Influence of somatic cell count and storage interval on composition and processing characteristics of milk from cows in late lactation

This study investigated the effects of somatic cell count (SCC) and storage interval on milk composition and processability. Milk samples were collected on 4 occasions, from 8 cow udder quarters with SCC <50 (n = 4) and >1000 × 10³Cells/mL (n=4). These milks were blended to form milks within the SCC ranges of 101-300, 301-500 and 501-900 × 10³/mL. These milk blends and the original milks of SCC <50 and >1000 × 10³cells/mL (i.e. 'control milks') were then analysed. In a separate experiment, 24 cows were selected, such that 4 groups of cows (n=6), each had bulk milk SCC in the ranges 30-60, 120-230, 300-370 and 500-650 × 10³ cells/mL. All milks were analysed for gross composition (fat, protein and lactose), N fractions, rennet coagulation properties and proteolytic activity. In addition, the bulk milk samples were analysed after O, 72 and 144 h storage at 4°C. Quarter milks of SCC <50 and 101-300 × 10³/mL had similar composition, renneting properties and proteolytic activities. Quarter milks of SCC 301-500 × 10³/mL had higher (p<0.05) total protein, casein and whey protein concentrations and a higher plasmin activity (p<0.05) than milks of SCC 101-300 × 10³/mL. Fat and lactose concentrations, N fractions and rennet coagulation properties of bulk cow milks were not affected by SCC up to 650 × 10³/mL. Plasmin activity, however, was increased (p<0.05) at SCC >300 × 10³/mL. Whey protein concentrations of milks were increased (p<0.001), and casein, casein number and curd firmness values were reduced (p<0.001) at 72 h and 144 h storage time. These results suggest that composition and processability of fresh milk is not adversely affected up to an SCC of 300 × 10³ cells/mL but may be compromised by storage for 72 h.