Yeast is an essential ingredient in both baking and the fermentation of alcohol. Unlike other chemical ingredients like salt or baking soda, yeast is a living organism that metabolises the other ingredients and makes carbon dioxide or alcohol. In bread making, yeast is a leavening agent, like baking powder, but it also adds flavour. In the fermentation of beer, wine and spirits, it digests sugar to make alcohol.
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What Is Yeast?
Yeast is a single cell form of fungus. Like humans, it feeds on carbohydrates and oxygen to produce carbon dioxide, water and waste. In bread baking, the carbon dioxide forms bubbles in the dough, which gives bread its fluffy texture. Yeast is also used to ferment alcohol, which is a waste product of its metabolism. Yeast is what gives both beer and bread its distinct flavour and smell. It's often sold in a dry form, which has been dehydrated in a lab and can be reactivated by adding water.
Dehydrated yeast is most commonly sold in supermarkets in small packets, jars or canisters. The yeast cells in this product are in a form of hibernation, meaning they're still alive. Dry active yeast needs to be activated in the kitchen by adding it to water and letting it come out of hibernation. Baker's yeast comes in general forms, known as strains, that have a higher tolerance for warmer temperatures than other yeasts, which allows them to continue to make bread rise while the dough is being baked.
Brewer's (Wine) Yeast
The yeast used in brewing beer and making wine is far more delicate and susceptible to high temperatures than baker's yeast. In fact, dozens of strains are available for different varieties of wine and beer. While brewer's yeast can quickly die at higher temperatures, it's bred with a high tolerance for alcohol. Most baker's yeast will die in an alcohol mixture of as little as 6 per cent or 8 per cent, but brewer's yeast can survive in alcohol concentrations of up to 18 per cent. It's important to note that baker's yeast and brewer's yeast aren't interchangeable.
Another form of baker's yeast, known as instant yeast, can be added directly into the dry ingredients of a recipe. It's most commonly used in bread machines because it doesn't need time to activate before mixing and kneading. Cooks can also purchase fresh yeast, which is living yeast suspended in a carbohydrate paste. It's use is typically limited to professional bakers because it's not as simple to use as dry active yeast or instant yeast. Yeast is also sold as a nutritional supplement, though it's been deactivated, which means it won't digest recipe ingredients to produce carbon dioxide or alcohol.
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