NIR light backscatter technology has been proven successful for monitoring cow milk coagulation and predicting cutting time but has never been tested with milk mixtures. In Spain similar to 40% of the cheese produced is made from cow, sheep and goat milk mixtures. The aim of this study was to evaluate if the proposed optical technology could be used to monitor milk coagulation and predict cutting time in milk mixtures. A randomized factorial design with three factors and three replicates was employed. Cow, goat and sheep milk was mixed in two different proportions. Milk mixtures were coagulated at constant calcium chloride addition level, pH and fat concentrations using two different enzyme concentrations and three coagulation temperatures (N = 36 tests). Milk coagulation was monitored using small amplitude oscillatory rheometry and a NIR fiber optic light backscatter sensor. Simultaneously, clotting time was visually evaluated. Optical parameter t(max) was highly correlated (0.80 < r < 0.99, P < 0.0001) with the rheological and visual parameters studied. Enzyme concentration and temperature had a significant effect (P < 0.05) on optically-, rheologically-, and visually-derived parameters. Milk mixture proportion was not significant for optical parameters related to clotting time but was significant for the aggregation rate and rheological parameters related to curd firming and syneresis. Models for predicting cutting time were developed successfully with R-2 = 0.93. Results strongly suggest that milk mixture proportion exerts an effect on gel assembly (i.e., on both aggregation and curd firming) and syneresis. This finding has important implications for inline process control when goat and sheep milk are used. (C) 2013 Elsevier Ltd. All rights reserved.