The influence of subclinical mastitis on the protein composition and protease activities of raw milk from lactating Thai-crossbred dairy cows

Background and Aim: Mastitis in dairy cattle is associated with a high rate of morbidity and death, which has major implications for milk production and quality. This study aimed to investigate the protein component and the activity of matrix metalloproteinase-2 (MMP-2) and -9 (MMP-9) in raw milk samples with different testing scores determined using the California mastitis test (CMT). Materials and Methods: Thirty cows were employed in the study, and milk from each quarter was tested for subclinical mastitis (SCM). According to the results of CMT, raw milk samples were classified into five categories: Healthy (score 0), trace (score T), weakly positive (score 1), distinctly positive (score 2), and strongly positive (score 3) for somatic cell count (SCC). The total milk protein was analyzed using the Bio-Rad protein assay, and the milk protein composition was determined using the sodium dodecyl sulfate-polyacrylamide gel electrophoresis technique. In addition, gelatin zymography was used to evaluate changes in proteolytic abilities. Results: Milk samples with CMT scores of 1 and 3 had the highest total milk protein levels (32.25 ± 12.60 g/L and 32.50 ± 7.67 g/L, respectively), while the samples from healthy cows (CMT score 0) were only 6.75 ± 1.64 g/L. Globulin and lactoferrin were significantly increased in samples with a CMT score of 3 compared with those with other CMT scores. The bovine serum albumin level in samples with a CMT score of 2 was significantly (p < 0.05) higher than those with other CMT scores. No significant differences in casein abundance were found among samples with different CMT scores. Results from analysis of proteolytic activities demonstrated that the level of MMP-9 in samples with a CMT score of 3 was significantly (p < 0.05) higher than those with other CMT scores. Conclusion: The protein content and gelatinolytic activity of milk were drastically altered by the number of SCC, mainly due to SCM. Keywords: milk protein, protease activity, subclinical mastitis, Thai-crossbred dairy cows.