Influence of Preservative Concentration, pH Value and Fat Content in Raw Milk at Detection Limit of Microbial Inhibitor Tests (DelvotestA® Accelerator) for Amoxicillin and Oxytetracycline

The presence of antibiotic residues in raw milk is a big threat not only to the dairy industry but also to public health. Therefore, it is very important to detect antibiotic residues in raw milk. There are many reliable antibiotic test used in dairy laboratories. Unfortunately, some factors can affect the results causing, so-called false-positive results. The aims of this paper are to check the detection limits of microbiological inhibitor test with test organisms Geobacillus stearothermophilus var. calidolactis, which are provided on DelvotestA (R) Accelerator instrument (DSM, The Netherlands) for antibiotics amoxicillin and oxytetracycline and to examine the influence of variations of milk fat contents, pH values and azidiol concentrations in raw milk on detection limits, e.g., test results. The obtained results showed that detection limit for amoxicillin is 3 mu g/L and oxytetracycline 250 mu g/L; concentration of preservative azidiol of 0.1 mL per 40 mL of milk does not affect the detection limits of the test; microbiological inhibitor test is extremely sensitive to low pH value of samples so, it is recommended to avoid testing milk samples with low pH value, or to adjust the sample's pH value by addition of some base; and that fat content in raw milk up to 5 % does not affect the detection limit of the methods for amoxicillin and for oxytetracycline.