The mechanism of the effects of glandular-level subclinical mastitis in dairy sheep on milk yield and on its composition as expressed in curd yield was studied. Thirty-six Israeli-Assaf dairy sheep with one udder half infected with identified coagulase-negative staphylococci and the contralateral gland free of bacteria were chosen. The milk yield of the infected halves was significantly lower than that of the uninfected ones (0.36 vs. 0.76 kg/milking). The somatic cell count and N-acetyl-beta-D-glucosaminidase activity were significantly higher in the infected halves than in the uninfected ones. The plasminogen activator and plasmin (PL) activities were significantly higher in the infected glands than in the uninfected ones, whereas plasminogen (PLG) activity and the ratio PLG:PL were significantly lower in the infected glands. Concentrations of Ca2+ did not differ, whereas Ca2+ activity was significantly lower and proteose peptone concentration was 2.4 times as high in the infected glands than in the uninfected ones. Curd yield was significantly lower in the infected glands than in the uninfected ones.

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Milk stasis triggers local stimuli, which make the tight junctions leak and trigger involution. The aim of the study was to test the hypothesis that casein hydrolyzates compromise tight junction integrity and dry-off milk secretion in dairy cows. Six repeated doses of casein hydrolyzates after each milking during 3 d caused drastic changes in mammary secretion and composition, which were associated with irreversible cessation of milk secretion. No such changes were recorded in the control glands that had been treated with nonhydrolyzed casein. Treatment with casein hydrolyzates disturbed tight junction integrity within 8 h (as indicated by changes in Na+ and K+ concentrations), reduced the concentrations of lactose precipitously, activated the plasmin activator-plasminogen-plasmin system, and induced the secretion of immunoglobulin type G and lactoferrin. At the end of the 3-d treatments, we stopped milking the experimental and control glands. Milk composition 19 d later was similar in the experimental and control glands and was consistent with the composition expected in fully involuted glands. We conclude that casein hydrolyzates are among the milk-borne factors that cause the disruption of tight junction integrity and induce involution in cows. The process induced by casein hydrolyzate was more rapid and synchronized than the involution induced at drying-off.

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The enzyme catalase is well-known to catalyze the disintegration of hydrogen peroxide to water and oxygen; however, this study shows that its main function in bovine milk is oxidation of nitrite to nitrate. This process depends on hydrogen peroxide, of which the main source appears to be hydrogen peroxide formation that is coupled to the conversion of purines--xanthine in the present study--to uric acid by milk xanthine oxidase. However, additional secondary sources of hydrogen peroxide appear to be important during the relatively long storage of milk in the gland cistern. This paper demonstrates that the oxidation of nitrite to nitrate is necessary to prevent accumulation of free radicals and oxidative products during storage of milk in the gland and during the unavoidable delay between milking and pasteurization in dairy plants. Recommendations for minimizing the deterioration in milk quality during commercial storage are presented.