic flame burns vapour above the pan rather than the alcohol in it.
• Long simmering removes most of it, and even hours of cooking leaves a trace.
• In bread, most of the alcohol evaporates during baking, and a trace remains — this is normal and true of all yeasted bread.
• Surface area and time matter most. A wide uncovered pan simmered for an hour removes far more than a covered pot for twenty minutes.
• If it matters for dietary or medical reasons, do not rely on cooking to remove it. Leave it out.

Controlling a yeast fermentation

1. Temperature is the main lever. Warmer is faster and produces fewer flavour compounds; cooler is slower and tastes better.
2. Less yeast plus more time beats more yeast plus less time, for flavour, in almost every case.
3. Sugar supply limits it. Yeast stops when the available sugar runs out, which is why an over-proved dough is pale — the crust has nothing left to brown with.
4. Salt slows yeast, which is part of why it is added to bread dough beyond seasoning.
5. Alcohol eventually inhibits the yeast that made it, which is what limits the strength of a fermented drink.
6. Oxygen changes the outcome — with plenty of air, yeast multiplies rather than producing much alcohol.
7. Anything fermented for drinking at home raises food-safety and, in some cases, legal questions. Whether a preserved food is safe to store, and for how long, depends on numbers a food safety authority sets. FoodHQ publishes none of them — follow current Food Standards Agency guidance.

Common questions