Why benzoic acid and salicylic acid is added to milk and what is the basis for the test used to detect the presence of these acids?

Benzoic acid and salicylic acid are added to milk as preservatives to inhibit microbial growth and extend shelf life. To detect their presence, tests such as colorimetric assays or chromatography can be employed, which rely on the characteristic absorption spectra or specific reactions of these acids. For instance, benzoic acid can be detected using a reaction with bromine water that produces a color change, while salicylic acid can be identified through its ability to form a colored complex with iron(III) ions.