Yeast is a type of fungus that reacts with sugar and produces carbon dioxide gas, which causes dough to rise. This process, known as fermentation, creates air pockets in the dough and gives baked goods their light and fluffy texture.

When yeast is combined with warm water and sugar, it becomes active and begins to multiply. As it feeds on the sugar, it releases carbon dioxide gas, which gets trapped in the gluten structure of the dough. The dough expands as the gas is produced, resulting in a light and airy final product.

There are several types of yeast available for baking, including active dry yeast, instant yeast, and fresh yeast. Active dry yeast is the most commonly used type and needs to be dissolved in water before use. Instant yeast, also known as rapid-rise or quick-rise yeast, does not require proofing and can be added directly to the dry ingredients. Fresh yeast, also called cake yeast, is moist and should be crumbled into the dough.

To ensure successful baking with yeast, it's important to follow a few key tips:

• Check the expiration date of your yeast before using it, as expired yeast may not produce the desired results.
• Use warm water between 100-110°F (37-43°C) to activate the yeast. Water that is too hot can kill the yeast, while water that is too cold may not activate it.
• Allow the dough to rise in a warm environment, such as a turned-off oven or a warm spot in your kitchen. Yeast activity slows down in cold temperatures.

If you don't have yeast on hand, there are a few substitutes you can use:

• Baking powder: Mix 1 teaspoon of baking powder with 1/4 teaspoon of baking soda and 1/2 teaspoon of cream of tartar. This can be used as a substitute for 1 teaspoon of yeast.
• Sourdough starter: Sourdough starter is a naturally fermented mixture of flour and water. It can be used as a substitute for yeast in bread recipes.

Avoid these common mistakes when working with yeast:

• Using water that is too hot or too cold to activate the yeast.
• Not proofing the yeast properly, resulting in dough that doesn't rise.
• Using expired yeast.
• Not giving the dough enough time to rise.

Yeast plays a crucial role in baking by leavening the dough and creating light and fluffy breads and pastries. Understanding how yeast works and following proper techniques can help you achieve perfect results every time. Experiment with different types of yeast and try out new recipes to discover the endless possibilities of baking with yeast.