Comparison between Colifast® Milk and the standard method for the detection of coliforms in pasteurised milk

Abstract

Colifast® Milk is a rapid screening test for the detection of total coliforms in milk based upon the measurement of change in fluorescence during an incubation period, due to the targeted -D-galactosidase activity on 4-methyl-umbelliferone--D-galactoside contained in a selective growth medium. In this work, 800 samples of homogenised pasteurised milk, with different fat content (1.5 and 3.5%) and contaminated with various concentrations of coliforms (from 0.03 to > 10000 CFU·mL -1 ), were analysed in order to compare the results obtained by Colifast ® Milk with those by the standard method. The effect of the incubation temperature (30 and 39 °C) was also investigated. For the totality of samples the correspondence between coliform counts obtained by the two methods was 64% (r 2 = 0.743; P = 0.88). The performance of Colifast ® Milk was notably affected by the level of contamination, since for samples with coliforms > 10 CFU·mL -1 the correspondence between the two methods achieved 86% (r2 = 0.837; P = 0.94) whereas for samples with coliforms 10 CFU·mL -1 it fell to 42% (r 2 = 0.073; P = 0.33). Fat content also influenced the response of the Colifast ® system since the correspondence decreased from 80% (r 2 = 0.767; P = 0.91) for semi-skimmed milk to 48% (r2 = 0.724; P = 0.63) for whole milk. Incubation at 30 °C improved the recovery of coliforms by Colifast ® Milk as the correspondence between values obtained with the two methods reached 72% (r 2 = 0.760; P = 0.89) if compared with 56% (r 2 = 0.735; P = 0.87) when the incubation temperature was 39 °C. Under these operating conditions the sensitivity showed by the fluorometric method appeared to be not sufficient for the detection of coliforms in pasteurised milk. Colifast ® Milk / coliform / method / milk