An approach for the estimation of somatic cells' effect in Sarda sheep milk based on the analysis of milk traits and coagulation properties

摘要

Milk traits and coagulation properties (MCP) were investigated in a population of 960 Sarda ewes distributed in 21 farms, in order to propose a limit of somatic cells (SCC) by using the statistical analysis of milk traits and MCP from a large population of ewes. At each farm, from 20 to 80 ewes were individually sampled (one sampling day for each farm), during the afternoon milking. Ewes were between two and seven months after parturition, and the first and seventh parity. Daily milk yield was recorded on the same day of sampling. Milk samples were analyzed to achieve traits of milk composition (fat, protein, casein, lactose, and pH), total bacterial count (TBC), SCC, traditional MCP (rennet coagulation time: RCT; curd-firming time: k(20); curd firmness after 30, 45 or 60 min from rennet addition: respectively a(30), a(45) and a(60)) and model parameters of curd-firming (curd firmness at an infinite time: C-FP; curd firming instant rate constant: k(CF); RCT estimated from the equation: RCTeq; syneresis instant rate constant: k(SR); maximum curd firmness: CFmax; and attainment time of the maximum curd firmness: t(max)). TBC and SCC were log transformed in their respective scores LBC and SCS. The Pearson product moment correlation coefficient was used to investigate the supposed linear relationship between LBC and SCS and a regression analysis to obtain the plots, model and the values of SCS predicted by using LBC. Results evidenced a positive correlation between SCS and LBC. Despite the very low R-2 from the regression analysis, two putative cut-off limit of SCC were calculated: 180,000 SCC/mL, corresponding to 500,000 TBC/mL, and 545,000 SCC/mL, corresponding to 1,500,000 TBC/mL. Later, three groups of SCC-related milk samples were obtained: Low-SCC group (L-SCC), with samples showing a SCC lower than the estimated limit of 180,000/mL; High-SCC group (H-SCC), samples with SCC higher than 545,000/mL; Intermediate-SCC group (I-SCC), samples showing a SCC between the two limits of L-SCC and H-SCC. The L-SCC group showed the highest daily milk yield and lactose content, and the lowest values of fat, protein and casein content and pH. H-SCC group showed delayed coagulation times (both RCT and RCTeq), k(20), t(max) and kCF. In conclusion, milk yield and coagulation properties registered significant worsening in those milk samples with SCC higher than 545,000 SCC/mL. This value could be the starting point for the improvement of milk traits and coagulation properties from the Sarda sheep.