The Analysis of Monomeric Anthocyanin by pH Differential Method is not Appropriate for Certain Anthocyanins

Abstract

: The light absorbance at pH 1 and 4 of 22 anthocyanin-source plant extracts was studied. Each one gram of fresh sample macerated in 4 ml 0.1 N HCl-Ethanol 96% (1:9) for an hour, then diluted in buffer solution pH 1 and 4 with various dilution factor. The extract spectrophotometrically scanned at visible region (400 – 700 nm), then the λmax, color intensity, browning index (BI), and violet index (VI) determined. The λmax of extracts were widely vary from 508 nm to 548 nm. Based on the BI the relatively high color quality at pH 4 exhibited by Clitoria ternatea (CT), Dendrobium sonia (DS), Ipomoea tricolor (IT), Dianella ensifolia (DE) and Melastoma malabathricum (MM) extract. Based on the VI, CT and DE exhibited bluish-purple color at pH 4, while DS was redish-purple, IT was purplish-red, and MM was red. Based on the light absorbance, the extracts might be classified into three types. The A-type exhibited very low intensity of flavylium cation (AH + ) species at pH 4 because of the hydration to colorless hemiketal. The B-type had relatively high intensity of AH + . The C-type showed the existence of purple and blue quinonoidal base species. The measurement of monomeric anthocyanin by pH differential method is based on assumption that the anthocyanin colorless at pH 4.5. Therefore, the method was not suitable for the B- and C-type anthocyanin-source plant extract.