By Harold McGee
Fiber is defined as the material in our plant foods that our digestive enzymes can’t break down into absorbable nutrients. These substances therefore aren’t absorbed in the small intestine, and pass intact into the large intestine, where some are broken down by intestinal bacteria, and the rest are excreted. The four main components of fiber come from plant cell walls. Cellulose and lignin form solid fibers that don’t dissolve in our watery digestive fluids, while pectins and hemicelluloses do dissolve into their individual molecules. Minor components of fiber include uncooked starch and various gums, mucilages, and other unusual carbohydrates (e.g., mushroom chitin, seaweed agar and carrageenan, inulin in onions, artichokes, and sunchokes). Particular foods offer particular kinds of fiber. Wheat bran—the dry outer coat of the grain—is a rich source of insoluble cellulose, while oat bran is a rich source of soluble glucan (a carbohydrate), and juicy ripe fruits are a relatively dilute source of soluble pectins.