How do we know?
↻ One of the pages with a moving picture
A scientist drew yeast cells under a microscope, in red chalk.
- Yeast is a single-celled fungus. It eats sugar and makes carbon dioxide gas and alcohol (fermentation).
- Two proteins in wheat flour join with water to make gluten. Kneading makes it stretchy.
- The gluten traps the gas like a balloon, so thousands of bubbles puff the dough up. Warmth helps.
- In the oven the gas expands and the bread rises more. The heat kills the yeast.
In 1680 Antoni van Leeuwenhoek studied fermenting beer under his microscope and sent red-chalk drawings of yeast cells to London, where the letter is still kept.
Yeast is alive. There are about 160 kinds of yeast, and many live all around us!
Yeast can even blow up a balloon, as you can see in the Try it! below.
Wow number
160 There are about 160 kinds of yeast, and many live all around us!
Myth or fact?
Yeast is a kind of tiny living fungus. Fact. It is a single-celled fungus that feeds on sugar.
Oven heat makes yeast grow faster. Myth. Too much heat kills the yeast.
Think about it
Why do you think dough rises faster in a warm kitchen than in a cold one?
Where in the world?
Delft, Netherlands where, in 1680, Antoni van Leeuwenhoek wrote a letter about yeast he saw under his microscope — 52.0116° N, 4.3571° E.
Where these facts come from
Every number on page 83, and who measured it
This page tells us that yeast is alive and that there are about 160 kinds.
Science of Bread: Bread Science 101 — The Exploratorium, a science museum in San Francisco
exploratorium.edu/explore/cooking/bread-science
We checked it on 31 October 2026
This page tells us how yeast makes gas, and how it can blow up a balloon.
Raising agents: biological (fermentation) — The Institute of Food Science and Technology, food scientists in the UK
ifst.org/lovefoodlovescience/resources/raising-agents-biological-fermentation
We checked it on 31 October 2026
This page tells us how gluten makes dough stretchy.
Gluten's Complex Chemistry Contributes to Delicious Baked Goods — Scientific American, a science magazine
scientificamerican.com/article/glutens-complex-chemistry-contributes-to-delicious-baked-goods
We checked it on 31 October 2026
This page tells us about Leeuwenhoek's 1680 letter about yeast, which is kept in London.
Search Results — The Royal Society in London, which keeps Leeuwenhoek's letters
catalogues.royalsociety.org/calmview/Record.aspx?id=EL%2FL1%2F57&src=CalmView.Catalog
We checked it on 31 October 2026
This page tells us more about that letter and the yeast drawings.
Wrote Letter L-108 of 1680-06-14 to Thomas Gale about beer, yeast, crab's eyes in vinegar, and little animals reproducing in hermetically-sealed tubes — Lens on Leeuwenhoek, a website about Antoni van Leeuwenhoek's letters
leeuwenhoek.net/content/wrote-letter-108-1680-06-14-thomas-gale
We checked it on 31 October 2026
Has this changed?
How dough rises is not going to change.
Leeuwenhoek’s drawings are still kept in London, nearly 350 years later.
Words from this page
- yeast
- A tiny single-celled fungus that eats sugar and makes gas.
- fermentation
- When yeast eats sugar and makes gas and alcohol.
- gluten
- A stretchy stuff made from wheat flour and water that traps gas.
Try it!
Blow up a balloon with yeast.
1. With a grown-up, put a pinch of yeast, a little sugar and warm tap water in a plastic bottle. 2. Swirl gently, then stretch a balloon over the bottle’s neck. 3. Keep it somewhere warm for 60–90 minutes. (Don’t drink it!)
What happens: it gets frothy and the balloon slowly fills with gas.
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