Phenolic content and antioxidant activity of donkey milk kefir fortified with sulla honey and rosemary essential oil during refrigerated storage

The aim of this work was to evaluate the phenolic content and antioxidant activity of donkey kefir fortified with sulla honey and rosemary essential oil, during refrigerated storage. The type of flavouring strongly influenced the antioxidant activity of the kefir: rosemary essential oil kefir showed the highest 2,2 '-azino-bis-(3-ethylbenzthiazoline-6-sulfonic acid values; sulla honey kefir showed the highest ferric-reducing antioxidant power values but, at the same time, in both fortified kefirs, the thiol content decreased, probably because of the formation of polyphenol-protein complexes that would have influence the availability of bioactive components. The antioxidant activity increased significantly during refrigerated storage, showing the highest values after 15 days. Sensory analysis highlighted the fact that the donkey kefir was well accepted by consumers. Addition of sulla honey resulted in the highest acceptability, while addition of rosemary essential oil kefir was less accepted by consumers. This knowledge provides a basis that could lead to the production of fortified donkey kefir with specific nutraceutical characteristics.