overestimating anthocyanins by a factor ∼7.5 with
respect to HPLC data. Methods 3 and 5 also yield
comparable results (mean content is 114 and 121 mg/
L, respectively), overestimating anthocyanins by a factor
∼1.8. Results of method 2 are similar to the HPLC ones
(mean content is 67.9 and 65.6 mg/L, respectively).
These results indicate that most of spectrophotometric
methods are affected by systematic errors. Possible
sources of these deviations are now separately examined