Monomeric anthocyanin pigments reversibly change color
with a change in pH; the colored oxonium form exists at
pH 1.0, and the colorless hemiketal form predominates at
pH 4.5. The difference in the absorbance of the pigments at
520 nm is proportional to the pigment concentration. Results
are expressed on a cyanidin-3-glucoside basis. Degraded
anthocyanins in the polymeric form are resistant to color
change regardless of pH and are not included in the
measurements because they absorb at pH 4.5 as well as
pH 1.0.