The main ingredient in rye bread is rye flour, which is made by grinding the grain of the rye plant. Rye is a cereal grain that imparts a distinct flavor and nutty aroma to the bread, differentiating it from wheat bread. Rye flour contains gluten, but in lower amounts compared to wheat, resulting in denser bread with a hearty texture. Rye bread is often darker in color and has a slightly sour taste due to fermentation, especially in traditional recipes. It's a staple in various cultures, particularly in Eastern Europe, where it is often enjoyed with meats and cheeses.