Quality evaluation of cheese from raw or pasteurized milk of ewes grazing sulla forage (Hedysarum coronarium L.) for different daily time

Abstract

Pasteurization is the more effective method for eliminating pathogens contaminating milk. However, the high temperature of pasteurization destroys either pathogens or lactic endemic flora, this latter involved in cheese ripening, thus impairing the development of sensory properties of cheese. It was previously observed that longer permanence at pasture allows the ewes to spend more time for eating; accordingly, ewes increase feed intake and milk yield. Aim of this experiment was to evaluate the effect of the prolongation of ewes daily grazing from 8 to 24 h and the effect of the milk pasteurization process on milk and cheese quality, with particular regard on fatty acid (FA) composition. In a semi-arid hilly area of Sicily, 14 Comisana ewes, averaging 157±57 days in milk and 44.8±7.3 kg of live weight, were divided into 2 groups. For 42 d in spring, both groups continuously grazed sulla under a stocking rate of 34 ewes/ha, one group for 8 h (8:00-16:00) (S8) and the other one for 24 h (S24). At 2-week intervals, 3 cheesemaking trials were conducted in controlled condition and according to the Pecorino Siciliano technology. From each group, the 48-h bulk milk was collected and divided equitably into 2 inox vats. In a vat, milk was heated at 33°C (S8 and S24); in the other one, milk was pasteurized at 72°C for 20 sec and added with starters (SP8 and SP24). Bulk milk was analysed for its components and coagulation properties. Determinations were executed on 15-day aged cheeses for chemical composition, colour, FA composition (by GC/MSD system with a flame ionization detector) and sensorial characteristics by triangular tests. Data were statistically analysed by GLM procedure of SAS 9.1.2 software, with grazing time, pasteurization and their interaction as effects. Ewes grazing for 24 h showed higher milk yield (929 vs. 817 g/d; P<0.05) and urea (58 vs. 49 mg/dl; P<0.01), and lower milk fat (6.3 vs. 6.8 %; P<0.01), somatic cells count (5.9 vs. 6.4 log10/ml; P<0.05), clotting time (r) (22.3 vs. 19.9 min; P<0.05) and curd firming time (k20) (2.1 vs. 1.4 min; P<0.05) than S8 ewes. Composition and properties of raw and pasteurized milk did not differ, except for bacterial count which was reduced by pasteurization action. Grazing time and pasteurization did not affected cheese yield and composition. Cheeses from pasteurized milk had a less intense yellow colour (b*= 15 vs. 18; P<0.05) than from raw milk. At triangular tests, significant differences emerged between cheeses in comparisons S8 vs. S24 (P<0.06), S8 vs. SP8 (P<0.01) and S24 vs. SP24 (P<0.05). Grazing time did not influenced significantly cheese FA profile, even though unsatured FA and CLA were slightly higher in S8 (31.46 and 1.57% FA, respectively) than in S24 (29.51 and 1.29% FA, respectively) cheese. Also the pasteurization treatment did not modify cheese FA composition, in accordance with other investigations on cheese from cows and goats pasteurized milk. In conclusion, longer permanence of ewes at pasture improved milk yield without modifying cheese yield and FA profile. In front of the advantage of better cleanness, pasteurized milk did not led to differences in cheese FA profile, in comparison to raw milk. However, milk pasteurization caused loss of typical taste and reduced yellow index of cheese, presumably due to variation in microbial flora.