IMPROVED FILTRATION METHOD TO ASSESS BACTERIOLOGICAL QUALITY OF RAW-MILK BASED ON BIOLUMINESCENCE OF ADENOSINE-TRIPHOSPHATE

To satisfy the increasing demand from the dairy industry for a simple rapid method of assessing the bacteriological quality of incoming raw milk, the ATP-F test was improved to make it more user-friendly. Raw milk was incubated at 40-degrees-C with a specially selected non-ionic detergent in a temperature-controlled filtration funnel with a membrane filter of diameter 8.3 mm. After concentration of the bacteria from the milk on the filter by vacuum filtration, the minifilter was transferred to a flat-bottomed cuvette and introduced into a luminometer. The bacterial ATP was extracted with a second extractant and assayed by addition of luciferase (EC 1.13.12.7) and luciferin. Measurement of light emission was integrated over 10 s and expressed as relative light intensity, I(r). The test took less than 6 min. The repeatability of the present ATP method was better than of the reference plate method. The standard deviation of repeatability of a single result, s(r), calculated for 102 samples after transformation of the duplicate values for ATP to the decadic logarithm of the numerical value of bacterial concentrations {C} in mL-1 by the regression line was 0.048. There was a good relationship between I(r) and {C}, as indicated by a low s(yx) (for 188 milk samples with a C above 10 000 mL-1, s(yx) was 0.312 log10 {C} and a good bacteriological grading of the milks. Because of the low detection level (C = 10 000 mL-1) the method is very useful as a sensitive and rapid semi-automatic method for fast microbiological screening of raw milk on arrival at the dairy plant.