Simultaneous determination of color additives tartrazine and allura red in food products by digital image analysis.

A method based on digital image is described to quantify tartrazine (E102), yellow, and allura red (E129) colorants in food samples. HPLC is the habitual method of reference used for colorant separation and quantification, but it is expensive, time-consuming and it uses solvents, sometimes toxic. By a flatbed scanner, which can be found in most laboratories, images of mixtures of colorants can be taken in microtitration plates. Only 400 µL of sample are necessary and up to 92 samples can be measured together in the same image acquisition. A simple-to-obtain color fingerprint is obtained by converting the original RGB image into other color spaces and individual PLS models are built for each colorant. In this study, root mean square errors of 3.3 and 3.0 for tartrazine and 1.1 and 1.2 for allura red have been obtained for cross-validation and external validation respectively. Results for repeatability and reproducibility are under 12%. These results are slightly worse but comparable to the ones obtained by HPLC. The applicability of both methodologies to real food samples has proven to give the same result, even in the presence of a high concentration of an interfering species, provided that this interference is included in the image analysis calibration model. Considering the colorant content found in most samples this should not be a problem though and, in consequence, the method could be extended to different food products. Values of LODs of 1.8 mg L-1 and 0.6 mg L-1 for tartrazine and allura red have been obtained by image analysis.