Chemistry
Fermentation
How tiny yeasts and bacteria turn sugar into bread, yoghurt and sauerkraut.
What you need first
A bread dough that turns thick and airy overnight. Milk that sets into yoghurt. White cabbage that becomes sauerkraut: behind all of this are tiny living things that convert sugar. This process is called fermentation, and you meet it in the kitchen more often than you think.
Yeasts convert sugar
Yeasts are tiny, single-celled fungi. When they get at sugar with hardly any air around, they break it down and gain energy from it. Two substances are formed in the process: the gas (CO₂) and ethanol, the alcohol in beer. One of glucose turns into two molecules of ethanol and two molecules of CO₂. The yeast keeps the energy for itself and releases the two products.
Why bread dough rises
In bread dough the yeast is trapped in the sticky dough. The carbon dioxide it produces cannot escape and collects in small bubbles. These gas bubbles blow up the dough, it rises and turns fluffy. During baking the bubbles burst and leave the many holes in the crumb. So the CO₂ turns flat dough into airy bread. And the ethanol? It almost completely evaporates in the hot oven, which is why bread does not taste of alcohol.
Not only yeast
Bacteria can ferment too. In sauerkraut and yoghurt it is lactic acid bacteria: they convert sugar into lactic acid. The acid makes yoghurt sour and firm and protects sauerkraut from spoiling. Fermentation is therefore the oldest biotechnology: people have used it for thousands of years to preserve food, long before they knew that living things were behind it.
Exercises
0 of 6 solvedTime to try it yourself. You can't break anything, every attempt counts.
Which starting material do yeasts need for fermentation?
How many different products does the yeast release during fermentation?
Which gas makes bread dough rise?
Put the steps in the right order for how bread dough rises and turns fluffy while baking.
- 1During baking the bubbles burst and leave holes
- 2The gas bubbles collect and puff up the dough
- 3The yeast breaks down the sugar in the dough
- 4The gas CO₂ forms in the process
How many oxygen atoms does one CO₂ molecule have?
Match what belongs to whom in fermentation.
Where this leads