Residual antibiotics in milk at maximum residual limits affects yoghurt fermentation but not yoghurt rheology

Background Antibiotic resistance is a challenge that negatively affects human, animal and environmental health. Beyond the negative impacts of antibiotic resistance on human health, it likely prolongs fermentation and causes quality losses in fermented milk products. Aim(s) The aim of the study was to investigate the effects of six different antibiotics (sulfamethazine, amoxicillin, ampicillin penicillin G, enrofloxacin and gentamicin) added to milk at maximum residue limits (MRLs) on yoghurt fermentation time and rheology. Methods Antibiotic‐free raw cow milk spiked with each antibiotic separately at the MRL level for each antibiotic was converted to yoghurt. The fermentation profile of each sample was monitored. The number of yoghurt bacteria, rheological and textural profiles of yoghurt samples were measured at day 1 and day 30. Major findings All antibiotic‐added milks had longer fermentation times compared with the control sample, being more pronounced in the ones spiked with gentamicin (100 μl/L) and penicillin G (4 μl/L). The time taken for the milks added with gentamicin and penicillin G to reach pH 4.6 was 73 and 74 min longer than the control sample, respectively. The sample with the closest fermentation time to the control sample was the sample added with sulfamethazine at 100 μl/L of milk (17 min longer than the control). Although there were differences in the rheological and textural characteristics of yoghurt samples, this was independent of the type of antibiotic used. Antibiotic had no effect on the number of Streptococcus thermophilus, but Lactobacillus delbrueckii subsp . bulgaricus counts were lower than those of the control sample regardless of the antibiotic type. Scientific or industrial implications This study revealed that the presence of the most commonly used antibiotics at MRL levels in milk prolongs the fermentation period and that the adverse effects of antibiotic residues may vary depending on the type of antibiotic applied.