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In this study, we characterized the bioactive properties of three important brown seaweed species, Sargassum thunbergii, Undaria pinnatifida, and Saccharina japonica, by subcritical water extraction (SWE), as these species are well known for their beneficial health effects. Their physiochemical properties, including potential antioxidant, antihypertensive, and α-glucosidase inhibitory activity, and the antibacterial activity of the hydroysates were also analyzed. The highest total phlorotannin, total sugar content, and reducing sugar content in the S. thunbergii hydrolysates were 38.82 ± 0.17 mg PGE/g, 116.66 ± 0.19 mg glucose/g dry sample, and 53.27 ± 1.57 mg glucose/g dry sample, respectively. The highest ABTS+ and DPPH antioxidant activities were obtained in the S. japonica hydrolysates (124.77 ± 2.47 and 46.35 ± 0.01 mg Trolox equivalent/g, respectively) and the highest FRAP activity was obtained in the S. thunbergii hydrolysates (34.47 ± 0.49 mg Trolox equivalent/g seaweed). In addition, the seaweed extracts showed antihypertensive (≤59.77 ± 0.14%) and α-glucosidase inhibitory activity (≤68.05 ± 1.15%), as well as activity against foodborne pathogens. The present findings provide evidence of the biological activity of brown seaweed extracts for potential application in the food, pharmaceutical, and cosmetic sectors.

Bioactive compounds were extracted from the brown seaweed Ecklonia stolonifera using subcritical water hydrolysis, a green extraction technique to valorize raw biomass. The reaction temperature for the hydrolysis ranged between 120 °C and 220 °C and the products were compared with those obtained by methanol extraction. Reaction temperature played a significant role in the release of active compounds such as sugars, phenolics, flavonoids, and antioxidants. The optimum extraction temperature was found to be 195 °C; at this temperature, the yield of reducing sugars was 61.83 ± 1.63 mg glucose, the total phenolic compound content was 26.02 ± 0.05 mg phloroglucinol, and the total flavonoid compound content was 8.40 ± 0.18 mg quercetin g−1 of E. stolonifera sample on a dry weight basis. Six different phenolic compounds were identified by high performance liquid chromatography (HPLC), of which gallic acid was the most abundant. Hydrolysates obtained using subcritical water extraction possessed strong antioxidant and antimicrobial activities. The results of this study will be useful for further valorizing brown seaweeds in applications with potential benefits for humans.