Kvass or kvas (from Old East Slavic квасъ, kvasŭ, meaning "yeast" or "leaven"; today, in Belarusian: квас, kvas, сиривець, siriviets; Lithuanian: gira; Russian: квас, kvas; in Ukrainian: квас, хлібний квас or сирівець, kvas, khlibnyy kvas or syrivets; Polish kwas; meaning "acid" in the 16th century), sometimes called a bread drink in English, is a fermented beverage made from black or regular rye bread. In the Ukrainian language, kvass is associated with either a sourness or a fermentation process depending on a region of Ukraine.
The colour of the bread used contributes to the colour of the resulting drink. It is classified as a non-alcoholic drink by Russian standards, as the alcohol content from fermentation is typically less than 1.2%.
It is popular in Russia, Belarus, Ukraine, Lithuania, Poland, Latvia, and other Eastern and Central European countries as well as in all ex-Soviet states, like Uzbekistan, where one can see many kvass vendors in the streets.
The alcohol content is low (0.05% - 1.0%). It is often flavoured with fruits or herbs such as strawberries or mint. Kvass is also used for preparing a cold summertime soup called okroshka.