Psychrotrophic vs. total bacterial counts in bulk milk samples

The objective of the study was to determine psychrotrophic bacteria counts (PBC) and total bacterial counts (TBC) in bulk milk samples collected during a longer period (1999 to 2000). Two sets of samples were analysed. Set 1, including samples collected in three herds, was analysed by the Central Laboratory at České Budějovice. TBC was determined using the apparatus Bactoscan, and PBC by culture according to IDF standards. Relative index pi expressing the PBC/TBC ratio was calculated for each sample. The value of pi for Set 1 was 0.09, ranging in herds included in this set from 0.05 to 0.20. The correlation between PBC and TBC was highly significant (r = 0.69; P < 0.01). Correlation coefficients for the individual herds ranged from 0.32 to 0.81. The correlation was highly significant (P < 0.01) in two herds, and significant (P < 0.05) in one herd. Microbiological milk quality is markedly influenced by housing and milking technologies. Another objective of this study was therefore to compare the microbiological quality of milk produced by cows housed in loose boxes and milked in parlours with that produced by cows housed in stanchion barns and milked with pipeline milking machines (Set 2). Bulk milk samples were collected and transported by standard methods and TBC and PBC were determined by culture on GTK agar supplemented with dried inhibitor-free milk according to the IDF standards. The results have confirmed favourable effects of milking in parlours on milk quality expressed in terms of counts of both technologically relevant groups of bacteria. The difference in TBC and PBC between the two milking technologies was significant (P < 0.05). Relative index for the milk obtained in milking parlours was pi= 0.18 and that for the milk from pipeline milking machines pi = 0.23. Correlation coefficients for PBC and TBC were almost identical (r = 0.92; r = 0.93; both P < 0.01).