Detection of colour adulteration in cochineals by spectrophotometric determination of yellow and red pigment groups

A rapid method was developed for the determination of yellow and red pigment groups in cochineals, a natural food colourant. The method was based on UV–visible spectrophotometric measurement at two characteristic wavelengths. The wavelengths 420 and 500 nm were selected as the most appropriate for simultaneous quantification of these pigment groups from a cochineal extract in methanol:water (65:35, v:v). Determination of the mean absorption coefficients for the yellow pigment group, at 420 nm, and the red pigment group, at 500 nm, yielded equations that serve to calculate the concentration of the two pigment groups (C420 and C500, respectively) and the ratio between them (C420/C500). Because the ratio between pigment groups can be considered constant (comprised between 0.93 and 1.10), this parameter can be used for quality control. Fluctuations outside the set limits may indicate that the colour of the cochineal has been enhanced by colourants foreign to the raw material. A study of adulterations showed the capacity of the proposed method to detect these adulterations. There were no significant differences when comparing the determination of the pigment groups by the proposed spectrophotometric method with the results obtained by liquid chromatography.