Chemical and Physical Properties of Starch

The factors that have made starch an ideal reserve material for green plants have also made it an attractive food for animals. Few nutrients have a higher energy content per gram, or per cc, than starch, fats being the main exception. Other polysaccharides such as cellulose are too insoluble, to be rapidly digested, and the more water-soluble gums have a low energy density. Proteins have a fairly high energy content per gram of dry material, but in natural foods they are often heavily hydrated. Moreover, the useful energy yield is reduced because of the energy requirement in the metabolism of protein breakdown products. Inasmuch as starch is such an ubiquitous plant constituent, all plant-eating animals have developed enzymic machinery for its digestion. The organization of starch into granules makes it somewhat resistant to amylase action, although pancreatic amylase will act on many raw starches. The branching in starch poses problems to the digestive enzymes, but it simultaneously renders starch much more soluble and therefore more digestible. Finally, many starch properties depend on the particular source of starch, as well as on its previous treatment. The chains of starch are capable of an infinite variety of molecular conformations and interactions. These affect starch's physical nature and hence its performance in a biological situation.