Aqueous extracts of Ascophyllum nodosum obtained by ultrasound-assisted extraction: effects of drying temperature of seaweed on the properties of extracts

Abstract

The purpose of this study was to determine the effects of convective air-drying at different temperatures (35, 50, 60 and 75 °C) on the colour and particle size distribution of Ascophyllum nodosum seaweed powders obtained after milling (mesh 500 μm). The antioxidant activity and polyphenolic and carbohydrate content of the aqueous extracts obtained by ultrasound-assisted extraction from air-dried and freeze-dried samples (−55 °C, 0.5 Pa) were also determined. Particle size characterization of the seaweed powders indicated that the highest mass fraction, for all drying temperatures, corresponded to a range of sizes of 250–500 μm. Significant differences of mean diameters of powder between samples dried below and above 50 °C were found (P ≤ 0.05). Ascophyllum nodosum seaweed powders exhibited a predominance of greenness and yellowness with differences between powders particle size obtained from A. nodosum dried at 35–50 °C (yellowish green) and 60–75 °C (brown). A study of the effect of drying temperature on polyphenolic content (TP), DPPH scavenging activity (SA) and carbohydrate content (CHO) of A. nodosum aqueous extracts obtained from all powders and freeze-dried samples showed a maximum TP (3180 mg PHL (100 g)−1 d.w.), SA (76.2%) and CHO (6465 mg GL (100 g)−1 d.w.) for samples dried at 50 °C. These values were higher than those measured for freeze-dried samples (1500 mg PHL (100 g)−1 d.w., 70.9% and 5136 mg GL (100 g)−1 d.w.). A relationship between SA of aqueous extracts and the colour of air-dried, milled A. nodosum was found. SA and CHO decreased with increasing air-drying temperature. The same trend with temperature was observed for aqueous extracts obtained only with the highest mass fraction, with lower values of TP, SA and CHO.