The success of lignocellulosic bioethanol will depend on the development of simple pretreatment technologies that effectively delignify a diverse portfolio of lignocellulosic biomass feedstocks. Reducing the enzyme cost while enhancing cellulose hydrolysis efficiency is another important consideration when developing suitable pretreatment technologies.

The enzymatic hydrolysis of lignocellulosic biomass is influenced by several factors, including lignin and hemicelluloses contents, cellulose crystallinity, degree of polymerization, accessible surface area, and pore volume. However, lignin has been believed to be a major hindrance in enzymatic hydrolysis. Because lignin forms a physical barrier to restrict the access of cellulases to the cellulose. Delignification increases cell wall porosity, rendering the biomass more amenable to enzymatic hydrolysis. Selective lignin removal can minimize cellulose degradation and thus enhance enzymatic hydrolysis.

… As cell walls in biomass feedstocks differ in structure and chemical composition, one pretreatment method will not necessarily fit all applications. Therefore, developing a pretreatment technology that is effective over a wide range of biomass materials is important.

—Wi et al.

Although there are a variety of pretreatment methods them seem to be promising—including steam explosion, ammonia fiber expansion (AFEX), dilute acid, and ionic-liquid pretreatment—there is currently no single pretreatment technology that is potentially acceptable for the multiple biomass conversion, according to the researchers.

The new HPAC method involves mixing H₂O₂ and CH₃COOH to form a reagent that effectively removes lignin from lignocellulosic biomass through partial hydrolysis of lignin bonds. The researchers evaluated pretreatment efficiency based on observations of lignin removal, enzymatic hydrolysis, and fermentation of rice straw, pine wood, and oak wood.

Proposed conceptual model for the mechanism of enhanced enzymatic saccharification of lignocellulosic biomass by delignification using the HPAC pretreatment. Wi et al. Click to enlarge.

As a result of their experiments, they found that an equal volume mixture (5:5) of H₂O₂ to CH₃COOH was the most effective. Optimal pretreatment conditions for all three biomass materials were a temperature of 80 °C and a treatment time of 2 h, which gave a high yield of total sugars.

Material balances for 1 kg of HPAC-pretreated lignocellulosic biomass materials extrapolated from the results of 10.0-g-dry weight scale experiments. Wi et al. Click to enlarge.

Compared to organo-solvent pretreatment under the same conditions, the HPAC pretreatment was more effective at increasing enzymatic digestibility. In addition, compared to a mechanical pretreatment (milling) and pretreatment with another acid (H₂SO₄), the HPAC pretreatment was more effective at converting all three biomass materials into glucose. Moreover, when a mixture of three lignocellulosic materials was treated, the HPAC pretreatment was very effective at increasing enzymatic digestibility. These results indicate that HPAC pretreatment is a highly efficient and effective process for converting lignocellulosic materials into fermentable sugars.

—Wi et al.

Resources

• Seung Gon Wi, Eun Jin Cho, Dae-Seok Lee, Soo Jung Lee, Young Ju Lee and Hyeun-Jong Bae (2015) “Lignocellulose conversion for biofuel: a new pretreatment greatly improves downstream biocatalytic hydrolysis of various lignocellulosic materials” Biotechnology for Biofuels doi: 10.1186/s13068-015-0419-4