Kvas (Cyrillic: квас) is a low-alcoholic cereal beverage which is traditionally produced from rye and barley malt, rye flour, and stale rye bread in many Balto-Slavic countries. In Poland it is known as kwas chlebowy (litt. “bread acid”), Latvians call it kvass, Lithuanians call it gira and Estonians kali. In some Russian cities the name kvas (квас) can also refer to another, unrelated popular fermented drink called kombucha, but traditional bread kvas goes back a long time in Russian history and hence is well-known in all outskirts of the country.
Kvas is similar to boza, a Turkish, low-alcoholic fermented beverage, both with respect to the composition of the final product as well as to the microflora present. Boza is well-known in the Turkestan region (Kazakhstan, Kyrgyzstan), Turkey, Bulgaria, Romania and the Balkan countries that were once part of the Ottoman Empire, such as Serbia and Bosnia-Herzegovina. The main difference with kvas is that boza seems to be mainly wheat-based (with a lot of local variations) while kvas is based on rye bread.
Two main kvas-making techniques exist using either stale sourdough bread, or malt as main raw materials. In kvas fermentations from stale sourdough bread (traditional Balto-Slavic type black rye bread), all sugars needed for yeast fermentation are derived from the bread-making process.
In the second technique, gelatinized starch is cleaved by malt enzymes. Prior to fermentation, the kvas batter is diluted in boiling water and clarified by sedimentation.
Sucrose is added to the kvas wort and fermentation is initiated by addition of baker’s yeast or a previous batch of kvas. Fermentation is terminated by chilling of the product to 4°C before nutrients are exhausted.
Kvas generally has a golden-brown color, but this can vary according to the ratio of rye to wheat present in the bread, and the length of fermentation. Kvas has the pleasant flavor of rye bread, a low sweetness and should not taste obviously alcoholic. It also should have produced sufficient carbon dioxide to give it a sparkle on the palate.
The final product contains carbohydrates, proteins and amino acids, lactic and acetic acids and vitamins originating from the raw materials or as a result of microbial activity.
Composition and characterisation of the microflora in kvas
Many consider the kvas to be spoiled if ethanol accumulates to higher levels, or at least not suitable any longer as a substitute for soft-drinks, as children are allowed to drink kvas. Others like myself prefer a somewhat stronger kvas although it is debatable whether such a product should still be considered kvas and not a kind of beer. I have at least heard on one occasion such an over-fermented product being smilingly referred to as “soldier’s kvas”.
The high cell counts of Saccharomyces cerevisiae present are obviously derived from the use of baker’s yeast to initiate the fermentation and the yeast produced the lion’s share of ethanol and carbon dioxide present. The lactic acid bacteria present likely originate from malt which is used as raw material, from baker’s yeast, or can originate from a bakery environment where sourdough bread is produced. Baker’s yeast is frequently contaminated with high cell counts of lactic acid bacteria that grow to high cell counts upon prolonged fermentation in cereal substrates, e.g. sponge doughs.
Cereal fermented foods are thus a suitable alternative to milk-based carriers for probiotic bacteria and kvas containing high cell counts of viable lactic acid bacteria may serve as a blueprint for the development of cereal-based probiotic functional beverages.
The presence of isomaltotriose as the major oligosaccharide formed in model kvas reflects the availability of glucose as dominant acceptor carbohydrate. Isomaltotriose was not detected, possibly because the overall carbohydrate turnover in kvas was mainly attributable to yeast metabolism. Sucrose hydrolysis by yeast invertase competes with
transglucosylation by dextransucrase.