Indirect detection of lipase in UHT milk by measuring methyl ester formation

A principle for indirect detection of lipase, through ester formation, is described. Methanol (1%, w/w) was added to UHT milk, leading to methyl ester formation. Ester formation was measured using solid-phase microextraction and gas chromatography mass spectrometry. Amano lipase DF15 was indirectly detected at approximately 4 ppb, detection limit in milk, due to its ability to catalyse ester formation. High sensitivity obtained was the result of the absence of an ester background in milk. Ester formation was proportional to both the amount of lipase added and storage time. Lipase was readily detected in milk after UHT treatment, while sensory defects occurred in such UHT milk only after 2 months of ambient storage. The principle underlying this test method should be further considered as a means to overcome difficulties in assaying heat-stable enzymes in dairy products.