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).

AbstrAct:The contamination of bulk samples of cow's raw milk (n = 491) by psychrotrophic lipolytic bacteria (PLiBC), total count of psychrotrophic bacteria (PBC) and mesophilic bacteria (TBC) was monitored for two years on eight dairy farms and the correlations among these groups of bacteria were analysed. An increase in TBC, PBC and PLiBC and in the values of free fatty acids (FFA) was tested experimentally in three milk samples in relation to time (analyses were done in 24-hour intervals until 96 hours) and storage temperature of milk samples (4; 6.5 and 10°C). Bacterial contamination of milk was determined by culture methods in accordance with IDF standards, the values of FFA were determined by an extraction-titration method. These mean values were determined in the set of samples (n = 491): PLiBC 659 CFU/ml, PBC 2 932 CFU/ml and TBC 18 932 CFU/ml. A high correlation was proved between values of PBC and PLiBC (r = 0.87; P < 0.001) while the correlation between TBC and PBC (r = 0.65; P < 0.001) and between PLiBC and TBC (r = 0.59; P < 0.001) was on a medium level. The proportional index p i for PLiBC/PBC was 0.20, for PLiBC/TBC 0.03 and for PBC/TBC 0.16. In seasonal dynamics a statistically significant difference (P < 0.001; P < 0.05) between the increased values of TBC in the summer season was proved compared to the winter and spring season. The differences in the seasonal variation of PBC and PLiBC values were not significant. Experimental investigation of an increase in the values of tested parameters showed that at temperatures of milk sample storage 4 and 6.5°C TBC did not exceed the permissible hygienic value (100 000 CFU/ml) even after 96 hours while at 10°C it amounted to 90 000 CFU/ml after 48 hours and the limit for TBC was exceeded several times after 96 hours. PBC, which is not inhibited by cold storage to such a large extent, did not exceed the hygienic limit value for PBC (50 000 CFU/ml) even after 96 hours when milk samples were stored at 4°C, but at 6.5°C after 72 hours and at 10°C already after 48 hours the values 6 and 20 times higher, respectively, than the hygienic limit were recorded. A similar trend was observed in PLiBC, which exceeded the hazardous limit (43 000 CFU/ml) at 6.5°C after 96 hours and at 10°C already after 48 hours whereas at 4°C the limit value was not exceeded even after 96 hours. The content of FFA also increased in relation to the storage time and temperature of milk samples but in comparison with the increase in the tested groups of microorganisms the increase in FFA showed a higher correlation with storage time compared to storage temperature. A medium correlation was calculated between PLiBC and/or PBC and FFA content (r = 0.52; r = 0.57; P < 0.001).

AbstrACt:The objective of the paper was to analyse the influence of dairy cow management technology, milking method, predipping and summer grazing on the contamination of cow's raw milk by mesophilic (TBC), psychrotrophic (PBC), lipolytic (PLiBC) and proteolytic (PPrBC) bacteria. The values of TBC, PBC, PLiBC and PPrBC in bulk milk samples were determined by the culture method according to IDF standards. Investigations were carried out in nine stables of seven dairy farms from January 2005 to June 2006. Summer grazing has the most marked influence on the values of studied parameters. Farms with summer grazing had a lower microbial contamination of milk compared to farms without grazing and the difference was statistically highly significant in all studied parameters (P < 0.001). A positive effect of predipping on a reduction in the values of milk microbial contamination was proved while the difference between farms with predipping and those without it was on a significance level P < 0.05 to 0.001 except PLiBC. A comparison of the influence of dairy cow management technology indicated the lowest values of all microbiological indicators in loose cubicle littered housing, higher values were determined in stanchion littered housing and the highest in loose slatted-floor housing. A statistical difference between the technologies was proved mainly in TBC (P < 0.001). Farms with milking in milking parlours had a lower microbial contamination of milk compared to farms that used the in-stall milking pipeline system but the difference was statistically significant only in TBC (P < 0.05).