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Subcritical water hydrolysis and carbonization of the biomass are an emerging green technology for seaweed biomass processing. In this work, a novel approach for co-generation of two energy streams from seaweed biomass (fermentable sugars and solid hydrochar) with subcritical water from a green macroalgae Ulva sp. was developed. It was found that for the released of glucose, xylose, rhamnose, fructose, and galactose, the process temperature is the most significant parameter, followed by salinity, solid load, and treatment time. For the formation of fermentation inhibitor 5-hydroxymethylfurfural (5-HMF), temperature also was the most important parameter, followed by residence time, salinity, and solid load. The optimum parameters for maximal release of total sugars under minimum formation of 5-HMF were 170 °C (800 kPa abs.), 5% solid loading, 40 min residence time, and 100% salinity. The hydrochar yield was 19.4% and hydrochar high heating value was 20.2 ± 1.31 MJ kg−1. These results provide new detailed information on the subcritical hydrolysis and carbonization of Ulva sp. biomass and show co-production of fermentable monosaccharides and hydrochar. [ABSTRACT FROM AUTHOR]

Edible green marine macroalgae, especially species of Ulva , can accumulate starch up to 30% of their dry weight; however, its physicochemical, functional, and digestibility properties are still unknown. In this study, we characterized molecular mass distribution, amylose-amylopectin ratio, crystallinity, hydration capacity, viscoelastic and pasting properties, and the digestibility of starch extracted from Ulva ohnoi and compared it to rice and potato starches. Ulva starch had a higher amylose content than rice and potato starches (55.0%, 34.5% and 24.3%, respectively). Ulva starch exhibited higher hydration capacity than rice starch (25.7 g water /g starch and 10.3 g water /g starch respectively), while potato starch exhibited the highest hydration capacity, of 41 g water /g starch , probably mainly due to presence of phosphate groups. Ulva starch had the lowest mass-average molecular weight ((1.17×106 g/mol), compared to rice (4.40×106 g/mol) and potato (5.80×106 g/mol) starches. Ulva starch exhibited the highest G′ and viscosity setback ratio after cooling-induced gelation following gelatinization, probably thanks to its uniquely high amylose content, which led to higher tendency for physical crosslinking by retrogradation. This may also explain the fact that Ulva starch exhibited somewhat lower digestibility after retrogradation, compared to rice and potato starches, which would be advantageous for forming resistant starch serving as a dietary fiber, and lowering glycemic index. The superior gel properties on the one hand and lower digestibility, which would be helpful in tackling obesity & diabetes, on the other hand, make U lva starch a unique and promising new functional food ingredient. [Display omitted] • Ulva ohnoi amylose content: 55% of the starch, more than most terrestrial starches. • Ulva starch exhibited higher hydration capacity than rice starch. • Gelatinized Ulva starch exhibited the highest G′ after cooling-induced gelation. • Ulva starch had somewhat lower digestibility than rice and potato starches. • Lower digestibility may lead to lower glycemic index, advantageous against diabetes. [ABSTRACT FROM AUTHOR]