Exploring the bioactive potential of algae residue extract via subcritical water extraction: Insights into chemical composition and biological activity

Gelidium sesquipedale is valued in the Spanish agar industry, but its production generates substantial waste, often discarded despite its nutritional and bioactive content. Subcritical water extraction (SWE) at 175 °C and 50 bar for 130 min was performed on this waste after agar extraction, comparing it to conventional ethanol extraction. The SWE extract exhibited superior nutritional profile, including proteins (170.6 ± 1.0 mg/gfreeze-dried-extract), essential amino acids (18.1%), carbohydrates (148.1 ± 0.3 mg/gfreeze-dried-extract), total phenolic content (57 ± 7 mg-EqGA/gfreeze-dried-extract), and also containing Maillard reaction compounds, such as 5-hydroxymethylfurfural, furfural, 2-furanmethanol, 1-(2-furanyl)-ethanone, and 5-methyl-2-furfural, influencing color, aroma and flavor. This extract showed better antioxidant and anti-inflammatory properties than the conventional extract, and higher xanthine oxidase, tyrosinase, and acetylcholinesterase inhibition activities. Toxicological assessment on human cells indicated the safety of the SWE extract. Therefore, SWE technology offers a promising method to valorize G. sesquipedale residue, yielding a bioactive and nutrient-rich extract suitable for food and nutraceutical applications. •SWE efficiently valorizes underexploited algal residue over conventional extraction•Superior nutritional profile, including Maillard compounds, is found in SWE extract•SWE extract has stronger antioxidant and anti-inflammatory benefits than ethanolic•Higher enzymes inhibition related to various pathologies is found in SWE extract•The SWE extract's safe profile makes it suitable for foods and nutraceuticals