A Sensitive and Robust Method for Direct Determination of Lipolytic Activity in Natural Milk Environment

Microbial lipases may be produced during milk storage and processing. This can lead to organoleptic changes in the corresponding dairy products. Thus, monitoring of lipase activity in milk is desirable. Turbidity of milk prevents a direct photometric measurement of lipase activity using chromophore- or fluorophore-based assays. Laborious pretreatments or alternative analytical methods normally have to be used. With the method for the determination of lipolytic activity (MeDeLi) proposed here, it is possible to measure lipase activity directly in the natural milk utilizing tailored fluorometric substrates. Only a defatting step is carried out initially for the MeDeLi. Then, the conversion of added lipase substrate is carried out in the unmodified milk without addition of any solutions or any enzyme extraction procedure which may influence the enzyme activity. Thereafter, the milk sample is treated with two solutions to remove the turbidity of milk by dissolution. A valid and sensitive fluorometric measurement is then possible. The applicability of the MeDeLi was demonstrated in comparison with tests published previously: The limit of detection for lipolytic activity measured by MeDeLi was the lowest, with 41 pkat/mL. Raw milk, milk products, and spoiled milk samples were also investigated with the MeDeLi.