Comparison between mechanical and near-infrared methods for assessing coagulation properties of bovine milk

The aim of the present study was to compare milk coagulation properties measured through a traditional mechanical device, the Formagraph (FRM; Foss Electric A/S, Hillerød, Denmark), and a near-infrared optical device, the Optigraph (OPT; Ysebaert SA, Frépillon, France). Individual milk samples of 913 Brown Swiss cows from 63 herds located in Trento Province (Italy) were analyzed for rennet coagulation time (RCT, min), curd-firming time (k20, min), and 2 measures of curd firmness (a30 and a45, mm) using the 2 instruments and under identical conditions. The trial was performed in the same laboratory, by the same technician, and following the same procedures. Extending the analysis by either instrument to 90 min permitted RCT and k20 values to be obtained even for late-coagulating milk samples. Milk coagulation properties measured using the OPT differed considerably from those obtained using the FRM. The average k20 values varied greatly (8.16 vs. 5.36 min for the OPT and the FRM, respectively), as did the a45 figures (41.49 vs. 33.66 mm for the OPT and the FRM, respectively). The proportion of noncoagulating samples for which k20 could be estimated differed between instruments, being less for the OPT. The between-instrument correlation coefficients were either moderate (0.48 for a30) or low (0.24 and 0.17 for k20 and a45, respectively) when the same traits were compared. The correlations between k20 and a45, and milk yield varied among instruments, as did the correlations between k20, a30, and a45 and milk composition, and the correlations between a45 and pH. The relative influence of days in milk on k20 and a45 varied, as did the effect of parity on a45 and that of the measuring unit of coagulation meter on k20 and a30. The RCT estimated by the OPT was the only milk coagulation property to show good agreement with the FRM-derived value, although this was not true for the data from late-coagulating samples.