In Vitro Measurement of Effects of Processing on Protein Nutritional Quality ¹

Effects of processing on protein structure and its nutritional consequences, and progress towards development of more rapid in vitro assays of nutritional quality are reviewed. Heat and/or alkali processing of proteins initiates Maillard and carbonylamine reactions, causes β-elimination of cystinyl and substituted seryl and threonyl residues, and causes racemization of certain residues. Depending on the extent of these reactions, as determined by the severity of processing conditions, resulting changes may adversely affect bioavailability. Chemical methods for assaying quality have been developed such as the "Chemical Score," which is based on amino acid analysis of acid hydrolysates, and methods for determining available lysine by reaction with FDNB. Recently a more rapid fluorometric method for measuring available lysine based on reaction with o-phthalaldehyde and mercaptoethanol was developed. Progress is also being made toward improvement of amino acid scores by replacing acid hydrolysis with total enzymic hydrolysis, which should be sensitive to chemical modifications of residues that are eliminated by acid hydrolysis. Reactors containing combinations of immobilized proteinases and peptidases are being characterized for this purpose. Some improvement of amino acid scores is also afforded by adjustments for protein digestibility. Studies of model digestive systems composed of immobilized gastric, pancreatic and intestinal mucosal proteinases and peptidases indicate that such systems may provide parameters reflecting bioavailability.