Sulphydryl (SH) groups and disulphide bonds are important in maintaining the structure and functional properties of food proteins. They play an important role in the formation of relatively rigid complexes as in protein gels and doughs. Heating affects the proportion of cysteine/cystine residues (ie. SH/S-S groups, respectively) and has also been found to reduce protein utilisation by animals. It has been postulated from studies which utilised fish protein that heat induced S-S linkages from SH group oxidation hamper the action of proteolytic enzymes and cause a reduction in protein and amino acid digestibility (Opstvedt et al.,1984). An examination of literature data on pigs also show that the amino acid cystine, is often among the least digestible amino acids. Secondly, proteins that are typically high in cystine or S-S bond content such as blood, feather and hair meals, are all known to have low in-vitro or in-vivo nitrogen digestibilities.