Question: Is rye an ingredient of Kvass?
About the article "Yeast", section "Nonalcoholic beverages".

Candidate passages:
Rye bread, including pumpernickel, is a widely eaten food in Northern and Eastern Europe.
Rye is also used to make crisp bread.
Rye flour is high in gliadin but low in glutenin.
It therefore has a lower gluten content than wheat flour.
It also contains a higher proportion of soluble fiber.
Alkylresorcinols are phenolic lipids present in high amounts in the bran layer (e.g.
pericarp, testa and aleurone layers) of wheat and rye (0.1-0.3 % of dry weight).
Rye is used to make alcoholic drinks, like rye whiskey and rye beer.
Other uses of rye include kvass and an alternative medicine known as rye extract.
Rye straw is used to make corn dollies. 
Kvass is made by the natural fermentation of bread, such as wheat, rye, or barley, and sometimes flavoured using fruit, berries, raisins, or birch sap collected in the early spring.
Modern homemade kvass most often uses black or regular rye bread, usually dried (called plural "suhari"), baked into croutons, or fried, with the addition of sugar or fruit (e.g., apples or raisins), and with a yeast culture and "zakvaska" ("kvass fermentation starter").
Commercial kvass, especially less expensive varieties, is occasionally made like many other soft drinks, using sugar, carbonated water, malt extract, and flavourings.
Better brands, often made by beer rather than soft drink manufacturers, usually use a variation of the traditional process to brew their products.
Kvass is commonly served unfiltered, with the yeast still in it, which adds to its unique flavour as well as its high vitamin B content. 
It is a drink of slightly burning yet piquantly pleasant taste, rye-bread-like aroma and clear dark brown colour.
One of the ingredients of the bitters that make it special is bread kvass. 
In Lithuania kvass is known as "gira", and is widely available in bottles and draft.
Many restaurants in Vilnius make their own gira which they sell on the premises.
Strictly speaking, gira can be made from anything fermentable — such as caraway tea, beetroot juice, or berries — but it is made mainly from black bread or barley/rye malt.
The only breweries in the United States which brew kvass all year round are the Beaver Brewing Company in Beaver Falls, Pennsylvania,where it is made with the addition of raisins and lemons and Kolokol kvass in Helena, MT.
using traditional ingredients (Water, Sugar, Bread{Rye}, Yeast).
In the United Kingdom, kvass is practically unknown as there are no cultural ties to it within the nation's history and there are no renowned kvass breweries in the country.
However, with the influx of immigrants following the 2004 enlargement of the European Union, a number of stores selling cuisine and beverages from Eastern Europe cropped up throughout the UK - many of them storing kvass on their shelves. 
Kvass has a long tradition in Russian culture.
In Fyodor Dostoyevsky's "The Brothers Karamazov", monastery kvass is mentioned in the dinner scene as being famous throughout the neighborhood.
In Leo Tolstoy's "The Death of Ivan Ilyich", kvass is made first thing on a holiday morning.
In Ivan Goncharov's "Oblomov" and in Tolstoy's "Anna Karenina", kvass is repeatedly mentioned.
In Tolstoy's "War and Peace", French soldiers are aware of kvass on entering Moscow, enjoying it but referring to it as "pig's lemonade".
In Anton Chekhov's "The Cherry Orchard", kvass is mentioned early in the play, "Bring me some kvass, would you?".
In Sholem Aleichem's "Motl, Peysi the Cantor's Son", diluted kvass is the focus of one of Motl's older brother's get-rich-quick schemes.
The Russian expression "Перебиваться с хлеба на квас" (literally "to clamber from bread to kvass") means to barely make ends meet, remotely similar to (and may be translated as) the expression "to be on the breadline".
To better understand the Russian phrase one has to know that in poor families kvass was made from stale leftovers of rye bread.
In Joris-Karl Huysmans' "Against Nature" "(À rebours)" the protagonist, Jean Des Esseintes serves kvass, along with porter and stout, for a funeral banquet "in memory of the host's virility, lately but only temporarily deceased."
In Jules Verne's 1896 novel "Michael Strogoff, Courier of the Czar", the titular character drinks kvass when he stops to change horses at post stations on his way to Siberia.