Food & Nutrition

Arab Journal of Food & Nutrition
12. A Study on the Use of the Yoghurt Starter Culture Bacteria for the Detection
of Antibiotic Residues in Milk (1994)
Lina M. A. AL-Kurdi\ University of Jordan
Supervisor: Dr. Mohammed Isam Yamani
The sensitivity of yogurt culture bacteria (Lactobacillus delbruekii subsp.
bulgaricus and Streptococcus thermophilus) to antibiotics was used to develop a test
for the detection of antibiotic residues in milk. In order to standardize milk
fermentation by yoghurt culture and to fix the time needed for curdling of the milk,
different combinations of culture concentration (2,3,4, and 5%) and milk pH values (6,
6.2, and 6.4) were tried to find out the shortest possible time for curd formation. The
best combination was the culture concentration 4% and milk pH 6. The developed test,
Yoghurt Culture Test (YCT), could be done by lowering the pH of 96 ml of the tested
milk to 6 using 1 N HCI, adding 4 g of yoghurt culture and incubation at 42°C for
2.5h. Negative result (absence of inhibitory substances in the milk) is indicated by
curd formation and change in the color of the pH indicator chlorophenol red (0.2% in
50% ethanol), of which 1 ml is added before or after the incubation.
The lowest concentration of penicillin enough to inhibit curd formation by
yoghurt culture bacteria in YCT was 0.03 lU/ml, whereas for chloramphenicol,
oxytetracycline, tetracycline, ampicillin, erythromycin and cloxacillin were: 2.0; 0.1;
0.2; 0.1; 0.3 and 0.3 g/g, respectively. YCT was more sensitive to chloramphenicol
and oxytetracycline than DelvotestP® and was of comparable sensitivity to
tetracycline and erythromycin, but of less sensitivity to penicillin and ampicillin.
YCT was used to examine 618 samples of milk received by the largest three
dairy plants in Jordan for antibiotic residues. 15% of the samples gave positive results
after 2.5 h of incubation. When extending the incubation time to 4 h only 2.1% of the
samples were positive, indicating the presence of other inhibitory substances in about
13% of the samples in marginal concentrations. When testing the positive samples to
YCT after 2.5 and 4 h by DelvotestP®, only 12.3% were positive after 2.5 h and the
results were similar to YCT after 4 h.
YCT is a simple and of low cost test. It has comparable reliability to
DelvotestP®. Testing milk samples for antibiotic residues by the same culture used in
the processing is an advantage, because it reflects the actual suitabilty of the tested
milk for processing as yoghurt. Thus YCT could be recommended as an alternative to
the commercial testing methods.
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Volume 11, No. 25, 2011
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