National University of Singapore
Food Science & Technology
Anthocyanin
Anthocyanins are water-soluble compounds, ranging in colour from deep purple to orange red. They are found primarily in fruits but also present in some vegetables, eg radish, eggplant, red cabbage and red potato. Anthocyanins are glycosides of anthocyanidins. Figure 1 shows the general structure of anthocyanin. The sugar attached to the structure is glucose; R and R’ groups may be H, OH or OCH3. They vary among anthocyanins from different sources and in mixtures from the same source.
Figure 1. Structure of Anthocyanin. R = H, OH or OCH3. Adapted from http://www.intechopen.com/source/html/41633/media/image3.jpeg
The colour of anthocyanins is sensitive to pH, whereby they tend to be red in acidic condition of pH <1, colourless in pH 4-5, purple in pH 6-7 and deep blue in the pH range of 7 to 8. Figure 2 shows the colour change of anthocyanin at various pH.
Figure 2. Colour changes of anthocyanin at various pH. Adapted from http://lecturedemos.chem.umass.edu/photos/anthocyanin2.jpg