Influence of molar ratio and pH on physicochemical characteristics of genipin-derived pigments formed at cold and mild temperatures

The reaction genipin-amino acids at acidic-alkaline conditions at 7; 37 °C was assessed and products were analyzed at the nanoscale (hydrodynamic size, concentration, and size distribution) for the first time, including FTIR analysis. Lysine and glycine pigments formed under all conditions except at pH 4.0, 7 °C. Lysine, tyrosine, and glycine at alkaline media showed a proportional increase in pigment concentration with temperature. Glycine pigments were bimodal and polydisperse (100 °C), whereas multimodal and polydisperse at 37; 7 °C. At acidic media, arginine, lysine, and glycine pigments were also multimodal and polydisperse. Leucine, tryptophan, tyrosine, and threonine did not react under unadjusted pH 7 °C but did so at basic media at 7; 37 °C, except threonine. Particle concentration from tryptophan and threonine pigments decreased from 7 - 37 °C but increased at 100 °C.

Only pH showed significant statistical differences in particle size and concentration, with the highest particle size at alkaline conditions (∼ 200 nm), higher than those under acidic media (∼ 170 nm). FTIR spectra confirmed pigment formation (∼1551 cm⁻¹, ∼1658 cm⁻¹, and ∼1400 and 1000 cm⁻¹), and PCA discriminated spectra at different pH. These results provide new insights for leveraging the production of genipin-derived pigments.