yam
A large tuber that grows in tropical and subtropical climates; it has starchy, pale-yellow flesh. The name yam is also given to the (botanically unrelated) sweet potato.

yeast
Microscopic fungus whose metabolic processes are responsible for fermentation. It is used for leavening bread and in the making of cheese, beer, and wine.

yogurt
Milk cultured with bacteria to give it a slightly thick consistency and sour flavor.

zest
The thin, brightly colored outer part of citrus rind. It contains volatile oils, making it ideal for use as a flavoring.