Physicochemical, rheological and digestibility characterization of starch extracted from the marine green macroalga Ulva ohnoi

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 Ulva starch a unique and promising new functional food ingredient.