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Application of innovative and rapid flow cytometry method for determination of psychrotrophic bacteria in raw milk

Cooling and longer storage of raw milk at low temperatures favors the growth of psychrotrophic bacteria which pose a serious problem for the dairy industry, as they affect the deterioration, sustainability and milk yield during processing. Therefore, the number of these bacteria in cooled milk compared to the number of mesophilic aerobic bacteria is significantly higher than their ideal ratio of 10%. For the industry it is of crucial importance to rapidly determine the number of psychrotrophic bacteria in raw milk. Flow cytometry method compared to the time consuming reference method is significantly different in the expression of the results. Flow cytometry method determines the total individual bacterial count (IBC) that represents the total number of living and non-living bacteria and needs to be converted to the total number of living psychrotrophic bacteria (CFU). The aim of this study was to establish the regression equation for psychrotrophic bacteria by which the IBC is converted into CFU. In 287 raw milk samples the IBC was determined by flow cytometry method and the CFU by reference method (ISO 6730). The conversion line y= 1, 0594x-0, 7874 was calculated by using linear regression (ISO 21187) where y represents the logarithmic value of CFU and x the logarithmic value of the IBC. The established conversion line will enable the dairy industry to quickly and reliably determine the total number of psychrotrophic bacteria in raw milk.

psychrotrophic bacteria, flow cytometry, IBC, CFU, conversion line

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