Physicochemical and Microbiological Properties of Yoghurt Made Form Defatted Milk Mixtures and Concentrated Buttermilk Fortified with Herbal Extract

This study evaluates the impact of plant-derived additives (Melissa officinalis L., Plantago major L., and Inula helenium L.) on the physicochemical, microbiological, and antimicrobial properties of yoghurt formulated with a combined base of milk and buttermilk concentrate (solid content 8–10%). Over 14 days of refrigerated storage, all fortified yoghurts maintained pH within acceptable limits, while Plantago and Inula slightly enhanced post-acidification without compromising quality. Notably, the buttermilk concentrate significantly boosted initial lactic acid bacteria (LAB) viability, and the yoghurt with Plantago major L. extract showed a 314% higher viable LAB count during storage compared with the control. In contrast, Inula helenium L. reduced LAB viability by 67.6%, indicating a trade-off between bioactivity and starter compatibility. Antimicrobial testing via disk-diffusion revealed selective activity against Gram-positive Staphylococcus aureus (maximum inhibition radius: 5.8 ± 0.5 mm for Plantago yoghurt), with no effect on Gram-negative E. coli or P. aeruginosa. Activity was driven primarily by acid-dependent metabolites, with Plantago retaining residual non-acidic bioactivity after neutralization. The combination of Plantago major L. with buttermilk concentrate emerged as optimal for functional yoghurt, enhancing higher viability of lactic acid bacteria during refrigerated storage. These findings support the use of compatible plant–dairy combinations to develop yoghurt products with improved physicochemical properties, LAB viability and microbiological stability under the tested conditions.