Propionic acid bacteria (PAB) are extremely important because of the metabolites they produce (vitamins, propionic acid, conjugated linoleic acid, and bioactive peptides). Nowadays, the increasing interest in healthy nutrition has led to the formation of the concepts of probiotics, postbiotics and functionality with studies. Vitamin B12 is one of the postbiotics produced by PAB and is known to be beneficial for human health. Due to its coenzyme feature, it plays a key role in the body process, as well as shows anti-inflammatory properties. The recommended dietary intake of vitamin B12 for adults is 4 μg/day. Although it is highly found in meat products, it is known that the bioavailability of B12 in dairy products is higher. Fermented dairy products, especially products that contain PAB in their natural microflora, are good tools for the production and intake of vitamin B12. In this study, the factors affecting the microbial production of vitamin B12 were examined.

This study investigated the occurrence of antimicrobial residues in bulk tank milk (BTM) from dairy herds in the Thrace region of Türkiye. A total of 122 samples were analysed using a broad-spectrum microbiological inhibition assay (BRT hi-sense) and a commonly used end-point screening test (Bioeasy 2IN1 BT) targeting β-lactams and tetracyclines. BRT-positive samples were further analysed using a Bioeasy 16IN1 multi-residue test. Antimicrobial residues were detected in 13 samples (11%) by BRT hi-sense, whereas no β-lactam or tetracycline residues were identified by the Bioeasy 2IN1 BT test. The 16IN1 test identified antimicrobial classes in 9 positive samples, with macrolides (n = 7) and sulfonamides (n = 4) most frequently detected. Two samples showed multiple antimicrobial residues. LC-MS/MS confirmed macrolide residues above maximum residue limits in selected two samples. These findings indicate limitations of mandatory end-point–based residue screening conducted at dairy plants in Türkiye and highlight the need for broader-spectrum screening to improve residue surveillance and milk safety.