A comparison of the Formagraph and low amplitude strain oscillation rheometry as methods for assessing the rennet coagulation properties of bovine milk

Rennet coagulation properties of milk can be used as indicators of subsequent cheese composition, recoveries of milk solids, and cheese quality. The transformation of milk during coagulation involves a number of physicochemical changes, the detection of which form the bases of methods for measuring aspects of rennet coagulation. The objective of this study was to compare the rheological changes in rennet-treated milk measured using 2 such methods, i.e. the Formagraph method, and low amplitude oscillation rheometry using a Cari-Med CSL2 Rheometer. These techniques have been used extensively in research, yet there are no published data comparing them. Milk samples from 31 cows were adjusted to pH 6.55 before analyses. Measurements made were: time until onset of gelation (GT, Rheometer, and RCT, Formagraph); curd firming rate (CFR and 1/K-20); and curd firmness 1 h after rennet addition (G'(60) and A60) There were significant (p<0.001) positive correlations between GT and RCT (r=0.850), CFR and 1/K20 (r=0.946), and G'(60) and A60 (r=0.789). The regression equations defining the relationships were: GT = (1.74 x RCT)-469, CFR = (28.54 x 1/K-20)-0.24, and G'(60) (4.07xA(60))+68. The ranges for GT, CFR and G'(60) were 659 to 2622 s, 0.48 to 8.04 Pa/min and 8.0 to 192.9 Pa respectively. These strong correlations indicate that the 2 techniques provide estimates of rennet coagulation parameters that are comparable. These data will allow continuity of data interpretation in studies where either method is used.