A stable isotope and chemometric framework to distinguish fresh milk from reconstituted milk powder and detect potential extraneous nitrogen additives.

Stable isotopes (δ²H, δ¹³C, δ¹⁵N and δ¹⁸O) of 232 milk powders and 88 fresh milk samples collected globally were used to distinguish fresh milk from reconstituted milk and qualitatively detect extraneous nitrogen in milk products. Nitrogen isotopes of six nitrogen-containing milk additives, including animal protein (peptone), soy protein, and four inorganic nitrogen additives (urea, melamine, ammonium chloride and biuret), were characterised to identify potential extraneous nitrogen (PENs) that could be fraudulently used to increase the protein N content of milk. Results showed that fresh milk and reconstituted milk can be distinguished by using stable isotopes combined with chemometrics (PCA, LDA and ANN) methods. LDA achieved accuracy rates of 94.9 % and 94.9 % for the testing and training sets respectively, while ANN had higher accuracy rates, up to 99.6 % for the training set, and 96.6 % for the testing set....