Measurement of Available Lysine in Heated and Unheated Foodstuffs by Chemical and Biological Methods

Abstract

The availability of lysine in meat, and in egg albumin before and after heat treatment in the presence of glucose, was determined by rat growth assay and by the fluorodinitrobenzene (FDNB) method. A 25% gluten diet was used as a basal diet to which graded levels of lysine or the protein under test were added. There was a high correlation between lysine consumed and weight gain of rats fed the standard diets for 2 weeks. Autoclaving of egg albumin to which 1 to 2% glucose had been added decreased the availability of lysine as measured by the FDNB method. However, the rat-growth assay gave a far lower value. This lowering was found not to be due to the formation of toxic compounds nor to loss of methionine. Similar results were obtained using autoclaved beef. Digestibility measurements of autoclaved egg albumin gave values similar to the value obtained by rat-growth assay. Fecal analysis showed that not only was lysine excretion increased but also excretion of other amino acids as well. The FDNB method does not appear suitable for the estimation of availability of lysine in heat-treated foods. It appears from fecal excretion studies that some lysine in heat-treated proteins measured as available by the FDNB method may be excreted as part of the undigestible residue in the feces.