Rye is another member of the wild grass family whose cultivation stretches back into antiquity. Rye is harder than wheat in challenging climates and is frequently grown either side by side or in rotation with wheat to ensure some sort of grain crop. More rye than wheat was grown during the Middle Ages, and it is still a favored grain in the colder European climates, especially the Scandinavian countries.
Whole rye flour does not have enough of or the right kinds of gluten-forming proteins to make light bread by itself. Rye does contain a roughly similar amount of protein, even both of the gluten-forming proteins, glutenin and gliadin, but in proportions smaller than in wheat. The gliadin in rye, however, doesn’t interact with the glutenin in the same way as wheat gliadin does in forming gluten. Most of the structure provided by rye flour in bread comes from the interaction of its proteins with pentosans (gumlike substances) and the gelatinization of its starch. Rye doughs containing more than 20 percent rye flour rely on the viscosity of starches and pentosans to trap carbon dioxide gas and provide structure. Any air trapped in the dough is not enclosed in gluten cells that can expand, but in unstable foam. As the loaf of rye bread enters the oven, in the absence of gluten, the gelatinized starch on the outside of the loaf forms a sort of skin that aids in gas retention. Wheat flour is added in varying amount to compensate for the gluten deficiency in rye.