Influence of Oxidation and Dialysis of Phlorotannins on Bioactivity and Composition of Ultrasound-Assisted Extracts from Ascophyllum nodosum

The isolation and chemical characterization of phlorotannins has gained special attention in recent years due to their specific health-promoting benefits. Flow-cell ultrasound-assisted extraction (90 W/cm2 of sonication power, 2 min of retention time and 20 g solvent/g algae of liquid–solid ratio) was carried out by using double-distilled water (WE) and acetone:water mixture (AWE) as extraction solvents. The AWE showed a higher total polyphenols content (TPC), carbohydrates (CHOs) and antioxidant activities than WE. However, when the WE was purified by using Amberlite XAD16 column, the purified WE (PWE) showed similar a TPC, decreased CHOs and increased antioxidant activity compared to WE. The oxidation of the PWE extract was evaluated under natural, forced and severe oxidation condition for 120 h. Only severe oxidation conditions were able to significantly reduce TPC and antioxidant activities. PWE was dialyzed (20, 10, 3.5 and 2 kDa). The main bioactive fraction of phlorotannins was obtained from 10 to 20 kDa. CHOs were distributed in fractions below 20 kDa. MALDI-TOF analysis was performed for PWE, PD20 and PD2 extracts to analyze the degree of polymerization of phlorotannins, which ranged from 4 to 17 phloroglucinol units/molecule. Fragmentation patterns allowed the proximate identification of several phlorotannins in Ascophyllum nodosum extracts.