Evaluation of Screening Test for Detection of Antimicrobial Residues in Ewe Milk

The effects of preservatives (potassium dichromate and sodium azide), heat treatment (untreated and 82°C/10min), and lactation stage upon the response of the microbial tests (BRT AiM and Delvotest) utilized for the detection of residues of antimicrobial substances in ewe milk were examined. Milk samples were collected from the morning milking of 50 Manchega ewes every 2wk, from 15 d postpartum until the end of lactation. A total of 2322 samples were analyzed by BRT AiM with prediffusion and Delvotest microbial tests. The specificity of preservative-free milk samples without heat treatment was high (96.3% for BRT and 97.7% for Delvotest), with results improving for those samples thermally treated at 82°C/10min (99.0% for BRT and 98.7% for Delvotest). Potassium dichromate produced a total inhibition of growth of Bacillus stearothermophilus with both methods. When acidiol was utilized, the specificity of the samples not treated thermally was lower compared with preservative-free milk samples for the BRT AiM (90.2%) and Delvotest (91.0%) methods, improving when the samples were thermally treated, both for BRT AiM (94.8%) and Delvotest (95.3%), given that the presence of the preservative increased the frequency of doubtful results. The lactation stage significantly affected the results of the methods, with a greater frequency of false-positive and doubtful cases toward the end of the cycle, especially in those samples preserved with acidiol. The greater selectivity in both methods was therefore obtained for preservative-free ewe milk samples with prior heat treatment taken at the beginning or in midlactation period.