Ultrasonic Pretreatment of Microalgal Biomass to Enhance the Enzymatic Hydrolysis for Sugar Recovery

Abstract

From the biotechnological point of view, microalgae are potential candidate of renewable and sustainable biomass resources for high-added value compounds such as bioethanol and enzymes due to their high levels of carbohydrates and low levels of lignin. There still remains significant room for improving the production of fermentable sugars from biomass in a cost-effective way that raises enzymatic saccharification efficiency. Although extraction of polysaccharides to convert fermentable sugar from microalga by acid or alkali pretreatments, and enzymatic hydrolysis are extensively studied, few studies exist that explore the use of ultrasonication (US) as emerging technology for the extraction of fermentable sugars (FSs) from microalgae biomass. To best of our knowledge no work has been reported on usage of ultrasonication as an extraction technique for the bioconversion of microalgae. The objective of the present study was to determine the effect of simultaneous ultrasonication and enzymatic hydrolysis on extraction of fermentable sugars from microalga biomass. C. vulgaris cultivated with red light in photobioreactors is efficient in producing algal biomass and sugar. The ultrasonication pretreatments were carried out in a glass bottle containing 0.3 g of dry microalgae in 100 ml of dilute acid solution. Based on preliminary experiments, the ranges in the pretreatment were selected for the variables of sulphuric acid (H2SO4) concentration (1, 2 and 3% (v/v)), amplitude (60, 75 and 90), and pretreatment time (10, 20 and 30 min). Then, enzymatic saccharification (cellulose and Viscozyme L at 50°C and 120 rpm for 48 h) were employed. Increased amplitude provided significant amount of sugar released. Among the variables studied, amplitude, acid concentration and time showed significant effects (p<0.05) on sugar released from alga biomass. The sugar released from alga pretreated biomass ranged at 158.14-447.09 mg/g dry weight microalga. The highest amount of biomass (0.38 g/L) and sugar released (447.09 mg/g) was obtained from simultaneous ultrasonication and enzymatic hydrolysis of microalga at a 75% amplitude, 2% H2SO4 and 20 min after 48 h. Thus, the ultrasonication appeared to be a successful alternative method when compared to the conventional pretreatment methods.