Abstract
Apple seeds are one of the main by-products of the food industry. This research aims to characterize free and bound phenols from apple seeds first defatted with three methods: (1) supercritical CO2, (2) ultrasound assisted and (3) Soxhlet extraction. Next, defatted seeds were extracted with hydroalcoholic maceration to give free phenols fraction and a solid residue. The latter was further extracted by alkaline hydrolysis to give bound phenols fraction. The time, temperature, concentration of NaOH and solid-to-liquid ratio were optimized for maximum extraction of bound phenols. Results showed that, regardless to the defatting method used, the highest phenol content was found mainly in the bound fraction (up to 31 mg g−1 GAE), which was eight times higher than the free fraction. Also, the antioxidant capacity (DPPH and ORAC) of the bound fraction was twice that of the free fraction. Such differences were next linked to the polyphenol profile (HPLC-DAD), and to the electron transfer properties (HPLC-coulometric detector). Results showed that the highest electron transfer properties of bound phenols were explained with higher content and lower half-wave potential of redox species like protocatechuic acid, salvianic acid A, and caffeic acid (identified by HPLC HR-MS/MS). Instead, the redox properties of free phenol extracts were attributed mainly to phlorizin. Overall, bound phenols in defatted apple seeds are a rich source of antioxidants that can be easily extracted by alkaline hydrolysis.