A simple test for the detection of antibiotics and other chemical residues in ex-farm milk

A simple Yoghurt Culture Test (YCT) for the routine detection of antibiotic and other residues in raw milk was developed. It involved acidifying a sample of the milk to pH 6.0, adding Chlorophenol Red as an indicator and inoculating the milk (4%, v/v) with a culture containing a balanced mixture of Streptococcus thermophilus and Lactobacillus delbrueckii sub-sp. bulgaricus . In the absence of inhibitory substances, incubation at 42 °C for 2.5 h gave rise to sufficient acid production for the milk to gel and the indicator to change colour (PASS), but low levels of the antibiotics tested increased the incubation time beyond 4 h (FAILURE). A survey of 618 samples of raw milk collected from three dairies in Jordan identified a failure rate of 15% with the YCT at 2.5 h whereas, due to differences in sensitivity, the Delvotest-P failed only 12.3% of the milks. It was concluded that the YCT was a reliable and inexpensive approach for testing ex-farm milks for inhibitory residues.