Kvass is a fermented cereal-based low-alcohol beverage with a slightly cloudy appearance, light-brown colour and sweet-sour taste.
Kvass originates from Northeastern Europe, where grain production is considered insufficient for beer to become a daily drink. The first written mention of kvass is found in the Primary Chronicle, describing the celebration of Vladimir the Great's baptism in 996. In the traditional method, kvass is made from a mash obtained from rye bread or rye flour and malt soaked in hot water, fermented for about 12 hours with the help of sugar and bread yeast or baker's yeast at room temperature. In industrial methods, kvass is produced from wort concentrate combined with various grain mixtures. It is a popular drink in Russia, Ukraine, Poland, Baltic countries, some parts of Finland, Sweden and China.
The word kvass is ultimately from Proto-Indo-European base *kwh2et- ('to become sour'). In English it was first mentioned in a text around 1553 as quass. Nowadays, the name of the drink is almost the same in most languages: in Belarusian: квас, kvas; Russian: квас, kvas; Ukrainian: квас/хлібний квас/сирівець, kvas/khlibny kvas/syrivets; in Polish: kwas chlebowy (bread kvass, to differentiate it from kwas, 'acid', originally from kwaśny, 'sour'); Latvian: kvass; Romanian: cvas; Hungarian: kvasz; Serbian: квас/kvas; Chinese: 格瓦斯/克瓦斯, géwǎsī/kèwǎsī; Eastern Finnish: vaasa. Non-cognates include Estonian kali, Finnish kalja, Latvian dzersis (beverage), Latgalian dzyra (beverage, similar to Lithuanian gira), Lithuanian gira (beverage, similar to Latvian dzira), and Swedish bröddricka (bread drink).
In the traditional method, either dried rye bread or a combination of rye flour and rye malt is used. The dried rye bread is extracted with hot water and incubated for 12 hours at room temperature, after which bread yeast and sugar are added to the extract and fermented for 12 hours at . Alternatively, rye flour is boiled, mixed with rye malt, bread yeast, sugar, and baker's yeast and then fermented for 12 hours at .