Why does bread need yeast? Bread doesn’t have to have yeast and there are many breads made without it. However, if you want the bread to rise, then it needs yeast.
Yeast is a single-celled organism called Saccharomyces cerevisiae. It is a fungus, which means they make their energy by decomposing other things. There are approximately 1,500 different species of yeast in the world at the moment and yeast appeared on Earth hundreds of millions of years ago. Yeast break down the sugar and starches in other organisms into carbon dioxide and alcohol. The process by which they do this is called fermentation. Not all yeasts work in the same way and some are anaerobic and some are aerobic. However, generally, they take the complex carbohydrates (sugars) in the organism and break them down into simple sugars. They use these simple sugars for energy and in the process release a small amount of heat, carbon dioxide, and ethanol.
If you want your bread to rise then it needs yeast. The yeast is added to the flour after it has been made into a dough. The process of adding water to the flour and making the dough releases enzymes in the flour that breaks the starch molecules apart into simpler sugars. The yeast is added to the dough and it starts to break down the sugars for energy. The yeast releases carbon dioxide, but the dough is very stretchy and the carbon dioxide cannot escape. Bubbles of it are trapped in the dough, which starts to expand. The yeast used in bread works best at a slightly warm temperature and it takes a few hours to break down the sugars, which is why you have to leave the dough somewhere warm for a while. When you bake the bread, the yeast is killed and the bread solidifies, trapping the carbon dioxide air bubbles. If you don’t want your bread to rise, such as unleavened bread, then there is no need to use yeast.
The first people to use yeast in bread were the Ancient Egyptians in about 1500 BC. How they discovered it is unknown, but it may have been a happy accident. Egypt has a warm climate and if any bread dough was left out, yeast would naturally start to grow on it because yeast has airborne spores.
Because yeast produces ethanol during fermentation, it has been used to make beer and other alcoholic drinks for thousands of years. The first beer was probably made about 6,000 years ago, but people have been drinking alcohol since before then. The first time people drank alcohol was probably from fruit that was decomposing. The yeast on the fruit, would have made it alcoholic.
Yeast in bread is used for the carbon dioxide. Although, the alcohol produced evaporates during the baking process and probably helps to make the bread rise. The yeast in beer is used for the alcohol. The carbon dioxide adds the gas to the drink, but that is not as important as the alcohol. For beer, the brewer makes a liquid called wort that contains sugar, grain, and water. Yeast is then added to the wort and it breaks down the sugars, creating carbon dioxide and ethanol. It may take about a week, but the yeast will break down all of the sugar in the beer. Once this is done, the yeast will start to clump together and the brewer will know the beer is finished. The brewer will remove the yeast for use in another batch and the beer will be ready.
Why do yeast produce alcohol? One mystery has always been why yeast actually produce alcohol? Breaking down sugars into carbon dioxide takes less energy than it does to break them down into carbon dioxide and ethanol. That means, when yeast break down sugar into carbon dioxide and ethanol, they are not able to use all of the energy from the sugar because they have to use some of it to make the ethanol. Biologists have long wondered why and it seems like something that evolution would have corrected for. The fact that evolution hasn’t got rid of it must mean that it has a purpose. It turns out that yeast do it to protect themselves. When they take the energy from the sugar, they grow and move a little. Once they have started to move, they begin producing ethanol. The thinking is that once they have taken some energy, they start to produce ethanol to get rid of the extra energy so that they don’t move too much and damage themselves. They make alcohol to protect themselves. And we thank them for it. And this is what I learned today.
Photo by Marianna OLE: https://www.pexels.com/photo/pile-of-sliced-wheat-breads-960662/
Sources
https://www.craftybaking.com/how-baking-works/yeast
https://www.thespruceeats.com/basic-yeast-information-304312
https://www.thekitchn.com/the-science-behind-yeast-and-how-it-makes-bread-rise-226483
https://en.wikipedia.org/wiki/Yeast
https://www.sciencedaily.com/releases/2019/01/190107112950.htm
https://www.exploratorium.edu/explore/cooking/bread-science
https://www.oculyze.net/does-yeast-die-after-fermentation/