Seasonal variations in plasmin activity in Swedish silo and UHT milk

Sammanfattning: Plasmin (PL) is a native proteinase in milk, which incorporates a complex system of inhibitors and activators, whose activity is known to cause changes in ultra-high temperature (UHT) treated milk during storage. The objective of this study was to investigate if there were any seasonal variations in PL and plasminogen (PG) de-rived PL activity in Swedish silo and UHT milk, and if enzyme activity was affect-ed by storage temperature and storage time. The reproducibility and detection limit also was calculated. Moreover, the relation between the somatic cell count (SCC) and PL and PG-derived PL activity was investigated. Milk collected monthly from November 2014 to November 2015 was ultra-centrifuged and analysed for PL and PG derived PL activities by a spectrometric assay using a chromogenic substrate. The reproducibility of the method was 10 % for the PL assay, and 9 % for the PG assay. The limit of detection was calculated to 0.59 U/ml. PL activity in silo milk was on average 3.35 U/ml ±15 % and PG-derived PL activity was on average 92.27 U/ml ±7 % over the year. Significant differences in PL and PG-derived PL activity between months indicated seasonal variations. The highest activity of PL activity was in milk collected during October, January and November 2014, and highest PG-derived PL activity in October, December and November 2015. Stage of lacta-tion is believed to cause the seasonal variation of PL activity. No PL activity was detected in UHT milk hence it was inactivated by the UHT treatment. The UHT treatment decreased the PG-derived PL activity by 75 % to 21.16 U/ml ±17 %. Neither storage temperature nor storage time affected PG-derived PL activity, and variations observed during storage were most likely due to within batch variation. There was no significant correlation between SCC and total PL and PG-derived PL activity, PL activity, or PG-derived PL activity in silo milk. Although SCC has been reported to co-vary with PL and PG-derived activity, the relatively low cell counts in the Swedish milk seems to be a plausible explanation for not observing this cor-relation.