Kvass is a traditional Slavic beverage made from fermented RYE bread. The main ingredient in kvass is RYE, which is why it is the answer to the crossword clue 'Kvass Ingredient'. RYE is a cereal grain that has been used for centuries in baking and brewing. It is known for its nutty flavor and is often used in breads, cereals, and other baked goods. RYE is also used to make beer, whiskey, and other alcoholic beverages. Kvass is a refreshing and slightly sweet drink that is popular in Eastern Europe and Russia.