Yeast, that magical microscopic fast-multiplying fungus that makes bread dough rise and beer and Champagne effervescent, comes in various fresh and dry forms. When mixed with flour and liquid at a warm temperature to make dough, yeast ferments, converting sugar and starch into ethanol and carbon dioxide gases that allow the bread to rise.

Fresh yeast: Fresh yeast is a type of baker's yeast that comes in squares. It is light beige and has a yeasty fresh smell (if the odor is too strong or the color is too dark, it is past its prime). It must be refrigerated. When you're ready to use it, fresh yeast is mixed with lukewarm water and sugar until it froths up from fermentation and can be mixed in with other ingredients. Once dough rises once, it is punched down and allowed to rise again before baking.

Active dry yeast: Dehydrated baker's yeast keeps longer than fresh yeast. Regular dry yeast is activated by adding lukewarm water and a bit of sugar and left to foam up before mixing with other ingredients and allowed to rise twice before baking. Instant, fast-acting or quick-rising yeast can be mixed in with other ingredients and only needs to rise once before baking.

Brewer's yeast: Not suitable for baking, brewer's yeast is a non-leavening yeast used to make beer and as a dietary supplement.

Nutritional yeast: This deactivated yeast is grown on mineral-enriched molasses, then pasteurized to kill the yeast; unlike live or brewer's yeast, it tastes good and is also an excellent source of protein, B vitamins and minerals that help stabilize blood sugar.

Yeast starter: The yeast starter was the leavening method for bread before commercial yeast was developed in the 19th century. It is a mixture of yeast, flour, sugar and water that is fermented and kept refrigerated until needed, then brought to room temperature and used to make a fresh batch of dough by refreshing it with water and flour.