The main sources of yeast are bread products, alcohol, soup stocks or gravies, salad dressings and ripe or fermented fruits. Most bread products contain baker's yeast, including bagels, pastries, muffins, crumpets and croissants. Some bread products that don't contain yeast are soda bread, crackers, pita bread and rice cakes.
Beer, wine, sake and cider contain yeast, as do most bouillon cubes and vinegar-based foods such as salad dressings and mayonnaise. Certain fruits such as grapes and plums have a naturally occurring whitish coating of yeast covering the skin. Yeast is also present in dried fruit, buttermilk and tofu, and it may grow on foods that are opened and stored indefinitely, such as jam or leftovers.