Is Acid Pretreatment Beneficial for the Saccharification and Fermentation of Alginate from Brown Macroalgae?

Brown macroalgae are actively considered for the production of biofuels and chemicals. In the overall process of utilizing brown macroalgae, the monomerization of alginate which is the main carbohydrate of brown macroalgae is critical and prerequisite. In general, the physicochemical pretreatment of biomass leads to increased enzymatic saccharification. To obtain a high reducing sugar yield from alginate, acidic pretreatment was performed at various temperatures such as 120–160°C for 15–45 min using 1% (w/v) sulfuric acid in this study. These acid pretreatment processes resulted in the formation of mannuronic and guluronic acids and oligosaccharides of alginate. The enzymatic saccharification of pretreated alginate by sequential treatments of endo- and exo-alginate lyases resulted in a 52.2% final reducing sugar yield, which was only 7.5% higher than when using unpretreated alginate as the substrate for the enzymatic saccharification. Rigorous analyses of reaction products of alginate by TLC, LC-MS, and MALDI/TOF-TOF MS showed that the depolymerization of alginate in acidic pretreatment occurs via hydrolysis and oligosaccharides saturated at their non-reducing ends are formed as a result of the acidic hydrolysis. The enzymatic saccharification of these saturated alginate oligosaccharides leads to the formation of mannuronic and guluronic acids which are not fermentable monomeric sugars. These result indicated that acidic pretreatment is not a desirable process for the saccharification and fermentation of alginate from brown macroalgae.