Suitability of methods used for the measurement of proteolysis in UHT milk

The objective was to study the suitability of the non-protein nitrogen (NPN ), Hull, trinitrobenzenesulfonic acid (TNBS) and O-phthaldialdehyde (OPA) m ethods for measuring proteolysis in fresh and recombined UHT milk samples d uring storage. They were also used to measure proteolysis in milk samples w ith added culture filtrate. The effect of heating milk on proteolysis measu rements was also studied. The OPA method showed higher proteolysis in fresh UHT milk than in recombined UHT milk samples while other methods indicated no clear difference. The measurements by the OPA method had low correlatio n with NPN measurements; however, it was highly correlated with hydroxymeth ylfurfural (HMF), indicating that it was affected by other changes in UHT m ilk during storage. After the addition of culture filtrate, the OPA method gave a higher percentage increase in proteolysis measurements for recombine d UHT milk than for pasteurised and fresh UHT milks, while the NPN method s howed a higher percentage increase in proteolysis measurements in pasteuris ed milk than in UHT milk. The percentage increase in proteolysis measuremen ts by the NPN, Hull and TNBS methods were highly correlated, while the resu lts of the OPA method had low correlation with those of the NPN method. Hea ting fresh UHT milk at 80 degreesC resulted in a significant increase in pr oteolysis measurements by all methods. NPN had the lowest and OPA had the h ighest percentage increase, and OPA had the highest correlation with the in crease in HMF concentration. The results indicated that the OPA method was not suitable for comparing proteolysis in different milk types. The NPN and Hull methods were more suitable than the TNBS method for comparing proteol ysis in different milk types and for following proteolysis in UHT milk duri ng storage.