# The Living Loaf

How wild yeast and bacteria in a sourdough starter make gas and acid that raise a loaf, from the jar to the hot oven\.

By Bibbler

Topic: Food & Drink

[Open the interactive Jig](https://ask.jig.com/j/daNHvU1E4wuC)

## Transcript

### 1. The Living Loaf

This crusty loaf was raised by living things too small to see\. They live in the jar of bubbly paste beside it, called a sourdough starter\. Let's find out how they turn flour and water into bread\. Drag anywhere to look around\.

Fermentation means microbes changing food as they feed\. Sourdough, yogurt and cheese are all made this way\.

### 2. A jar full of life

A starter is just flour and water\. But wild yeast and bacteria from the flour and the air move in and feed on it\. Bakers keep it alive for years by feeding it fresh flour and water\.

The rubber band marks where the starter sat right after its last feed\.

### 3. Meet the microbes

Under the dome is a model of the microbes, made thousands of times bigger\. The round cells are wild yeast; some are budding off daughter cells\. The little rods are lactic acid bacteria, and they far outnumber the yeast\.

A real starter can hold from about 15 to several hundred bacteria for every yeast cell\.

### 4. Feed the starter

After a feed the starter sits at the rubber band\. The microbes eat sugars freed from the flour and multiply\. In a warm kitchen the starter can double in height within hours\.

Warmth speeds things up\. Around 26 °C \(79 °F\) the yeast grows fastest\.

### 5. Bubbles and sour

Yeast breathe out carbon dioxide gas, which makes the bubbles\. Bacteria make lactic and acetic acids, which give the sour taste\. The test strip shows the starter turning more acidic, its pH falling from about 6 to about 4\.

pH measures acidity: lower means more sour\. Most harmful germs cannot grow below about pH 4\.6\.

### 6. Dough holds its breath

Mix starter into flour, water and salt, and the dough comes alive too\. Folding builds gluten, a stretchy web of wheat proteins\. Its thin walls trap the gas, so small bubbles swell as the dough rises\.

The bubbles come from the microbes' gas, not from air beaten in\.

### 7. Into a hot oven

In an oven at about 230 °C \(446 °F\), the gas expands and water turns to steam\. The loaf springs up and the cut on top opens into an ear\. The crust browns as sugars and proteins react\.

The loaf can grow by half or more in the first minutes\. The microbes die at about 60 °C, so the baked bread is no longer alive\.

### 8. Keep a little alive

Every hole in a slice is a bubble the microbes made, set in place by the heat\. The loaf is finished, but the starter in the jar lives on\. Feed it again, and it will raise the next loaf\.

In an open crumb, most holes are about 3 to 15 mm across, with a few bigger ones\.