Heat-treated milk differentiation by a sensitive lactulose assay

A sensitive enzymatic assay for determining lactulose (detection limit=2.5 mg/l; RSD <4%) was applied to 90 samples of various milk categories, such as, pasteurized milk (fresh and high-temperature), UHT milk (indirect, direct, by infusion and by injection), and in-container sterilized milks. Results showed that it was possible to distinguish, by lactulose content, not only in-container sterilized milk (744 mg/l), indirect UHT (341 mg/l) and direct UHT (165 mg/l), but also UHT milk produced by mild technologies such as milk treated by infusion (107 mg/l), high-temperature pasteurized milk (58 mg/l), and low temperature pasteurized milk (3.5 mg/l).