Apart from biodiesel, bioethanol is another attractive biofuel that is used as a substitute for gasoline. Biomass that contains sugar, starch, or cellulose is used by yeast as a substrate dur­ing the fermentation process, releasing bioethanol as the product and CO2 as the byproduct (Brennan and Owende, 2010). Since algae are able to accumulate significant amounts of carbohydrates (mainly referred to as starch) inside their cells, the potential to utilize the car­bohydrate for bioethanol production is high (Harun et al., 2010). Among the algal strains that have been identified as having high carbohydrate content are Chlamydomonas reinhardtii (53%), C. reinhardtii (45%), Chlorella vulgaris (12-37%), Chlorella sp. (21-27%), and Scenedesmus sp. (13-20%) (John et al., 2011).

Unlike terrestrial plants, algal cells are buoyant and thus do not require lignin and hemi — celluloses for structural support (John et al., 2011). Therefore, carbohydrate extraction from algal biomass is expected to be simpler than carbohydrate extraction from lignocellulosic materials; avoiding the complicated pretreatment steps to remove lignin and the economic feasibility of bioethanol production can consequently be improved. Nevertheless, the extracted algal carbohydrates need to be hydrolyzed further (hydrolysis process) to simple reducing sugars (e. g., glucose) for yeast to effectively convert the sugar into bioethanol during the fermentation process.

Recently, several effective carbohydrate hydrolysis methods for algal biomass, such as using dilute acid solution (Harun and Danquah, 2011), dilute alkaline solution

(Harun et al., 2011), and enzymatic methods (Choi et al., 2010; Harun and Danquah, 2011), have been reported in the literature, as shown in Table 12.5. The data in the table show that the bioethanol yields are comparable to the yields attained from sugar and lignocellulosic feedstocks, indicating that it is technically viable to produce bioethanol from algal biomass. Reutilization of the algal residue after lipid extraction for bioethanol conversion instead of using the freshly dried algal biomass is also possible (Lam and Lee, 2011). This would be a more realistic approach because two different types of biofuels are simultaneously produced from the algal biomass, and thus the life-cycle energy balance of algal biofuels can be further strengthened.

This concept has been proven viable in a recent study in which lipids from Chlorococum sp. were preextracted with supercritical CO2 at 60 °C and subsequently subjected to fermentation with the yeast Saccharomyces bayanus (Harun et al., 2010). From the study, algal biomass with pre-extracted lipids gave 60% higher bioethanol concentration for all samples than the dried algal biomass without lipid extraction. This is because during supercritical CO2 extraction of lipids, the algal cell wall is ruptured due to the high temperature and pressure required for the supercritical process (Harun et al., 2010). The rupturing of the cell wall leads to the release of carbohydrates and subsequently being hydrolyzed to simple reducing sugar. Thus the algal residues after lipid extraction are readily available for fermentation with yeast. Based on this study, a maximum bioethanol yield of 3.83 g/L was achieved from 10 g/L of lipid-extracted algal residue.