How Does Yeast Make Bread Rise?

Yeast makes bread rise by fermenting sugars in the dough and releasing carbon dioxide gas, which collects in tiny air bubbles held inside a stretchy gluten network, inflating the dough like thousands of little balloons. In the oven, that gas expands even more before the heat kills the yeast and sets the crumb in its puffed-up shape. Yeast also produces alcohol and flavour compounds, which is why well-fermented bread tastes so much better than a rushed loaf.

What is yeast, exactly?

Baker's yeast is a living single-celled fungus called Saccharomyces cerevisiae, the same species used to brew beer and make wine. A single gram of fresh yeast holds billions of cells. Dried, they go dormant; given water, food and warmth, they wake up and get to work.

Yeast needs three things:

• Water, to rehydrate and to move nutrients in and waste out.
• Food, in the form of simple sugars.
• The right temperature, warm but not hot.

Unlike baking soda or baking powder, which make gas by a single quick chemical reaction (see baking soda vs baking powder for that side), yeast is a slow, living factory. That's why yeast doughs need hours, not minutes.

Where does yeast get its sugar?

Plain bread dough is just flour, water, salt and yeast, with no sugar added. So what does the yeast eat?

Flour contains a small amount of free sugar, but the main food comes from starch. During milling, some starch granules get cracked. Natural enzymes in the flour, called amylases, break that damaged starch down into maltose, a sugar made of two glucose units. Yeast has its own enzyme, maltase, that splits maltose into glucose, which it can use. If a recipe does include table sugar, another yeast enzyme, invertase, splits it into glucose and fructose.

How does yeast turn sugar into gas?

When yeast has plenty of oxygen, it breaks sugar down completely into carbon dioxide and water. Inside a ball of dough, though, oxygen runs out within minutes. So the yeast switches to alcoholic fermentation, a less efficient process that turns each glucose molecule into two molecules of ethanol and two of carbon dioxide.

Here's where those products go:

1. Carbon dioxide first dissolves into the water in the dough. Once that water is saturated, the gas moves into the tiny air bubbles that were folded in during mixing and kneading.
2. The bubbles grow. Yeast doesn't create new bubbles. It inflates the ones that are already there. That's why mixing and kneading matter so much: they create the bubble nuclei and build the gluten network that holds the gas without letting it leak out.
3. Ethanol and flavour compounds stay in the dough. Most of the alcohol bakes off in the oven, but along the way yeast and bacteria also produce organic acids and aromatic esters, which give bread its complex, slightly fruity, wheaty aroma.

Chef Fizz says: Yeast is the world's tiniest balloon artist. It can't make a balloon from scratch, but give it one and it'll blow it right up.

What temperature does yeast like?

Yeast is fussy about temperature, and the right temperature makes a huge difference to both timing and flavour.

Most bread recipes aim for a dough temperature of about 24 to 26°C (75 to 78°F). If your kitchen is cold, use slightly warmer water. If it's hot, use cool water so the dough doesn't race ahead.

To activate active dry yeast, use water at about 40 to 43°C (105 to 110°F). It should feel comfortably warm on your wrist, like a warm bath, not hot.

Want to practise the timing? The Cooking Scientist app has a lesson on waking up yeast, plus one on slow, cold fermentation, both with quick quizzes.

Which type of yeast should I use?

To swap between them, use roughly these equal amounts: 1 g instant yeast = about 1.25 g active dry yeast = about 3 g fresh yeast. A standard 7 to 8 g (1/4 oz) packet of dry yeast holds about 2 1/4 teaspoons.

What happens during rising and proofing?

Most yeast breads rise twice.

Bulk fermentation

After mixing, the whole batch of dough rests and rises, usually until roughly doubled. At about 24°C (75°F) with a typical amount of yeast, that takes 1 to 2 hours. This is when most flavour develops, and the gas gently stretches and strengthens the gluten.

Shaping and the final proof

The dough is knocked back gently, shaped and left to rise again. Shaping redistributes the yeast and its food, and divides big bubbles into many smaller ones for a finer crumb. The final proof is usually shorter, 30 minutes to an hour at room temperature.

Slow and cold fermentation

You can slow everything down by putting dough in the fridge overnight, often 12 to 72 hours. The yeast slows dramatically, but enzymes and bacteria keep producing acids and flavour compounds. The result is a more complex, slightly tangy loaf with a better crust. Sourdough takes this further, using wild yeast and lactic acid bacteria together.

How do I tell when it's ready?

Press a floured fingertip about 1 cm (1/2 in) into the dough.

• Springs back fast and completely: under-proofed. Wait longer.
• Springs back slowly, leaving a slight dent: ready to bake.
• Doesn't spring back at all, or the dough sighs and sinks: over-proofed. The gluten has stretched too far and the bubbles are merging. Gently reshape it and give it a short second proof.

What is oven spring?

The final burst of rising happens in the first 10 to 15 minutes in the oven, and it's called oven spring. A well-proofed loaf can grow by a further 20 to 30% here.

Several things drive it at once:

1. Gas expands as it heats. Warmer gas takes up more space.
2. Dissolved CO₂ comes out of solution. Hot water holds less gas, so it's pushed into the bubbles.
3. Water and alcohol vaporise. As the dough heats, some of its water and ethanol turn to vapour, swelling the bubbles further.
4. Yeast has a last frenzy. As the dough warms toward 50°C (120°F), yeast works faster than ever, until it dies around 60°C (140°F).

Then the structure sets. Starch gelatinises and gluten proteins firm up, locking the bubbles into crumb. The crust dries, hardens and browns. Lean loaves are usually done at an internal temperature of about 93 to 99°C (200 to 210°F).

Bakers add steam to the oven during the first minutes, or bake in a lidded pot, to keep the crust soft and stretchy for longer so the loaf can expand fully. Scoring the top with a sharp blade gives the expanding dough a controlled place to open.

Once baked, the loaf's starch begins a slow change that will eventually make it stale, which is its own story in why bread goes stale.

Try it at home

Watch yeast inflate a balloon.

1. Get two empty plastic bottles (500 ml / 16 oz) and two balloons. Stretch the balloons a few times so they inflate easily.
2. Into bottle A, pour 120 ml (1/2 cup) of water at about 40°C (105°F), 1 tablespoon of sugar and 1 packet (7 to 8 g / 2 1/4 tsp) of dry yeast. Swirl gently.
3. Into bottle B, pour 120 ml of ice-cold water, 1 tablespoon of sugar and another packet of yeast.
4. Stretch a balloon over each bottle's neck and set them side by side.

Within 10 to 15 minutes, bottle A will turn foamy and its balloon will start to stand up. After 30 to 45 minutes, it should be noticeably inflated. Bottle B will barely change. That balloon full of carbon dioxide is the same gas puffing up your dough; the cold bottle shows why a chilly kitchen makes for a slow rise. Want a third bottle? Try one with very hot tap water to see how heat stops the yeast cold.

Common mistakes

• Using water that's too hot. Water above about 50°C (120°F) can damage yeast, and hotter will kill it. Aim for comfortably warm, 40 to 43°C (105 to 110°F), for active dry yeast.
• Trusting old yeast. Yeast loses power over time, especially once opened. Test it: in warm water with a pinch of sugar, it should foam within about 10 minutes. Store opened yeast in the fridge or freezer.
• Rising by the clock instead of the dough. Kitchen temperature changes rising times dramatically. Watch for roughly doubled size and use the poke test.
• Proofing in a hot spot. Putting dough on a radiator or in a very hot oven makes it rise fast but taste flat, and can kill the yeast. Aim for 24 to 27°C (75 to 80°F).
• Over-proofing. Dough left too long exhausts its food and over-stretches its gluten, then collapses in the oven. If the dent from a poke doesn't spring back at all, reshape and bake sooner.
• Adding lots of sugar to speed things up. A little sugar helps, but large amounts, more than about 10% of the flour's weight, actually slow yeast down by drawing water out of the cells. Sweet doughs need more time, more yeast or osmotolerant yeast.
• Skipping steam. A dry oven sets the crust too early and limits oven spring. Use a lidded pot, or put a tray of hot water in the oven for the first 10 to 15 minutes.