An objective measurement technique is needed to simplify the determination of total milk-clotting activity (a(t)) of rennets and other milk coagulating enzymes. IDF Standard 157: 2007/ISO 11815 is the current standard method for bovine rennets and measures milk-clotting activity by visually determining the Berridge clotting time (i.e., time of appearance of flocks of renneted standard milk substrate on the wall of a rotating test tube). The IDF Standard 157: 2007 method is somewhat subjective because it depends on an operator's skill to consistently identify milk flocculation. An optical method based on changes in infrared light back-scatter at 880 nm during milk coagulation is proposed as an alternative, objective method to Berridge clotting time to eliminate the operator's subjectivity from the IDF Standard 157: 2007. The Berridge clotting time method and a proposed optical method were compared using 29 replications to determine the precision of a(t) measurements. The light backscatter profiles collected during the coagulation process were analyzed with five optical time parameters determined for each profile. The a(t), expressed in International Milk-Clotting Units (IMCU mL(-1)), was calculated using Berridge clotting time and each of the five optical time parameters. The calculated milk-clotting activities were compared statistically to determine differences between measurement methods. No significant differences were found between the a(t) determined using the Berridge clotting time and the optical time parameters, t(2min) and t(2min2). Preferred optical time parameters and t(max) and t(2max) were recommended. The results show that the proposed optical method based on measurement of changes in infrared light backscatter is a potential objective method for a(t) measurement. (C) 2011 Elsevier Ltd. All rights reserved.