Detection of non-dairy fat in milk based on quantitative assay of anionic detergent using azure A dye

A spectrophotometric method for ascertaining the presence of anionic detergent in milk using azure A dye is described. The method requires brief mixing of milk, azure A dye and EDTA solutions, followed by addition of chloroform and subsequent centrifugation. The blue colour in the chloroform phase was measured at 635 nm. The limits of detection (LOD) and limit of quantification (LOQ) for sodium dodecylbenzene sulfonate in milk were 10 and 20 mg L−1, respectively and for a laboratory grade anionic detergent, labolene, 50 and 100 mg L−1, respectively. Recovery of detergent in spiked milk samples was quantitative. The presence of other additives, i.e., NaOH, NaHCO3, KNO3 and NaCl at levels of 250, 750, 500 and 500 mg L−1 milk, respectively, did not interfere with estimation of detergent. The method is also applicable to milk samples stored in the presence of formalin and sodium azide for 39 days at 4 °C.