ANALYSIS OF ENZYMATIC-HYDROLYSIS OF KAPPA-CASEIN IN MILK USING DIFFUSE-REFLECTANCE OF NEAR-INFRARED RADIATION

A technique for monitoring and predicting enzymatic hydrolysis of kapp a-casein in milk using diffuse reflectance of near-infrared radiation (NIR) at the wavelengths of 820, 940, 1250, and 1650 nm was developed and tested. Macropeptide concentration which is traditionally referred to as nonprotein nitrogen (NPN) was used as an indicator of enzymatic reaction. The NPN data were fitted to a first order diffusion model t o determine the enzymatic reaction constant. It was found that protein concentration, temperature, and enzyme concentration had a significan t effect on the diffusion model kinetic constant, k(f). The NIR reflec tance response was used to develop a model to predict enzymatic hydrol ysis. Normalized NPN was found to correlate with several combinations of reflectance ratios indicating that changes in reflectance during th e the induction period contained information on the extent of enzymati c hydrolysis. A linear regression between the measured enzymatic hydro lysis time constant, tau and the induction period measured from reflec tance curves, had a correlation of 95.7, 97.0, 98.1, and 97.0% for the wavelengths of 820, 940, 1250, and 1650 nm, respectively. A nonlinear regression for predicting the NNPN by substituting the time constant with induction period had a correlation R(2) of 94.2, 94.81, 95.1, and 94.8% with standard errors of 0.07, 0.07, 0.06, and 0.07 for respecti ve wavelengths.