A novel and rapid qualitative method for detecting anionic detergent in milk

Abstract

Milk adulteration with anionic detergents poses a significant food safety concern and requires rapid, simple and field-applicable detection methods. In the present study, a rapid liquid-based assay using ortho-toluidine blue (OTB) and chloroform was developed for detecting detergent adulteration in milk. The method is based on the formation of a dye–detergent ion-pair complex that partitions into the lower chloroform phase and produces a distinct colour transition from pink in control milk to purple/blue in detergent-spiked milk. The developed liquid assay showed a limit of detection (LOD) of 0.02% for commercial detergents and 0.002% for sodium dodecyl benzene sulfonate (SDBS), with a response time of 1 min. The assay demonstrated high sensitivity and specificity, good repeatability and reproducibility and no observable interference from common adulterants such as glucose, sucrose, urea, neutralisers, starch, formalin, hydrogen peroxide, boric acid, nitrate and salt. The visual response remained effective across a pH range of 5.0–8.5 and in boiled, pasteurised, and sterilised milk samples. A complementary spectrophotometric method based on the same OTB extraction principle was also developed and showed strong agreement with the reference Azure A method. The developed assay is simple, rapid, economical and suitable for routine screening of detergent adulteration in milk at laboratory, industrial and field levels.