When does the protein profile in milk normalize after antibiotic treatment against clinical mastitis?

Sammanfattning: Concentration of protein in bovine milk is one of the most significant milk quality parameters, to a large extent determining the price for milk to the producer. Mastitis is a common disease among dairy cows, negatively affecting not only milk yield but also milk protein composition. Milk from mastitic cows tends to have lower cheese yield, negatively affected processability properties and sensory quality, due to changed protein quality and composition. Poorer milk protein quality would have an economically negative impact on dairy industry. Only few studies have investigated the short term effects of mastitis on milk protein composition. Considering the severe secretory damage on the epithelial gland cells caused my mastitis, the risk of altered milk protein concentrations immediate after withdrawal time is imminent. The objective with this study was to follow the changes in milk protein composition from the time of mastitis treatment during the withdrawal period and until five days after the withdrawal time has expired. The proteins studied are α-lactalbumin and β-lactoglobulin, αS2-casein, αS1-casein, αS0-casein, κ-casein and β-casein. The hypothesis was that protein composition is still affected after the elapse of the withdrawal time. Sample collection and analysis were performed during summer and autumn 2009. The animals used were seven cows with mastitis, of which five Swedish Red (SRB) and two Holstein breed. Cows were housed at Jälla agricultural school and at Kungsängens Research Station, close to Uppsala, Sweden. Collection of udder composite milk samples started on the first milking after administration of the antibiotic treatment and proceeded during treatment and withdrawal periods until five days after end of withdrawal time. From each animal, approximately 30 samples were taken in connection with morning and afternoon milking over 15 days. The experiment period was divided into three parts: treatment period, withdrawal period and the five days after the end of the withdrawal period. Protein analysis was performed with capillary electrophoresis (CE). CE separates protein due to their physiochemical properties by an electric gradient making proteins migrate through a capillary in different velocities depending on their respective charge in a determined pH. Protein quantification was obtained by comparing electropherogram protein peak areas with those of Sigma standards. Concentrations of individual proteins during treatment period, withdrawal period and post withdrawal period respectively were compared with concentrations in reference bulk tank milk. Also, milk from cows infected by different pathogens was compared regarding protein composition. Concentrations of whey proteins analysed, α-LA and β-LG (α-lactalbumin and β-lactoglobulin) did not change noticeable during the period of mastitis treatment and recovery. Concentrations of individual caseins analysed (αS2-CN, αS1-CN, αS0-CN, κ-CN and β-CN) were found to be highest during treatment period, lower during the withdrawal period and lowest after the withdrawal period. All casein concentrations except that for κ-CN were lower during the first five days after the withdrawal time had expired than in normal bulk tank milk. The hypothesis that protein composition is still affected after the withdrawal time has expired cannot be rejected. The question when protein concentrations in milk normalize after treatment can still not be answered. For most animals and most proteins analyzed in this study, protein levels seemed to continue decreasing or increasing by the last sample taking. In future studies, the length of the experiment period must be extended to find the time when protein concentrations may normalize or stabilize. Concentrations of individual proteins showed large variation between animals. Various pathogens causing the mastitis may also affect milk protein concentrations differently. Therefore, to be able to draw more reliable conclusions from results in future studies, number of animals studied must be greater.