Tapai is a traditional fermented preparation of rice or other starchy foods, and is found throughout much of Southeast Asia, especially in Austronesian cultures, and parts of East Asia. It refers to both the alcoholic paste and the alcoholic beverage derived from it. It has a sweet or sour taste and can be eaten as is, as ingredients for traditional recipes, or fermented further to make rice wine. Tapai is traditionally made with white rice or glutinous rice, but can also be made from a variety of carbohydrate sources, including cassava and potatoes. Fermentation is performed by a variety of moulds including Aspergillus oryzae, Rhizopus oryzae, Amylomyces rouxii or Mucor species, and yeasts including Saccharomyces cerevisiae, and Saccharomycopsis fibuliger, Endomycopsis burtonii and others, along with bacteria.

In food processing, fermentation is the conversion of carbohydrates to alcohol or organic acids using microorganisms—yeasts or bacteria—under anaerobic (oxygen-free) conditions. Fermentation usually implies that the action of microorganisms is desired. The science of fermentation is known as zymology or zymurgy.