Comparison between Colifast (R) Milk and the standard method for the detection of coliforms in pasteurised milk

Colifast(R) 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 beta-D-galactosidase activity on 4-methyl-umbelliferone-beta-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(R) Milk with those by the standard method. The effect of the incubation temperature (30 and 39 degreesC) 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(R) 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% (r(2) = 0.837; P = 0.94) whereas for samples with coliforms less than or equal to10 CFU . mL(-1) it fell to 42% (r(2) = 0.073; P = 0.33). Fat content also influenced the response of the Colifast(R) system since the correspondence decreased from 80% (r(2) = 0.767; P = 0.91) for semi-skimmed milk to 48% (r(2) = 0.724; P = 0.63) for whole milk. Incubation at 30 degreesC improved the recovery of coliforms by Colifast(R) 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 degreesC. Under these operating conditions the sensitivity showed by the fluorometric method appeared to be not sufficient for the detection of coliforms in pasteurised milk.