How Does Yeast Work?

Baker's yeast is a species called Saccharomyces cerevisiae, and it has been used by humans for thousands of years. Each cell is a complete organism that feeds, grows and reproduces by budding off copies of itself.

With plenty of oxygen it respires and multiplies. With little oxygen, which is the situation inside dough, it switches to fermentation, breaking sugars down into carbon dioxide and ethanol. In bread we want the gas and lose the alcohol to the oven. In brewing we want the alcohol and let the gas escape.

A basic bread dough may contain no added sugar at all, so yeast has to find its own. Flour contains enzymes called amylases that break starch into simpler sugars the yeast can use.

This is one reason long fermentation improves bread. Given time, more starch is converted, the yeast produces more gas steadily, and side reactions generate flavour compounds. A fast rise with lots of added sugar produces volume but much less taste.

Yeast activity is highly temperature dependent, and knowing the ranges gives you control over timing:

• •Below 4 degrees Celsius, activity nearly stops, which is why dough can be held in the fridge
• •Around 20 to 25 degrees, fermentation is slow and flavour develops well
• •Around 27 to 32 degrees, activity is fast and rises are quick
• •Above about 50 degrees, the yeast dies

Fresh yeast is moist, perishable and mostly used by professionals. Active dry yeast has larger granules and is traditionally dissolved in warm water first. Instant yeast has finer particles and can be mixed straight into flour.

Sourdough starter is different in kind. It is a stable community of wild yeasts and lactic acid bacteria, and the bacteria are what produce the sour flavour. It works more slowly and less predictably than commercial yeast, but that slowness is exactly where the flavour comes from.

Dried yeast loses viability over time, and a failed loaf is usually a dead yeast problem. Proofing is the test: mix the yeast with warm water and a pinch of sugar and wait about ten minutes.

If it foams noticeably, the yeast is active. If nothing happens, it is dead and no amount of extra kneading will help. Storing dried yeast sealed in the fridge or freezer greatly extends its life.

Baker's yeast, brewer's yeast and most wine yeast are all strains of the same species. What differs is which characteristics have been selected over generations of use.

Baking strains are chosen for fast, vigorous gas production. Brewing strains are chosen for alcohol tolerance, flavour compounds and how readily they settle out of the liquid. Some can be substituted in an emergency with mediocre results, which is a good demonstration of how much selective breeding can achieve within a single species.

Yeast makes the gas, but the dough has to catch it. When flour is mixed with water and kneaded, proteins in the flour form gluten, a stretchy network that can expand like a balloon. Carbon dioxide from the yeast collects in tiny air pockets trapped during mixing, and the gluten stretches around them as they grow. If the network is weak, gas escapes and the loaf stays dense. If dough rises for too long, the gluten becomes overstretched and the dough can collapse, a problem bakers call over-proofing.

Bakers often judge readiness with a poke test. Press a floured finger gently into the risen dough. If the dent springs back slowly and only partly, the dough is usually ready. If it springs back immediately, it needs longer, and if it does not spring back at all, it may be over-proofed. In the oven, the loaf expands quickly during its first minutes, known as oven spring. Heat makes the trapped gas expand, the yeast works briefly faster before it dies, and water and alcohol turn to vapour.

For a long time it was not obvious that fermentation was done by living things at all. In the nineteenth century, some leading chemists, notably Justus von Liebig, argued that it was a purely chemical breakdown. Louis Pasteur showed through careful experiments in the 1850s and 1860s that fermentation required living yeast cells, and that unwanted microbes caused wine and beer to spoil. His work helped establish that microorganisms drive many processes people had assumed were simply chemistry.

The story then took a twist. In 1897 the German chemist Eduard Buchner ground yeast with sand, pressed out the juice and found that this cell-free extract could still ferment sugar into alcohol and carbon dioxide. The yeast was dead, yet fermentation happened. The work came to be recognised as showing that substances inside cells, which we now call enzymes, carry out the reactions of life, and Buchner received the Nobel Prize in Chemistry in 1907. The word enzyme itself comes from Greek words meaning in yeast.

Bakers often write recipes in baker's percentages, in which every ingredient is given as a share of the flour's weight, counted as 100 percent. A simple loaf might use 500 grams of flour, 325 grams of water, 10 grams of salt and 5 grams of instant yeast. That is 65 percent water, 2 percent salt and 1 percent yeast. Writing a recipe this way makes it easy to scale up or down, and it lets you compare doughs at a glance: an open, airy ciabatta usually has far more water than a firm sandwich loaf.

The yeast figure is the one to adjust when you want to change timing. Using less yeast does not ruin bread; it simply slows the rise, and a dough made with a small pinch of yeast and left overnight in the fridge often tastes better than a fast one. Salt matters too, because it tightens gluten and slows fermentation, so a dough without salt rises quickly but tastes flat. When swapping types, fresh yeast is usually used at roughly two and a half to three times the weight of instant yeast, because it is mostly water and has fewer active cells per gram.

It is tempting to think yeast only ferments when oxygen runs out, but baker's yeast often ferments even with plenty of air, as long as sugar is abundant. This is called the Crabtree effect. Making ethanol is a wasteful use of sugar, releasing far less energy per molecule than full respiration, but it is fast, and the alcohol poisons many competing microbes. When the sugar runs out, the yeast switches to respiring the ethanol it made earlier, a change called the diauxic shift. Biologists describe this as a make, accumulate, consume strategy, well suited to ripe fruit where sugar is plentiful but contested.

The same organism became one of biology's most important laboratory tools. It grows quickly, is cheap to keep and, unlike bacteria, its cells are built on the same eukaryotic plan as our own. In 1996 it became the first eukaryote to have its whole genome sequenced, revealing about 6,000 genes, many with close human counterparts. Leland Hartwell used baker's yeast to find genes controlling cell division, work recognised by the 2001 Nobel Prize in Physiology or Medicine, shared with Paul Nurse, who studied a related yeast, and Tim Hunt. Yeast is also used to manufacture medicines, including the hepatitis B vaccine.

Yeast works by feeding on sugars and releasing carbon dioxide and alcohol, its speed is controlled almost entirely by temperature, and slower fermentation produces better flavour because it gives the chemistry time to develop.