A new cyanine from oxidative coupling of chlorogenic acid with tryptophan: Assessment of the potential as red dye for food coloring

A red pigment was prepared by reaction of chlorogenic acid (CGA) with tryptophan (TRP) in air at pH 9 (37% w/w yield) and evaluated as food dye. The main component of pigment was formulated as an unusual benzochromeno[2,3-b]indole linked to a TRP unit, featuring a cyanine type chromophore (λmax 542, 546 nm, 1% extinction coefficient of the sodium salt = 244 ± 2). The chromophore showed a minimal pH dependence and proved stable for at least 3 h at 90 °C, both at pH 3.6 or 7.0, whereas red wine anthocyanins showed a substantial (30%) and betanin a complete abatement after 1 h at the acidic pHs. An intense coloring of different food matrices was obtained with the pigment at 0.01 % w/w. No toxicity was observed up to 0.2 mg/mL on hepatic and colonic cell lines. These data make this dye a promising alternative for red coloring of food.