Improved Generation, Physicochemical Characteristics, and Food Application Studies of a Red Colorant Obtained from Oxidative Coupling of Chlorogenic Acid and Tryptophan.

Due to a widespread consumer reluctance toward synthetic food dyes, the interest in natural compounds from plants has increased. This study aimed to optimize the oxidation process between chlorogenic acid (CQA) and tryptophan (Trp) using sodium periodate (NaIO4) to obtain a red-colored pigment. The impact of temperature and different ratios of Trp to NaIO4 on the reaction progress was investigated. After the best conditions for the reaction were established, three pH values were tested. The reaction time could be reduced from 72 to 24 h with a yield of 46 ± 2% w/w based on the quantity of CQA. After the first purification step of the product by size exclusion chromatography, the pigment obtained was characterized for its solubility, and its hydrolyzed form was used for investigations into the stability at different pH values, storage under light and in the dark (period of 28 days), in the presence of reducing agents, and for heat resistance. Finally, several food matrices were successfully colored with the natural pigment in amounts from 0.005 to 0.01% (w/w). In conclusion, the present study provides new insights into the feasible production and comprehensive characterization of a red pigment derived from oxidative coupling of CQA and Trp, as well as its application in food systems.

Generation and structure elucidation of a red colorant formed by oxidative coupling of chlorogenic acid and tryptophan.

In view of the poor acceptance of synthetic food colorants by consumers, there is intense interest in novel natural compounds, preferably from plant-derived sources. We oxidized chlorogenic acid using NaIO4 and reacted the resultant quinone with tryptophan (Trp) to obtain a red-colored product. The colorant was precipitated, freeze-dried, purified by size exclusion chromatography, and subsequently characterized using UHPLC-MS, high-resolution mass spectrometry, and NMR spectroscopy. Additional mass spectrometric studies were performed on the reaction product generated with Trp educts labeled with 15N and 13C. The data obtained from these studies allowed the identification of a complex compound consisting of two Trp and one caffeic acid moieties, and the proposition of a tentative pathway of its formation. Thus, the present investigation expands our knowledge about the formation of red colorants based on the reaction of plant phenols and amino acids.