Amino acyl residues, effect racemization

Racemization of amino acyl residues in food proteins is a reaction that can take place during processing and cooking. This review deals with the occurrence and detection of alkali- and heat-induced racemization in proteins. Differences between calcium hydroxide-and sodium hydroxide-induced racemization and the effects of treatment with these alkalis on protein bioavailability is discussed. 

Recent studies have shown that in addition to the structure of the amino acyl residue, the position of the residue in the peptide (or protein) can have a major effect on racemization (69). Therefore, at the end of an exposure to alkali, and depending on the severity of the treatment, a mixture of the original protein and several D-amino acyl residue-containing proteins is likely to result. The latter are not necessarily Identical, i.e., the D-amlno acyl residues may be located at different positions along the primary structure of the protein, thereby giving rise to a heterogenous mixture of racemized proteins. 

While several laboratories have shown that severe racemiza-tion of proteins can occur during treatment with sodium hydroxide (6,18,22-24,61,62) the effects of other alkalis used in food processing are documented less well. Jenkins, et al. (70) have observed substantial differences in the degree of racemization caused by lime or caustic soda treatment of zein. Lime causes only 50% to 90% of the racemization observed for several amino acyl residues compared to when caustic soda is used. Because a substantial amount of calcium ion remained bound to the protein (approx. 10,000 ppm) compared to l/20th that amount of sodium ion for the caustic soda-treated zein, it is possible that divalent calcium may stabilize the protein making it less susceptible to racemization. Tovar (14) observed increases of 40% to 50% in serine and phenylalanine racemization and a decrease of 30% aspartate racemization for caustic soda-treated fish protein concentrate compared to lime-treated protein (see Table II). These studies indicate that different alkalis have different effects on racemization of proteins specifically, lime may cause less racemization than caustic soda at a similar pH. 

---