Conversion of lipid-extracted Nannochloropsis salina biomass into fermentable sugars

The primary focus of research and development toward algal biofuels has been the production of fuel from the lipid fraction, with the non-lipid biomass used for production of biogas, electricity, animal feed, or fertilizer. Since the non-lipid fraction comprises approximately half of the algal biomass, the development of processes to produce additional liquid fuel or higher value products is of interest. We evaluated several hydrolysis methods for the deconstruction of cell wall carbohydrates in residual algal biomass. The hydrolysate, which contains the released sugars, can be used as a fermentation feedstock. For all methods, hydrolysis rates and yields of released sugars were measured. The effects of temperature, acid concentration, and biomass loading on acid hydrolysis were studied. Combined severity factors, indicators of treatment intensities, were evaluated for their correlation to the hydrolysis outcome. An optimal enzyme mixture, which released sugars with an acceptable yield and rate, was found. The ability of the resulting hydrolysates to support the growth of an industrial yeast strain was tested and the levels of common fermentation inhibitors were examined. Of the conditions tested, the highest yield of released sugar (243.2 mg/g biomass) was obtained with a one-step sulfuric acid process with 10% acid concentration at 90 °C for 5 h, while the maximum sugar release rate was obtained with 10% hydrochloric acid under the same conditions. This is the first process for conversion of residual algal biomass that does not require pretreatment and that results in a hydrolysate on which yeast can be grown with no added nutrients. Adapting a biorefinery concept by conversion of algal residue to value-added products may improve the process economics.