A rapid colorimetric method for the detection of carminic acid in samples based on visible color change

Carminic Acid (CA), an insect-derived red color, is widely used as a colorant and additive in food and non-food items. The detection of CA is of great concern since it is unacceptable for vegetarians and vegans consumers. Therefore, it is important for food authorities to have a rapid detection method for CA. We describe here a simple and rapid method for the qualitative detection of CA, using Pb²⁺ for complex formation. As a result, the sample solution shows a visible change from pink to purple (bathochromic shift) which could also be analyzed through a spectrophotometer at λ_max = 605 nm. The structure of the CA-Pb²⁺ complex was also studied through advanced spectroscopic techniques. Moreover, the presence of iron results in the formation of a stable CA-Fe²⁺ complex without any significant color change, as Fe²⁺ has a stronger binding affinity with CA. Thus, sodium fluoride (NaF) was used to prevent CA-Fe²⁺ complex formation. Therefore, two methods were developed based on the absence (method I) and presence (method II) of NaF. The LOD and LOQ for the method I was 0.0025 and 0.0076 mg mL^-1, and for method II, values were 0.0136 and 0.0415 mg mL^-1, respectively. The methods were also validated by intra and inter-day analyses. A total of 45 commercials, including food and non-food samples, were screened for the detection of CA. The developed methods are applicable for the effective and rapid surveillance of CA in various samples without the use of high-tech instruments.