Protein-Bound Maillard Compounds in Foods: Analytical and Technological Aspects

By means of suitable analytical techniques, the formation of protein-bound Maillard compounds in foods was studied. The lysine derivative pyrraline could be detected in enzymatic hydrolysates of milk and bakery products at levels up to 3700 mg/kg protein by ion-exchange chromatography (IEC) with photodiode-array measurement and by ion-pair RP-HPLC. During storage of freeze-dried milk, the rate of formation of pyrraline correlated directly with the water content of the samples. In acid hydrolysates, sensitive determination of the fluorescent arginine-lysine crosslink pentosidine was achieved using IEC with direct fluorescence detection. Levels of pentosidine in various foods ranged between ‘not detectable’ (< 50 μg/kg protein) and 35 mg/kg protein, indicating that pentosidine does not play a major part in crosslinking of food proteins. A previously unknown protein-bound arginine derivative, NΔ-(5-methyl-4-oxo-5-hydroimidazol-2-yl)-L-ornithine, was isolated from acid hydrolysates of bakery products. The ornithinoimidazolinone is formed by direct condensation of the guanido group of arginine and the sugar degradation product methylglyoxal, thus representing a new post-translational modification of food proteins. For certain foods (baking products, roasted coffee), between 20 and 50% of the arginyl residues might react with methylglyoxal during food processing.