A novel approach to detect milk adulteration based on the determination of protein content by smartphone-based digital image colorimetry

A novel alternative for the detection of milk adulteration by dilution based on the determination of the protein content is proposed. The proposed method involves the precipitation of the proteins by the salting-out effect of copper sulfate and the proportional adsorption of Cu(II) on the precipitate. Subsequently, photometric measurements of the remaining amount of Cu(II) by smartphone-based digital colorimetry are conducted to determine the protein content. The proposed procedure is rapid (sample throughput of 32 assays/h), precise (coefficient of variation = 3.0%, n = 20), environmentally friendly, and allows the detection of as low as 1.0% v/v water in adulterated milk. The main species used to mask the protein content do not interfere, thus permitting the direct detection of adulteration. The procedure was successfully applied for the classification of milk samples as compliant and non-compliant as well as for the indirect determination of the protein content in milk. The quantitative results agreed with those obtained by the reference method (near infrared spectroscopy) at a 95% confidence level.