Your turn
Each lab is one part of the video. Change things and see what happens.
Each lab isolates one idea from the video. The models are simplified, and the speeds are illustrative.
Tap the buttons and see what happens.
Tap anywhere on the bread to zoom in. What is hiding in the dough?
Each hole in the crumb is a gas bubble. Zoom in to find the cells that made it.
Tap the bread. Who is inside?
Tap a yeast cell to make it bud, or double them all. How fast do the numbers grow?
Budding doubles the population each generation. Watch the count compound.
Tap a yeast. Watch it make a new one!
Sped up: in a lab each doubling takes about 90 minutes.
Tap the grapes, plums, flower, soil, wasp and air. Where is the yeast hiding?
Wild yeasts live on fruit skins, in nectar, soil and bark, in the air and on wasps. Find all six.
Tap the fruit, the flower and the wasp. Find the yeast!
Switch the gluten net on and off. Does the dough rise or stay flat? Then bake it.
Without a gluten network the carbon dioxide escapes. Toggle it, then bake to set the bubbles.
Turn the net on. Does the bread puff up?
Turn the air pump on and off. When does the yeast grow fastest?
Oxygen speeds yeast growth. Compare the growth rate with and without aeration.
Pick the water and the food. Which balloon grows?
Vary temperature and sugar: gas output tracks how active the yeast is.
Pick warm or cold water. Does the balloon grow?
Yeast is a tiny living thing. It is a cousin of mushrooms, but each yeast is so small you need a microscope to see it.
Wild yeast lives all around us, wherever there is sugar. It lives on grapes and plums, in flowers, in the dirt and on trees. It even floats in the air. Wasps carry it around too!
A yeast grows a little bump on its side. The bump grows and pops off. Now there are two! They do it again and again.
Yeast eats the sugar in flour and puffs out a gas. Squishing the dough makes a stretchy net inside. The net catches the gas in little bubbles, so the dough puffs up. The oven makes the bubbles stay. Those are the holes in your bread!
Yeast comes from nature. It is a tiny living fungus made of just one cell, and wild yeasts live almost everywhere: on the skins of fruit like grapes and plums, in the sweet nectar of flowers, in soil, on tree bark, drifting in the air, and even riding carried by insects such as wasps (even inside their guts).14812 The yeast in a packet from the shop is the same kind of living thing. Factories take one carefully chosen strain of baker's yeast and grow huge amounts of it in tanks of sugary molasses.1
wild yeast → chosen strain → grown on molasses + air → pressed or dried → yeast packet
So nobody "makes" yeast from nothing, just as nobody makes a tomato plant from nothing. You start with living yeast and give it food, warmth and air so it multiplies.
Yeast is a single-celled fungus. Fungi are the same big family as mushrooms and moulds, but a yeast is just one cell living on its own.26 Scientists know of about 1,500 species of yeast.1 The one bakers use is called Saccharomyces cerevisiae, a name that means "sugar fungus".5
A yeast cell is roughly egg- or lemon-shaped and about the same size as one of your red blood cells, so you need a microscope to see one.3 A pinch of dried yeast holds a huge number of them.
Most yeasts multiply by budding. A small bump grows on the side of a cell. The bump swells, gets its own copy of the cell's DNA, and then breaks off as a brand-new cell.17 You can even see little scars on a yeast cell where buds broke away.3
In a lab, with plenty of food and warmth, baker's yeast can double its numbers in about 90 minutes.7 That's why one cell could become about 65,000 cells in a day, and about a million in 30 hours. (In the player's budding toy, each tap is one doubling. The toy speeds things up to make it easy to see.)
| Doublings | Time (at ~90 min each) | Cells from one |
|---|---|---|
| 1 | 1.5 hours | 2 |
| 4 | 6 hours | 16 |
| 10 | 15 hours | 1,024 |
| 20 | 30 hours | about 1 million |
An idealised count: real yeast slows down when food runs out or space gets crowded.
Yeasts love sugar, so they're easiest to find wherever plants make sweet things.14
Yes, and this is one of the most surprising discoveries about yeast. In 2012, scientists in Italy (Stefanini and colleagues) showed that social wasps, including hornets and paper wasps, carry baker's yeast. The wasps carry yeast cells inside them and help spread them from place to place; their yeasts are closely related to those found on grapes.12
Even better, wasp queens that sleep through the winter can keep yeast cells alive inside their bodies from autumn to spring, and then pass them on to their young.12 So wasps act like a winter home for yeast, and a delivery service for it too.
When you mix flour, water and yeast, the yeast wakes up and starts to feed on sugars from the flour. As it feeds, it gives off a gas called carbon dioxide. This is called fermentation.14
Gas alone would just bubble away. The trick is gluten. Wheat flour contains two proteins, glutenin and gliadin, which join with water to make gluten. Kneading makes gluten stretchy, like a net of tiny rubber bands. The net traps the gas in thousands of bubbles, and the dough puffs up.138 Wheat flour makes the stretchy gluten net that holds the gas in most raised loaves; gluten-free breads use other ingredients to hold it.10
In the oven, the heat kills the yeast and the bubbles set firm.413 The holes in your slice of bread are those bubbles, frozen in place.
| Stage | What the yeast does | What the dough does |
|---|---|---|
| Mixing | Wakes up in the warm, wet dough | Gluten starts to form |
| Kneading | Starts feeding | Gluten net gets stretchy and strong |
| Rising | Eats sugars, gives off carbon dioxide | Bubbles grow; dough puffs up |
| Baking | Killed by the heat | Bubbles set into holes; crust forms |
The yeast you buy is grown, a bit like a crop. Here's how it works:114
This factory method grew up after scientists such as Louis Pasteur showed that living yeast cells do the work of fermentation. Britain introduced special yeast-growing vats in 1879, and around 1900 factories in the United States began using spinning centrifuges to concentrate the yeast.414 (The air-pump toy in the player uses made-up speeds, just to show that air helps.)
Sourdough is the old way of making bread rise. A baker mixes flour and water and leaves it for a few days, feeding it with more flour and water. Wild yeasts from the flour and air move in, along with friendly lactic acid bacteria. The bacteria give sourdough its tangy taste, and the yeast makes the gas.101516 This bubbly mixture is called a starter, and bakers keep it going for years. Microbes and fermentation make other foods too. Bacteria turn milk into cheese, and cocoa beans are fermented on the way to becoming chocolate.8
People used yeast for thousands of years before anyone knew it was alive. The earliest clear records of yeast-raised bread come from ancient Egypt, where archaeologists have found baking chambers and drawings of bakeries about 4,000 years old.414 Researchers think a flour-and-water mix was left out on a warm day, wild yeasts got in, and the result was a lighter, tastier bread.14 One of the oldest sourdough loaves ever found, from about 3700 BC, was dug up in Switzerland.15
You can watch yeast at work with an adult to help.617
Try a second bottle with cold water, or one without sugar, and compare. Never use kettle-hot water: too much heat kills yeast, just like the oven does.4 The balloon sizes in the player's toy are made up, just to compare the three setups. Your real balloons will vary.
Want to meet more tiny helpers? See how bacteria reproduce by splitting in two, a different trick from yeast's budding.
Wild yeasts live almost everywhere there is sugar: on the skins of fruit such as grapes, plums and apples, in flower nectar, in soil, on tree bark, floating in the air, and carried by insects such as wasps (even inside their guts). Mixing flour and water and leaving it for a few days catches some of them. That is how a sourdough starter begins.
Nobody knows exactly. The earliest clear records of yeast-raised bread come from ancient Egypt. Researchers think a mix of flour and water was left out longer than usual, wild yeasts from the flour and air got in, and the dough rose into a lighter, tastier bread.
Yeast isn't made from scratch. It is grown. Factories start with a pure strain of baker's yeast and grow it in big tanks of molasses (a sugary syrup) with minerals, bubbling in air so the yeast buds quickly. The yeast is then separated from the liquid, washed, and pressed into soft cakes or dried into granules.
Yes. Yeast is a living single-celled fungus. Dried yeast is dormant, like it's asleep. It wakes up when it gets warm water and food. Baking kills the yeast, so there is no living yeast left in a baked loaf.
Yes. Yeasts are fungi, distant cousins of mushrooms and moulds, but each yeast is a single cell. About 1,500 species are known. The one used for bread is Saccharomyces cerevisiae.
Baker's yeast is safe to eat in bread, and its use goes back thousands of years. Some other kinds of yeast live on and in our bodies and are usually harmless. A few can cause infections when the body's balance is upset, which a doctor can treat.