Why does bread rise?
― Making the gas is easy. Not losing it is the hard part
Fresh-kneaded dough is a heavy, dense lump. Within a few hours it more than doubles in size, then swells further in the oven, and when you cut it open it's riddled with tiny holes. Nobody whipped air into it. A gas is being made inside. But the real challenge isn't producing that gas — it's trapping it so it can't escape.
Whether it's sandwich loaf or a baguette, slice it open and you'll find tiny holes packed everywhere. Press it with a finger and it shrinks; let go and it springs back.
What's inside those holes? People assume it's air, but it isn't. Nobody kneaded air into the dough.
The holes are actually a gas newly made inside the dough. The maker is a microbe called yeast, which eats the sugar in the dough and releases carbon dioxide.
If that were all, the same thing would happen in beer or soda water. What makes bread special is that the bubbles stay put instead of escaping.
It breaks down sugar in the wheat and releases carbon dioxide and alcohol. Part of bread's smell comes from that alcohol.
Wheat proteins form a rubbery mesh. This mesh stretches and holds its shape while wrapping around the bubbles.
There are plenty of ways to make gas. The hard part is step 2 — something only wheat can do. Let's look at each in turn.
The gas itself is easy to make — there's plenty to spare
Yeast breaks down the sugar in dough and releases carbon dioxide and alcohol. That faint sweet-sour smell drifting from a bakery doorway is partly this alcohol (most of it burns off during baking).
As for how much gets made: about two teaspoons of sugar can produce nearly two 1.5-litre bottles' worth of carbon dioxide. The full calculation is in the collapsible section at the end, but it's more than five times what's needed to make the dough rise.
In other words, there's no shortage of gas. There's a surplus. Yet dough made from other flours often won't rise properly. The problem was never about producing the gas.
Not losing them is hard.
The "mesh" only wheat can make
When you add water to wheat flour and knead it, two proteins in the flour link up to form a mesh. Stretch this mesh and it spreads thin; let go and it shrinks back. It behaves like a rubber balloon.
Thanks to this mesh, the gas stays wrapped as bubbles, pushing outward and inflating the whole dough. Without the mesh, gas would simply escape through the surface and the dough wouldn't rise.
Rice flour and buckwheat flour are hard to bake into bread because they lack, or have very little of, the protein that forms this mesh. Gas is released, but nothing traps it. It turns out these were never "rising flours" — they were flours that could or couldn't hold onto bubbles.
And kneading exists purely to build this mesh. Simply mixing the ingredients doesn't link the proteins together. Applying force and stretching the dough repeatedly gradually assembles the mesh.
This might be surprising, but gas almost never forms a bubble in a completely empty space. It needs some tiny existing gap to start from.
In bread, that starting point is invisibly tiny air pockets folded in during kneading. The carbon dioxide from yeast first dissolves into the dough, then gathers into these tiny pockets and slowly grows bigger.
So the number of holes in bread is essentially fixed at the kneading stage. New bubbles don't appear later — existing bubbles simply grow. That's why how you handle the dough changes the bread's texture.
The same thing happens with volcanic magma and carbonated drinks. "Dissolved gas turning into bubbles once it gets a starting point" is exactly the same mechanism.
The biggest rise happens in the first few minutes of baking
Put fermented dough into the oven, and it swells by another size. In fact, this final rise is what shapes the finished loaf.
Three things overlap to cause it.
As it warms, its activity speeds up sharply, producing a burst of carbon dioxide. But it gives out once temperatures pass around 50°C.
This is the biggest force of all. Even a small amount of water expands to about 1700 times its volume when it becomes steam.
The third factor is that the gas already present simply expands as it heats up. Going from 30°C to 100°C alone makes it about 20% bigger.
Run the numbers, and steam's contribution turns out larger than the carbon dioxide made by fermentation. People say "yeast makes bread rise," but the final push actually comes from water.
And right at the right moment, the framework sets. Starch absorbs water and firms up, and the protein mesh sets under heat. The shape gets locked in at the point of maximum rise. That's why baked bread doesn't shrink back as it cools.
It's tempting to pinch off a bit of dough while kneading, or a bit of cookie dough. It's best avoided.
Eggs aren't the only concern. Raw wheat flour itself also needs care. Because flour goes from field to bag without being heated, it can occasionally carry bacteria that cause food poisoning. Think of it as an ingredient that only becomes safe once cooked.
If a young child is helping in the kitchen, it's worth telling them in advance not to put dough in their mouth, and not to touch their mouth with hands that have touched it.
Something you can check in your own kitchen
- Into a 500 mL plastic bottle, add 200 mL of lukewarm water, 2 teaspoons of sugar, and 1 teaspoon of dry yeast, and swirl gently
- Stretch a deflated balloon over the mouth of the bottle
- Leave it somewhere warm (around 30°C) for 15–30 minutes
- The balloon will start to inflate. That's the carbon dioxide the yeast has made
- For comparison, it helps to also set up a bottle with no yeast at the same time
Step 5 is the key to this observation. It lets you rule out the idea that "it only inflated because it got warm." Don't seal the bottle with a cap — the balloon must be there to give the gas an escape route. Sealing it builds up pressure and is dangerous. Keep the water from being too hot (above 60°C the yeast dies and won't produce gas). When you're done, pour it down the sink and rinse well.
Summary
Bread rises because yeast eats sugar and releases carbon dioxide, and a mesh made of wheat protein wraps around it without letting it escape. There's more than enough gas to spare. The hard part is trapping it, and that's something only wheat can do. Then, during baking, water turns to steam and delivers the final stretch, right at the moment the framework sets.
Yeast is what makes the gas,
but the mesh and the water decide the shape.
Want to go deeper? — Terms, numbers, and how this connects to the textbookFrom middle-school science to topics still being researched — each level is labeled clearly
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Chemistry" / "Basic Biology"
- High school+High-school "Chemistry" / "Biology," or textbook advanced/sidebar content
- UniversityNot covered in high school — university-level specialist content (food science)
- ResearchNot even settled fact at university level — something researchers are actively investigating
Middle schoolTerms: words around bread
- Yeast: a microbe that breaks down sugar into carbon dioxide and alcohol. It's in the same family as molds and mushrooms.
- Alcoholic fermentation: breaking down sugar without using oxygen. Common to bread, sake, and beer.
- Gluten: the mesh formed when two wheat proteins are kneaded with water. This is the "mesh" referred to in the article.
- Gelatinization: starch firming up with water and heat. This is also what happens when rice is cooked.
- Baking powder: a powder that releases carbon dioxide directly via a chemical reaction, without microbes. It doesn't need time.
High schoolChecking with a formula: how much gas comes from sugar
The article said "there's more gas than needed, with plenty to spare." Exactly how much extra can be worked out with a calculation.
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
| C₆H₁₂O₆ glucose | 180 g per mol |
| C₂H₅OH ethanol | 46 g per mol |
| CO₂ carbon dioxide | 44 g per mol |
In words: "for every 1 unit of sugar, you get 2 units of alcohol and 2 units of carbon dioxide." Just to be sure, let's check the masses add up.
| Mass of alcohol produced | 2 × 46 = 92 g |
| Mass of carbon dioxide produced | 2 × 44 = 88 g |
| Total (should match the original sugar) | 92 + 88 = 180 g |
It matches. The sugar didn't disappear — it just split into a gas and a liquid.
| Sugar used | 10 g (about 2 teaspoons) |
| How many moles | 10 ÷ 180 ≒ 0.056 mol |
| Carbon dioxide is twice that | 0.056 × 2 = 0.112 mol |
| Volume of 1 mol of gas | about 24 L |
| Carbon dioxide produced | 0.112 × 24 ≒ 2.7 L |
2.7 litres. Nearly two 1.5 L bottles' worth. From just two teaspoons of sugar.
So how much is actually needed? For 500 g of dough to double in size, an increase of about 0.5 L is enough.
| Amount that can be produced | 2.7 L |
| Amount actually needed | 0.5 L |
| Margin | 2.7 ÷ 0.5 ≒ 5.4 times |
More than 5 times as much as needed. So "not rising because there isn't enough gas" basically never happens. When bread fails to rise, the cause is almost always on the trapping side.
* In practice, not all the sugar gets broken down — some is used by the yeast itself to grow. Also, table sugar (sucrose) is broken down before it's used. This is a rough calculation to gauge the upper limit.
Let's break the final rise in the oven into three parts and estimate each.
| A) Water turning to steam | If 5% of the dough's 100 g of water turns to steam |
| Mass of water that turns to steam | 100 × 0.05 = 5 g |
| Volume expands about 1700-fold, so | 5 × 1700 = 8500 mL |
| Converting to litres | 8500 ÷ 1000 = 8.5 L |
| B) The existing gas expanding with heat | From 30°C (303 K) to 100°C (373 K) |
| Expansion factor | 373 ÷ 303 ≒ 1.23 times |
The carbon dioxide from step ② was 2.7 L; the steam from A was 8.5 L. The steam is more than three times as much.
People say "yeast makes bread rise," but by volume, the final push actually comes from water. Yeast's real job is building a fine network of bubbles before baking even starts.
This "about 1700-fold expansion when water turns to steam" figure is the same number that came up in the article about water hitting hot frying oil. The force that raises bread in the kitchen and the force that sends up a fireball are the same phenomenon. The only difference is whether it's contained or released all at once.
* The evaporation ratio, the dough's water content, and the baking temperature all vary a lot with conditions. This is a rough estimate meant to show which effect dominates.
High school+Why does it set at just the right moment?
While baking, several separate things happen inside the dough in sequence. This sequence is what shapes the loaf.
- Around 40–50°C: yeast activity peaks, then it dies off
- Around 60°C: starch absorbs water and starts to set (gelatinization)
- Above 70°C: the protein mesh sets under heat and stops stretching
- Around 100°C: water turns to steam, giving the strongest push of all
- Above 140°C (surface only): crust colour and aroma develop
The peak rising force and the setting of the framework happen at almost the same moment. If setting happened too early, the rise would be cut off before it finished. If too late, the structure would lose its support and collapse.
If you take bread out of the oven partway through baking, it can deflate. That's because the framework hasn't fully set yet.
Also, carbon dioxide becomes less soluble in water as temperature rises. The portion that was dissolved in the dough's water gets driven out by heat and joins the bubbles. This also contributes to the final rise. As with the article on pressure cookers, how much can stay dissolved depends on conditions.
UniversityDough is neither solid nor liquid
Bread dough deforms when pushed (like a liquid) but springs back a little when released (like a solid) — an in-between property. There's a field that studies this kind of behaviour, and it separately measures "how much it stretches" and "how much it springs back" to turn dough quality into numbers.
What makes this hard is that this property changes over time. Dough right after kneading and dough after resting are essentially different materials. And the rate of change itself varies with temperature, water content, and type of flour. "The same recipe" doesn't guarantee "the same dough."
The bubbles present their own problem too. Large bubbles absorb small ones and grow. Left alone, bubble sizes become uneven, leading to bread full of large holes. The step of folding the dough at the right moment to release some gas has the effect of resetting the bubbles to a more even size.
ResearchSame recipe, different bread every time
- Exactly how the gluten mesh assembles isn't fully understood at the molecular level. How the two proteins tangle together, and which bonds matter most — there's a rough picture, but "kneading makes it stronger" hasn't been fully traced through in molecular terms.
- Reproducibility is still an issue. Wheat is an agricultural crop, so the amount and quality of its protein varies by growing region, variety, and that year's weather. Factories measure and adjust the recipe every time, but the very indicators used to measure flour quality are known not to fully predict how the finished bread turns out.
- The microbial balance in natural sourdough starters is complex. Yeast and lactic acid bacteria coexist and influence each other. Even starters split from the same source end up with different compositions depending on their environment. Why they remain stable over time is still being studied.
- Some points remain debated about ill effects from eating wheat. While some conditions have a clearly understood immune mechanism, for complaints that don't fit that pattern, whether the cause is gluten or some other component is still debated. Please don't restrict your diet based on self-diagnosis — consult a medical professional.
Bread is a food humanity has been making for thousands of years. Yet we still can't predict from molecules alone "why this particular flour rises well." Having made something for a long time and truly understanding it are two different things.
Connections to the textbook (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: living things & microbes / gases | What yeast does, carbon dioxide |
| High school | Basic Chemistry: reaction equations & moles | Calculating gas from 10 g of sugar |
| High school | Basic Biology: respiration & fermentation | Alcoholic fermentation |
| High school+ | Chemistry: gas laws / protein denaturation | Thermal expansion, order in which the mesh sets |
| University | Food science: rheology | Dough viscoelasticity, bubble coarsening |
| Research | Cereal science (unresolved) | Gluten formation, reproducibility, microbial balance |
| ― | Food safety | Not eating raw dough |
- Explanations of the breadmaking process from the Japanese Society for Food Science and Technology (日本食品科学工学会) and standard baking textbooks.
- Cauvain, S. P. & Young, L. S., Technology of Breadmaking (a standard textbook on baking technology).
- Shewry, P. R. et al., a series of studies on wheat proteins and gluten formation.
- De Vuyst, L. et al., studies on sourdough microbial communities.
- Public advisories from Japan's Ministry of Health, Labour and Welfare (厚生労働省) and Consumer Affairs Agency (消費者庁) on undercooked flour and eggs.
* The amount of gas produced, the proportion of water that evaporates, and the temperature at each stage vary greatly with recipe and conditions. This article presents commonly cited approximate figures.
※ This article is a general science explainer. For guidance on food handling, please follow information from Japan's Ministry of Health, Labour and Welfare, the Consumer Affairs Agency, and similar bodies. Do not self-diagnose food allergies or dietary restrictions — consult a medical professional. Figures given here are approximations meant to aid understanding of the mechanism.