Trihydroxybenzacridine pigments by oxidative coupling of chlorogenic acid with amino acids and proteins: toward natural product-based green food coloring

Introduction: The occasional greening of sweet potatoes and other plant tissues observed during cooking was shown to arise from the autoxidative coupling of chlorogenic acid (5-caffeoylquinic acid, CQA) with amino acid/protein leading to trihydroxybenzacridine pigments. To explore the potential of this reaction for food coloring, we report herein the optimized preparation of trihydroxybenzacridine pigments from CQA and glycine, lysine, proteins or protein-rich food matrices. Material & Methods: The reaction of CQA with aminoacids or bovine serum albumin (BSA) was carried out at pH 9. The resulting green mixtures were purified by gel filtration chromatography. Pigments were analysed on an UPLC system with diode array detector and ESI/ion trap analyser . Results: UHPLC-MS/MS analysis of the purified pigments confirmed the presence of trihydroxybenzacridines.. Similar pigments were obtained by reaction of CQA with BSA or Chichen Egg White (CEW). For all materials, no significant toxicity against Caco-2 and HepG2 was observed at doses compatible with common use in food coloring. Additions of the pure pigments or pigmented CEW to different food matrices imparted intense green hues exhibiting satisfactory thermal stability up to 90°C. Conclusions: The results obtained reinforce the potential of this class of pigments for food coloring.