How Does Yeast Work?
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Yeast is a living single-celled fungus that eats sugar and produces carbon dioxide and alcohol. That is the whole mechanism. Everything a baker or brewer does is really about giving this organism the right conditions and then getting out of its way.
Yeast is a living thing
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.
Where the sugar comes from
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.
Temperature is the main control
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
The forms you can buy
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.
Checking whether it is alive
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.
The same species, different jobs
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.
The takeaway
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.