Which word does NOT belong with the others?

Loaf, sourdough, and pumpernickel are types of bread. A rye is not a bread type. Loaf - Bread that is shaped and baked in one piece and usually sliced before being eaten. Pumpernickel - Dark, dense German bread made from coarsely ground whole-grain rye. Sourdough - Leaven for making bread, consisting of fermenting dough, typically that left over from a previous batch. Rye - A wheatlike cereal plant that tolerates poor soils and low temperatures.