Optimisation of Ultrasound Frequency, Extraction Time and Solvent for the Recovery of Polyphenols, Phlorotannins and Associated Antioxidant Activity from Brown Seaweeds

Abstract

This study investigates ultrasound assisted extraction (UAE) process parameters (time, frequency and solvent) to obtain high yields of phlorotannins, flavonoids, total phenolics and associated antioxidant activities from 11 brown seaweed species. Optimised UAE conditions (35 kHz, 30 min and 50% ethanol) significantly improved the extraction yield from 1.5-fold to 2.2-fold in all seaweeds investigated compared to solvent extraction. Using ultrasound, the highest recovery of total phenolics (TPC: 572.3 ± 3.2 mg gallic acid equivalent/g), total phlorotannins (TPhC: 476.3 ± 2.2 mg phloroglucinol equivalent/g) and total flavonoids (TFC: 281.0 ± 1.7 mg quercetin equivalent/g) was obtained from Fucus vesiculosus seaweed. While the lowest recovery of TPC (72.6 ± 2.9 mg GAE/g), TPhC (50.3 ± 2.0 mg PGE/g) and TFC (15.2 ± 3.3 mg QE/g) was obtained from Laminaria digitata seaweed. However, extracts from Fucus serratus obtained by UAE exhibited the strongest 1,1-diphenyl-2-picryl-hydrazyl (DPPH) scavenging activity (29.1 ± 0.25 mg trolox equivalent/g) and ferric reducing antioxidant power (FRAP) value (63.9 ± 0.74 mg trolox equivalent/g). UAE under optimised conditions was an effective, low-cost and eco-friendly technique to recover biologically active polyphenols from 11 brown seaweed species.

Keywords: antioxidant capacity; conventional extraction; macroalgae; phlorotannin; polyphenols; ultrasound assisted extraction.

Figure 1

Effects of ultrasound assisted extraction (UAE) conditions (solvent concentration (30%, 50% and 70% ethanol), ultrasonic frequency (control, 35 kHz and 130 kHz) and UAE treatment time (10 and 30 min)) on the extract yield obtained from F. vesiculosus. Different letters indicate statistical differences (p < 0.05) on the yields of seaweed extract obtained using different solvents at each US frequency: control (m-n), 35 kHz (o-p) and 130 kHz (q-s). ^abc columns with similar letters are not significantly different (p < 0.05) treated for 10 min; ABC columns with similar letters are not significantly different (p < 0.05) treated for 30 min. The extraction yield is calculated by using following formula: Extraction yield (%) = (weight of dry extract / weight of dry sample) × 100.

Figure 2

SEM images of F. vesiculosus (a) untreated samples (intact and dried macroalgae), (b) control samples (50% ethanol, 30 min, no ultrasound) and (c) UAE treated samples (35 kHz, 50% ethanol, 30 min). Scale bars: 300 µm (magnification 150×), 200 µm (magnification 250×) and 50 µm (magnification: 1000×).

Figure 3

Total polyphenol (a), total phlorotannin (b) and total flavonoid (c) content from 11 seaweed species obtained using UAE (grey bars) and conventional solvent extraction (white bars) technologies. The statistical differences in bioactive compounds extracted using UAE or conventional solvent extraction technologies for each seaweed are represented as * p < 0.05, ** p < 0.01 and *** p < 0.001. Different letters indicate statistical differences (p < 0.05) in the yields of bioactive compounds between seaweed obtained by UAE (uppercase letters) or conventional solvent extraction (lowercase letters). TPC (total phenolic content), TPhC (total phlorotannin content) and TFC (total flavonoid content) are expressed as mg gallic acid equivalents (GAE)/g dried weight extract, mg phloroglucinol equivalents (PGE)/g dried weight extract and mg quercetin equivalents (QE)/g dried weight extract, respectively. Abbreviation of seaweed species are as follows: PC (Pelvetia caniculata), FV (Fucus vesiculosus), LS (Laminaria saccharina), LH (Laminaria hyperborea), FSp (Fucus spiralis), AN (Ascophyllum nodosum), FSe (Fucus serratus), HE (Himanthalia elongata), HS (Halidrys siliquosa), LD (Laminaria digitata) and AE (Alaria esculenta).

Figure 4

Antioxidant capacity measured as 1,1-diphenyl-2-picryl-hydrazyl (DPPH) activity (a) and ferric reducing antioxidant power (FRAP) (b) of 11 seaweed extracts obtained from UAE (grey bars) and conventional solvent extraction (white bars) techniques. The statistical differences in antioxidant activity extracted by using UAE or conventional solvent extraction for each seaweed are represented as * p < 0.05, ** p < 0.01 and *** p < 0.001. Different letters indicate statistical differences in the antioxidant activity among seaweed species obtained by UAE (uppercase letters) or conventional solvent extraction (lowercase letters). DPPH and FRAP: expressed as mg trolox equivalent (TE)/g of dry weight extract. Abbreviation of seaweed species are as follow: PC (Pelvetia caniculata), FV (Fucus vesiculosus), LS (Laminaria saccharina), LH (Laminaria hyperborea), FSp (Fucus spiralis), AN (Ascophyllum nodosum), FSe (Fucus serratus), HE (Himanthalia elongata), HS (Halidrys siliquosa), LD (Laminaria digitata) and AE (Alaria esculenta).