Non coagulating sheep milk characterization through Fourier-Transform IR spectroscopy

Milk coagulation ability is important for the sheep dairy industry because all sheep milk is destined for cheese processing. The aim of this work is to investigated relationships between milk composition, physical properties and Fourier-Transform Infra-Red (FTIR) spectra and the occurrence of non-coagulating (NC) sheep milk. Milk was collected from 1018 Sarda ewes farmed in 47 flocks located in Sardinia. Measured variables were milk yield, compo-sition and coagulation properties (MCP): rennet coagulation time (RCT), curd firming time (k20) and curd firmness (a30). The FTIR spectra were also acquired. About 9% of samples did not coagulate within 30 min. A linear mixed model (LMM) analysis was performed to test whether milk composition differs between coagulating and NC samples; fixed effects of parity (3 levels), DIM (5 dim classes), lambing month (4 levels) and the random effect of flock-test-date (67 levels) were included in the model. Principal component (PC) analysis was carried out on FTIR spectra. PC scores were used as the response variables with the same MLM. Coagulating and NC showed differences for all traits (p < .01), except for milk yield and fat%. NC samples showed higher values for protein, casein, somatic cells score (SCS), chlo¬ride and pH, and lower for lactose and urea, respectively. Seven PC explained 90% of the variance. Interestingly, PC2 and PC3 were associated to the coagulation class (p < .05). Two regions (2290–2657, 3663–3732 cm−1) were highly correlated with PC2. PC3 loadings showed values higher than 0.5 in two spectral regions (1096–1469 and 2668–2826 cm−1). High loadings of PC2 and PC3 correspond to regions highly correlated with fat and casein, and with lactose and chloride contents, respectively, including a large region (from 1780 to 2825 cm−1) often not used in prediction models as scarcely informative. Significant differ¬ences in the milk composition and FTIR spectra between normal and NC milk samples were observed in the present study, likely related to the variables often associated with the mammary gland health status, that seems a very important factor influencing milk coagulability. The study of the individual milk FTIR spectra is crucial in order to predict the coagulability of future samples. This may contribute to the development of useful applications for the dairy sheep industry.