Influence of processing on the digestibility of amino acids and starch in cereals and legumes in non-ruminants

Abstract

Non-ruminant livestock diets in most regions of the world are based on cereals and plant proteins that are particularly important in view of the current ban on animal proteins within the European Union, although they are still valuable raw materials elsewhere. The major component of cereals is starch that makes a significant contribution to energy-yielding value. Starch has traditionally been viewed as being very well digested, although, increasingly, this statement is being challenged. Thus, native (raw) starch is, to a varying degree depending on the origin, crystalline which is less well digested than is amorphous starch. Processing (invariably heat treatment) reduces the degree of crystallinity of starch, leading to better digestibility, particularly in young animals. For newly weaned pigs, processing can overcome, to an extent, the post-weaning growth check. Extrusion can improve the coefficient of apparent digestibility of starch in wheat three-quarters along the small intestine, from a range of 0.50 to 0.85 (raw) to 0.95. Plant proteins invariably contain naturally occurring anti-nutritive factors, principally trypsin inhibitors that are particularly important in soya beans but also occasionally in peas. The inhibitors are heat labile and denatured by heat. There are several technologies available for processing plant proteins, but a key message is that equipment operates under variable conditions of temperature, duration and moisture addition. Over-processing risks protein being denatured; for example, a trypsin inhibitor activity of 1.5 mg/g is associated with a reduction in amino acid digestibility. It is crucial that processing conditions are defined accurately rather than simply providing the name of the equipment.