Ian Spencer Hornsey. A history of beer and brewing, page 8. Royal Society of Chemistry, 2003. "A similar, low alcohol (0.5–1.0%) drink, kvass … may be a 'fossil beer'".
Kvass in Oxford English Dictionary. c 1553 Chancelour Bk. Emp. Russia in Hakluyt Voy. (1886) III. 51 Their drinke is like our peny Ale, and is called Quass.
Олег Николааевич Трубачёв и др. Этимологический словарь славянских языков. Академия наук СССР, Москва, т. 13 (1987), с. 153 (Oleg Trubachyov et al. Etymological dictionary of Slavic languages. USSR Academy of Sciences, Moscow, vol. 13 (1987), p. 153; in Russian)
rinet.ru
starling.rinet.ru
Max Vasmer. Russisches etymologisches Wörterbuch. Winter, Heidelberg, 1953–1958 (in German). Russian translation by Oleg Trubachyov: Этимологический словарь русского языка. Progress, Moscow, 1964–1973. квас