Mechanisms of heat damage in proteins

Abstract

I . Bovine plasma albumin (BPA) containing approximately 14 % moisture, when heated for 27 h at I I~O suffered an appreciable loss of cystine and a small loss of lysine; at 1 4 5~ all the amino acids except glutamic acid and those with paraffin side-chains, showed considerable losses. Isoleucine also showed some loss through racemization to alloisoleucine.2. BPA heated at I 1 5~ evolved H2S; at 145' other sulphur compounds were released as well, all coming from the breakdown of cystine. Possible mechanisms for this are discussed.3. Ammonia was also liberated from BPA heated at I I 5'. The degree of correlation of lysine binding in different proteins with ammonia liberation and amide changes has led us to suggest that the main reaction of €-amino lysine groups is with amide groups of asparagine and glutamine. Reaction of e-amino groups with carboxylic groups is thought to be less important. 4.Model experiments have shown that a reaction between amide groups and the e-amino group of lysine in proteins can occur at practical drying temperatures. .Reactions of the e-amino group of lysine with destruction products of cystine is also considered to be partially responsible for the lysine binding in heated proteins.It has long been known that the Maillard reaction between sugar aldehyde groups and the free amino acid groups of proteins can explain much of the damaging effect of heat on the protein quality of dried milk. However, heat damage can also occur if meat or fish products are heated excessively and these are essentially carbohydratefree. This too is accompanied by a loss of reactivity of the s-amino groups of lysine. Any oxidizing fat present may contribute carbonyl groups for a Maillard-type reaction but high-temperature heat damage cannot be considered as being just due to reaction with other non-protein compounds present, since it will occur to the same extent with pure protein preparations (Carpenter, Morgan, Lea & Parr, 1962). The changes also occur in an atmosphere of nitrogen as well as of oxygen (Carpenter, Ellinger, Munro & Rolfe, 1957).This study was designed to investigate the chemical changes which must occur during the heating of proteins as such, and be responsible for the nutritional damage. Because of the multiplicity of functional groups and of the many possible reactions which may occur during the heating, analytical work is difficult, and very little is known about what actually happens. Apart from amino acid destructions only the binding of lysine is well established. It has been suggested that the binding of lysine is caused by a reaction between the €-amino group of lysine and carboxylic groups in the protein and that such cross-linking reduces the nutritional availability of all the essential amino acids (cf. Mecham & Olcott, 1947 J. BJARNASON AND K. J. CARPENTER I970 E X P E R I M E N T A L Test materialsBovine plasma albumin (BPA) and lactalbumin were from the same source as in our first study (Bjarnason & Carpenter, 1969) and heated samples were prepared in the same way. The vacuum-d...

NO and N2O precursors (NH3 and HCN) from biomass pyrolysis: Co-pyrolysis of amino acids and cellulose, hemicellulose and lignin

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Effect of reaction conditions on product distribution from the co-pyrolysis of α-amino acids with glucose

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Protein, energy and carotene content of maize at six stages in the preparation of Ufa by village processing methods

“…The milling process, the final step in the production of flour, has chemical effects 276 T. NINJE AND F.J. WEAVER in that heat is produced which may cause some ammonia to be liberated (Bjarnason and Carpenter, 1966). Heat may also directly destroy some amino acids (Hoffman et al, 1977).…”