Monitoring Antibiotic Residues in Extensively Managed Raw Sheep Milk for Enhanced Food Quality and Safety

Introduction: Extensive livestock grazing of native sheep breeds plays a vital role in rural sustainability and the production of traditional Idiazabal Protected Designation of Origin (PDO) cheese. However, these raw milk-based production systems are vulnerable to veterinary drug residues. Antibiotic residues in raw ewe milk pose a critical public health concern regarding antimicrobial resistance dissemination and threaten the dairy industry by compromising technological microbiota viability. Therefore, this study evaluates antibiotic residues at the farm level by combining commercial rapid screening assays with chromatographic confirmation.

Methods: A total of 51 raw ewe milk samples were collected from 26 extensive farms distributed across the Basque Country and Navarre (Southwestern Europe). Antibiotic residues were screened using the microbiological inhibition test Eclipse Farm^3G (Zeulab S.L., Zaragoza, Spain). Positive and doubtful samples were subsequently analysed by confirmatory LC-QqQ-MS/MS, as previously described (Santamarina-García et al., 2024).

Results and Discussion: Screening results showed that 10% of the samples tested positive and 2% were close to the detection limit. Following confirmatory LC-QqQ-MS/MS analysis, Penicillin G was quantified in a single sample at levels below 1 μg/kg, safely below the European Maximum Residue Limit (MRL) of 4 μg/kg. The discrepancy between techniques is attributed to the broader detection spectrum of the microbiological assay and the presence of active antibiotic metabolites undetected by specific chromatography. Despite being sub-MRL, these concentrations could still favour resistant bacteria development and affect cheesemaking.

Conclusions: This workflow demonstrates that while rapid assays are excellent frontline screening tools, continuous enhancement and combination of both techniques are required to ensure food quality and safety.