Ultrasound-assisted extraction and structural characterization by NMR of alginates and carrageenans from seaweeds

•Alginates and carrageenans were extracted from seaweeds using ultrasound.•Extraction parameters were optimized.•Ultrasound allows shortening of the extraction time and enhances extraction yields.•SEC, FTIR and NMR were used to characterise extracted biopolymers.•Ultrasound treatment for up to 30min does not alter polysaccharides. Polysaccharides from seaweeds are interesting materials for food and pharmaceutical applications such as drug delivery due to their biocompatibility and biodegradability. Extraction of these biopolymers is usually performed during several hours to obtain a significant extraction yield. In this paper, we report on a new process to extract alginates from brown seaweeds (Sargassum binderi and Turbinaria ornata) and carrageenans from red seaweeds (Kappaphycus alvarezii and Euchema denticulatum) with the assistance of ultrasound. The effect of several parameters (pH, temperature, algae/water ratio, ultrasound power and duration) was investigated to determine optimal extraction conditions. The extracted polysaccharides represented up to 55% of the seaweeds dry weight and were obtained in a short time (15–30min) as compared to 27% in 2h for conventional extraction. NMR, FTIR and SEC analysis were used to characterise the extracted polymers. Ultrasound allowed the reduction of extraction time without affecting the chemical structure and molar mass distribution of alginates and carrageenans.