Enhancing Set-Type Yoghurt With Olive Leaf Extract: A Comparison of Powdered Addition and Surface Spraying on Quality, Functionality, and Sensory Attributes

In this study, the effects of using powdered olive (Olea europaea L.) leaf extract (POLE) in yoghurt at different treatments and concentrations on the physicochemical, textural, functional, microbiological, and sensorial aspects of yoghurt were investigated. Accordingly, the enrichment of yoghurts with POLE was carried out by spraying aqueous solutions of POLE (0.4% and 0.6% solution) on the yoghurt surface after fermentation and adding POLE (0.4% and 0.6% w/v) to yoghurt before fermentation. While both POLE treatments caused no difference in the total solids, fat, protein, and ash contents of yoghurt, yoghurts sprayed with olive leaf extract aqueous solution exhibited similar results to the control regarding color and textural properties. Nevertheless, yoghurt enriched with POLE before fermentation showed higher total phenolic content and antioxidant properties. POLE treatments of yoghurt provided the expected antimicrobial effect without causing any change in the viable counts of yoghurt bacteria among the samples during 28 days of storage compared to plain yoghurt. Furthermore, surface spraying of POLE aqueous solution prevented the occurrence of an undesirable bitter taste in yoghurt resulting from the addition of POLE before fermentation. These results reveal that olive leaf, which is a cost-free waste product of the olive industry, can be used as a potential antimicrobial and functional agent. In addition, the positive sensory results obtained by spraying POLE aqueous solution on the surface after fermentation suggest that this application may be an alternative method for using herbal extracts.