Oxidative Browning of Amadori Compounds from Amino Acids and Peptides

Abstract

The oxygen-dependent browning (oxidative browning) of Amadori compounds from amino acids or peptides was studied since it was an important reaction contributing to the discoloration and deterioration of some foodstuffs containing sugars and amino compounds during storage in contact with atmospheric oxygen.

The browning of fourteen Amadori compounds derived from amino acids and peptides was determined under the influence of metals or oxygen. All Amadori compounds exhibited remarkable browning during storage at 37°C for 5 days when both 40 ppm of Fe²⁺and oxygen were present, but exhibited little browning without Fe²⁺or oxygen. Every mixture of a parent sugar and amino compound showed no browning even though Fe²⁺and oxygen were present. In particular, Amadori compounds composed of aromatic or heterocyclic amino acids were very reactive in oxidative browning, and this type of browning was synergistically accelerated by the presence of both Fe²⁺and Mn²⁺. The Amadori compound derived from pentose such as xylulose-glycine browned more rapidly than that from hexose such as fructose-glycine. Oxygen was thought to accelerate the breakdown of Amadori compounds to liberate parent amino acids and glucosone in the oxidative browning reaction.

In the browning reaction between glucose and seven peptides, glycylglycine, glycylleucine, leucylglycine, glycyltyrosine, glycylphenylalanine, glycylproline and glycylglycylglycine, the liberation of C-terminal amino acids by the cleavage of peptide bonds was observed. The amino acids were suggested to be liberated from the peptide in Amadori compounds, because the peptide bond in Amadori compounds was found to be more labile from the peptide.