Yeast is a single-celled organism that reproduces asexually and has been used in baking and brewing for thousands of years. There are at least 1,500 species of yeast, all of which are technically living organisms. Yeast occurs naturally in the environment and is in the same biological family as fungi such as mushrooms.

The most common use of yeast has been in the making of bread. The yeast reacts with oxygen and helps leaven bread, or make it rise. During Passover, Jewish people will remove the yeast from bread to make flatbread. There is evidence the ancient Egyptians used yeast to make bread around 4,000 years ago.

Brewing wine and beer has also used yeast for centuries to ferment the mixture to make it alcoholic. Different kinds of yeast are used to make ale, lager, spirits and wine. It makes alcohol by reacting with the sugars that naturally occur in these drinks.

Root beers and other soft drinks use yeast to add flavor, but the fermentation process is stopped before the drink becomes alcoholic. This means the drinks are much sweeter than their alcoholic counterparts and contain more carbon dioxide. They also normally have a very low alcohol content, although this is usually around 0.1 percent.

Due to the cellular makeup of yeast, many scientists use it to learn more about human genetics. Studies of yeast cultures led directly to the mapping of the human genome.

Most recently yeast has been used in the production of biofuels. This is because the yeast turns sugar into ethanol which can be used as a diesel substitute in vehicles. The process it goes through is identical to that of making beer or wine.

Many of the probiotic drinks available now use yeast as a supplement. Many vegetarians use yeast as a supplement due to low protein and vitamin amounts in their normal diet.

Yeast can also be processed to create yeast extract. This is then used in a variety of food products such as Marmite and Vegemite.