초록
Three types of seaweed-sea lettuce, chigaiso, and agar weed-were used as representatives of green, brown, and red seaweeds, respectively, and methods for obtaining high concentrations of ethanol (bioethanol) from these seaweeds were investigated. Starch, β-1,3-glucan, and cellulose were present as glucans in the seaweeds examined, though the cellulose content was relatively low compared to that in lignocellulose. However, little to no lignin was present in the seaweeds to interfere with the hydrolysis of polysaccharides, making the polysaccharides in these seaweeds easily hydrolyzable. Glucans are the only polysaccharides in sea lettuce and chigaiso that can be hydrolyzed to fermentable sugars. Successive saccharification with an enzyme, in which the hydrolysate from a primary saccharification, after removing the residue, is successively used as the hydrolyzing liquid for a secondary saccharification, was effectively used to obtain high concentrations of glucose from these seaweeds. An ethanol yield of more than 3% was obtained from these seaweeds. Agar weed, on the other hand, contains both galactan and glucan. For this reason, it was possible to obtain a high concentration of ethanol from agar weed using combined saccharification, which is the acid hydrolysis of galactan to produce galactose followed by the enzymatic hydrolysis of glucan to obtain glucose. The resulting sugars were fermented to produce ethanol by Saccharomyces cerevisiae IAM 4178, and a concentration of ethanol as high as 5.5% was produced. This is higher than the 4-5% ethanol concentration that was declared as an economically feasible concentration for distillation.