Effect of protein shortage and conjugated linoleic acid supplementation on quality traits and modelling of coagulation, curd firming and syneresis of Holstein-Fresian milk

Aim of the present study was to evaluate the effect of diets with optimal (CP 15% DM) or suboptimal (CP 12.3% DM) protein content, supplemented (CLA+) or not (CLA-) with rumen-protected conjugated linoleic acid (rpCLA) on some cheese- making properties. Twenty Holstein-Fresian mid lactating dairy cows have been reared following a 4×4 Latin square experimental design of 4 periods, 3 weeks each. Individual milk samples, collected during the third week of each period, were analysed for chemical composition, traditional milk coagulation properties (MCP: RCT, k20 and a30) and for recording curd firmness (CF) every 15 s over a 90 min period. Data acquired from each sample were used to model CF over time calculating the following parameters: rennet coagulation time (RCTeq), asymptotic potential CF (CFP), CF rate constant (kCF), syneresis rate constant (kCF), maximum CF achieved within 90 min (CFmax) and time to CFmax (tmax). Data were analysed using period, diet and group (random) as sources of variation. Cows evidenced a strong individual variability within groups and were classified as early (RCT<20 min) or late (RCT>20 min) coagulating cows. Dietary protein shortage reduced milk protein and lactose content, while rpCLA supplementation depressed milk fat synthesis. Results showned that traditional MCP parameters were worsened by reduction of dietary protein in the case of milk produced by early coagulating cows, while rpCLA supplementation affected negatively all three traits on all cows. The study of CF model parameters evidenced that CP12 diets have improved CF (CFP and CFmax) respect to CP15 when fed to late coagulating cows while worsened CF (CFP and CFmax) and reduced kCF when fed to early coagulating cows. The results of the present study underline the complex relationship between dietary fat and protein and their consequences on milk technological properties highlighting the need for further investigations.