Stability studies and structure identification of anthocyanins from Tradescantia pallida

Anthocyanins are an important type of natural red pigments for use in replacing the artificial red colorants in foods. The major problem with anthocyanins is their instability. The search for more stable anthocyanins, especially at near neutral pH, was the motivation for this research. The anthocyanins from the leaves and stems of Tradescantia pallida were found to be more stable than conventional anthocyanins of grapes and blackberries at near neutral pH, such as pH 5.5, in a model beverage storage study of more than 54 weeks. The high stability of this type of anthocyanin may eventually expand the application of anthocyanin from high acid to low acid foods. Furthermore, the stability of T. pallida anthocyanins(TPA) was significantly reduced by tannic acid, ascorbic acid and their interaction in a 2-factor 3-level factorial experiment, in contrast to the enhancement by tannic acid, of the color and stability of the conventional anthocyanins from grapes and blackberries. Because of the significant stability of TPA, understanding of their chemical structure became necessary. A new approach of identifying anthocyanin structure was developed, which includes both qualitatively identifying the deacylated anthocyanins, with comparison to the known deacylated anthocyanins of Zebrina pendula through HPLC and TLC, the sugars with TLC, and the acyl groups with TLC and HPLC, and quantitatively measuring the ratio of sugar to the aglycone with the dinitrosalicylic acid method and the acyl/aglycone ratio with HPLC. The pigments of T. pallida were found to consist of mainly two anthocyanins, one in much higher abundance than the other. The major one was identified as cyanidin-3,7,-diglucosylferuloyl-3$\sp\prime$-glycosylferuloylglucoside, and the minor one cyanidin-3,7,3$\sp\prime$-triglycosylferulic acid. This study indicated the possibility of applying this chemical approach of anthocyanin structure identification especially to those anthocyanins whose acyl groups were qualitatively identified but not quantitatively measured