Automatic screening method for the rapid and simple discrimination between synthetic and natural colorants in foods

Abstract

A simple screening method was developed to discriminate between synthetic and natural colorants present in foods in order to reduce the use of expensive instruments such as a liquid chromatograph with diode array detection. A rapid flow system was proposed in which samples containing natural and synthetic colorants in an acetic acid medium were passed through a wool/cotton column, where only synthetic colorants were retained and were then eluted with dilute ammonia. Yellow, red and green–blue–brown additives can be monitored at 400, 530 and 610 nm, respectively, with a conventional spectrophotometer. Complete discrimination (no false positives) between natural and synthetic colorants can be obtained for molar concentrations of natural colorants (in the absence of synthetic ones) up to 2000 (yellow), 2000 (red) and 10 000 (brown) times that of the detection limit (DL) of synthetic additives. The reliability of the method was established at five concentrations (between 0.5 and 3 DL) of the synthetic colorants tartrazine (yellow), erythrosin B (red) and brilliant black BN (brown). For a cut-off concentration of 2 DL, the percentage of false negatives ranges from 8 to 12%. Finally, the method was applied to screening several fruit drinks and candies for the determination of synthetic colorants with a sampling frequency of 10 h −1.