Onopordum nervosum ssp. platylepis Flowers as a Promising Source of Antioxidant and Clotting Milk Agents: Behavior of Spontaneous and Cultivated Plants under Different Drying Methodologies

Abstract

This study is a contribution to the valorization of the Onopordum nervosum ssp. platylepis flowers in the agri-food sector. It aims to evaluate the effect of different drying methodologies (room, microwave, convective, oven and freeze-drying) on the chemical composition, the microbiological quality of aqueous extracts and their biological activities, namely clotting milk and antioxidant activities for both spontaneous and cultivated plants. The results showed that the drying methodology has significantly affected the phenolic composition. The extract obtained from freeze-dried flowers showed the best amounts of total phenols (6.19 ± 0.50 mg GAE/g), flavonoids (2.35 ± 0.07 mg QE/g) and condensed tannins (0.038 ± 0.002 mg CE/g) followed by the microwave-dried flowers. However, microwave-dried flowers presented the lowest protein content with only 3.58 ± 0.62 mg BSAE/g. Finally, the biological activities were significantly dependent on the used drying process. The extract of freeze-dried flowers was the most potent to inhibit free radicals (diammonium 2,2′-azino-bis-(3-ethylbenzothiazoline-6-sulphonate) (ABTS), 1,1-diphenyl-2-picrylhydrazyl (DPPH)) and to reduce iron with the efficient concentrations EC50s of 0.48 ± 0.02 mg/mL, 0.16 ± 0.04 mg/mL and 1.52 ± 0.19 mg/mL, respectively. It also showed the highest clotting milk activity followed by room-dried and forced convective-dried flowers. As a consequence, freeze drying was the best method for preserving most chemical and biological properties of Onopordum nervosum ssp. platylepis flowers. However, in consideration of production costs and drying duration, the convective dryer is recommended as a more practical choice. All the obtained results do not reveal a significant difference between cultivated and spontaneous plants, which can suggest that the domestication of Onopordum nervosum ssp. platylepis can be useful for industrial applications.