Smartphone-based digital images as a novel approach to determine formaldehyde as a milk adulterant

Milk is an important source of protein and its consumption has gained relevance because of new evidence for its nutritional and functional role for humans. On the other hand, milk quality has been hindered by tampering, including the addition of formaldehyde to mask product deterioration and increase its shelf life, although this substance may cause harm to consumers. Taking these aspects into account, a novel, fast, greener, portable, and cost-effective method is proposed for the determination of formaldehyde in milk. This method relies on smartphone-based digital image colorimetry of the product formed upon reaction with the Schiff reagent, followed by conversion of the images into RGB values using free application software. Sample preparation only involved protein precipitation in an acidic medium and analyte determination required only a few micrograms of the reagents. The proposed method achieved a limit of detection as low as 120 μg L−1 formaldehyde with a linear response up to 1250 μg L−1 and good precision (CV = 2%, n = 20). Matrix effects were negligible after protein precipitation with recoveries of formaldehyde spiked milk samples within 92–106%. The accuracy was assessed using a reference procedure (HPLC with UV detection after analyte derivatization and liquid–liquid extraction) and the results agreed at the 95% confidence level. The procedure is applicable to whole, semi-skim, and skim milk processed using pasteurization or ultra-heat treatment (UHT).