The effect of various concentrations of papain on the properties and hydrolytic rates of β-casein layers

Recently, much attention has been focused on the investigation of the surface biocatalysis of proteases. In this study, papain, a proteolytic enzyme was used to hydrolyze a bovine β-casein (β-CN) layer, which was monitored by a quartz crystal microbalance with dissipation (QCM-D). The changes of the β-CN layers before and after hydrolysis were characterized by atomic force microscopy (AFM) imaging, grazing angle infrared spectroscopy (GA-FTIR) spectra, and contact angle measurement. Our results demonstrated that the proteolytic reaction was enzyme concentration-dependent and started with the hydrophobic C-terminal sequence of the β-CN. The remaining β-CN layer became thinner, smoother, stiffer, and hydrophilic after hydrolysis. These results are conducive to the further understanding of the catalysis of papain on β-CN layers in liquid–solid interfaces and the promotion of biocatalytic applications.