Yeasts have two main uses in food production: baking and making alcoholic beverages. They have been used in this way since ancient times – there is evidence that ancient Egyptians used yeast in breadmaking, and we have been making fermented drinks like beer and wine for millennia.

Baking
Baked goods like bread rise because of the presence of yeast as a raising, or leavening, agent. The most common yeast used in breadmaking is Saccharomyces cerevisiae. It feeds on the sugars present in the bread dough, producing the gas carbon dioxide. This forms bubbles within the dough, causing it to expand. Other ingredients in the mixture have an effect on the speed of the fermentation – sugar and eggs speed it up; fats and salt slow it down.

Brewing
Several different yeasts are used in brewing beer, where they ferment the sugars present in malted barley to produce alcohol. One of the most common is Saccharomyces cerevisiae, the same strain used in breadmaking; this is used to make ale-type beers and is known as a top-fermenting yeast as it forms a foam on the top of the brew. Bottom-fermenting yeasts, such as Saccharomyces pastorianus, are more commonly used to make lagers. They ferment more of the sugars in the mixture than top-fermenting yeasts, giving a cleaner taste.

Winemaking
The alcohol in wine is formed by the fermentation of the sugars in grape juice, with carbon dioxide as a byproduct. Yeast is naturally present on grapeskins, and this alone can be sufficient for the fermentation of sugars to alcohol to occur. A pure yeast culture, most often Saccharomyces cerevisiae, is usually added to ensure the fermentation is reliable. Sparkling wine is made by adding further yeast to the wine when it is bottled. The carbon dioxide formed in this second fermentation is trapped as bubbles.

Do you know?
Yeasts are also common in kefir products, in semi-soft ripened cheeses and fermentation of soy drinks.

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