The red pigment prepared from aucubin and amino acid

Aucubin (AC) is an iridoid glucoside contained in Aucuba japonica Thunb. and Plantago asiatica L. AC was hydrolyzed by β-glucosidase to aucubigenin (ACN), a reactive aglycone, and it reacted with a certain amino acid to form a red pigment. The conditions of this coloring reaction were similar to those of the red pigment formed from geniposidic acid (GSA) and amino acid. It was suggested that ACN became a dialdehyde form in the aqueous solutions and then reacted with amino group of amino acid to form the intermediate by amino-carbonyl reaction. Further, the coloring reaction was promoted with heating and acidity by organic acid, and the reaction mixture deepened with the passage of time and finally gelled. The molecular mass of the pigment prepared from AC and glutamic acid showed higher than that of the pigment produced from GSA under the same reaction condition. The color of the pigment produced from AC and glutamic acid showed high stability in a pH range of 4-7, however, it precipitated at pH 3. The color deepened and decreased the red tone with heat treatment. Light exposure caused hypsochromic shift and fading of the color. The red pigment produced from a mixture of AC and GSA showed red color tone depending upon the proportions of these iridoid glucosides and was not separated into each pigment originated from AC or GSA by a gel chromatography. These results suggested that the forming mechanism of the red pigment produced from AC was a similar to that of GSA.