Optimization of the Reaction between 5-O-Caffeoylquinic Acid (5-CQA) and Tryptophan—Isolation of the Product and Its Evaluation as a Food Dye

The food industry is seeking a stable, non-toxic red dye as a substitute for synthetic pigments. This can result from the reaction between 5-O-Caffeoylquinic acid (5-CQA) and tryptophan (TRP). This study explores the reaction kinetics under ultrasound conditions and investigates reaction parameters, such as pH, temperature, and reactants’ concentrations, to accelerate the reaction. At the end of the reaction, the solution was either spray-dried or acidified to isolate the pigment, which was evaluated for its potential as a food dye. Using ultrasound at 40 °C led to a significant acceleration of the reaction that was completed in 8.5 h, marking a 300% improvement compared to literature. The caffeic acid, and not the quinic acid, moiety of 5-CQA seems to be partly responsible for the formation of the red pigment. The pH had a profound impact on the reaction rate, with an optimal value of pH = 9.5. Increased TRP concentrations led to increased reaction rates, while higher 5-CQA concentrations led to significant deviations from redness (a* value). The pigment, lacking significant antimicrobial activity, exhibited remarkable thermal stability (pH 3–9), delaying food oxidation and color deterioration. The results indicate that the reaction can be significantly accelerated by ultrasound, which will be useful for the scale-up of the process and giving the produced pigment the potential for use as an alternative to artificial coloring.