By Harold McGee
Rye contains a large quantity, up to 7% of its weight, of carbohydrates called pentosans (an old term; the new one is arabinoxylans). These are medium-sized aggregates of sugars that have the very useful property of absorbing large amounts of water and producing a thick, viscous, sticky consistency. Thanks to its pentosans, rye flour absorbs eight times its weight in water, while wheat flour absorbs two. Unlike starch, the pentosans don’t retrograde and harden after being cooked and cooled. So they provide a soft, moist texture that helps gives rye breads a shelf life of weeks. Rye pentosans also help control appetite; the dried carbohydrates in rye crisps swell in the stomach, thus giving the sensation of fullness, and they are slowly and only partly digested.