A Cookbook for Bioethanol from Macroalgae: Review of Selecting and Combining Processes to Enhance Bioethanol Production

PURPOSE OF REVIEW: The depletion of fossil reserves and environmental challenges associated with fossil fuels are major drivers of the search for sustainable renewable energy sources. Bioethanol production from macroalgae is one of the promising alternatives to reduce use of fossil fuels and achieve energy security and ecological sustainability. The purpose of this review is to critically discuss the options to optimize the process parameters for steady production of bioethanol from macroalgae. RECENT FINDINGS: A comprehensive literature review reveals that bioethanol production from macroalgae not only depends on the macroalgae type but also on the selection of pretreatment, hydrolysis, and fermentation options. Unlike the first- and second-generation feedstocks, macroalgae contains low concentrations of glucans. Thus high bioethanol concentration cannot be achieved by converting only glucans. Therefore, it is important to produce bioethanol from other carbohydrate components of macroalgae, such as alginate, sulphated polysaccharides, carrageenan, mannitol, and agar. The selection of the right hydrolysing agents (e.g., enzyme and/or acid) and steps to minimize formation of inhibitors during the process were found to be important factors affecting the efficiency of hydrolysis process. The hydrolysis enzymes currently used were developed for lignocellulosic and starch-based biomass, not for macroalgae, which is different in polysaccharide structure and composition. Also, the lack of appropriate fermenting microorganisms capable of converting heterogeneous monomeric sugars in macroalgae is a major factor limiting bioethanol yield during the fermentation process. This review systematically discusses the implications of selecting different macroalgae types. The optimization of process parameters of different bioethanol production steps such as pretreatments, hydrolysis, and fermentation is discussed. It can be concluded that high bioethanol yield can be achieved by considering macroalgae type and composition, selecting appropriate pretreatment, hydrolysis, and fermenting microbes, and with effective bioethanol purification.