Eight milk-clotting enzyme prepartations were standardized to equal clot time and used to coagulate pasteurized whole milk. Diffuse reflectance profiles were monitored for 60-min using a fiber optic sensor sensitive to infrared light at 950 nm. Modified M. miehei and M. pusillus protease, recombinant chymosin and calf rennet produced similar profiles. Rates of increase in diffuse reflectance were E. parasitica recombinant chymosin > calf rennet > modified M. miehei, M. pusillus var. Lindt > 50:50 blend of calf rennet and bovine pepsin > unmodified M. miehei > pepsin. Monitoring milk coagulation as described may be useful during cheese making and allow setting optimal conditions for milk-clotting enzyme preparations.