While legumes are regarded as good sources of protein they are poorly digested so improved digestibility would be a desirable objective in a breeding programme. From the point of view of protein quality they are limited by the sulphur amino acids (SAA) so an increase in these is a primary nutritional objective of a breeding programme and their lysine content is regarded as being of secondary importance. However, some traditional methods of preparation are far more severe than that necessary to cook the food and may result in loss of lysine. These two problems are being investigated. (1) True digestibility of the nitrogen (TD-N) of cooked, white haricot beans (Phaseolus vulgaris) was measured by feeding to rats at 20, 40 and 80% levels in the diet (5, 10 and 20% protein), for 10 days periods when 23 days old and again when 63 days old. TD-N at the three dietary levels was 80, 74 and 67% for the young animals and 63, 55 and 51% when more mature. True digestibility of dry matter (TD-DM) was very much higher at 97, 92 and 83% for younger and 95, 90 and 81% for older animals. The considerable difference between TD-N and TD-DM together with the apparently greater TD-N by the younger rats suggests that the extra faecal N does not come from the undigested food but possibly from residues of digestive juices. Results suggest that it is necessary to standardise the conditions for measuring TD. Five legumes were boiled for 2 h and 8 h; available lysine fell by 5-10% after 2 h and by 15-20% after 8 h. Improvement in flavour appeared to parallel development of the Maillard reaction and loss of available lysine. SAA were still limiting so that NPU does not reveal loss of available lysine. However, in diets where legumes are used to supplement cereals in such proportions that lysine is limiting this loss may become of nutritional importance. © 1979 Dr. W. Junk b.v. Publishers.
