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Biochemical pathways for biofuel production involve the use of microorganisms and biological processes to convert lignocellulosic biomass into ethanol through enzymatic hydrolysis and fermentation

Summary​

The biochemical pathway utilizes microorganisms and biological processes to convert lignocellulosic biomass into ethanol through enzymatic hydrolysis and fermentation. After pretreatment, cellulose is broken down into glucose using enzymes, which is then fermented into ethanol by yeast or other microorganisms. A major challenge is handling xylose from hemicellulose, which cannot be fermented by standard yeast and can degrade into fermentation inhibitors called furfurals. Current research focuses on developing genetically modified strains for xylose fermentation and co-fermentation processes to improve efficiency and ethanol yields from lignocellulosic feedstocks.

The biochemical pathway utilizes microorganisms and biological molecules to produce alcohols such as bioethanol (C₂H₅OH). The process of converting lignocellulosic biomass into alcohol involves two key steps after the pretreatment process which allows for access to the fermentable sugars.

Enzymatic (sometimes acidic) hydrolysis of cellulose​

After pretreatment, the cellulose is separated from the hemicellulose (now in the form of 5-carbon sugars know as pentose) and the lignin. Cellulose (C₆H₁₂O₆)ₙ is usually not broken down into its individual sugar component glucose (C₆H₁₂O₆) in the pretreatment process, so it needs to be broken down by enzymatic hydrolysis. The hydrolysis reaction often involves a handful of enzymes that break apart the cellulose chains at different points. The pentose is generally removed prior to the hydrolysis process to avoid inhibiting the hydrolysis process (see pentose (xylose) fermentation section for more details). The overall reaction can be shown in the following equation, where n is the number of glucose units in the cellulose chain which translates into the number of individual glucose molecules.

(C6H10O5)n+nH2O→nC6H12O6\begin{align*} \mathrm{(C_6H_{10}O_5)_n + nH_2O \rightarrow nC_6H_{12}O_6} \end{align*}

Fermentation​

After the cellulose is broken down into glucose, it can then be fermented into ethanol, usually by yeast but also with bacteria and mycelium (mold) 1. After fermentation, the ethanol can be distilled to remove the water and other impurities. The fermentation reaction is shown in the following equation:

C6H12O6→yeast2C2H5OH+2CO2\begin{align*} \mathrm{C_6H_{12}O_6 \xrightarrow{\text{yeast}} 2C_2H_5OH + 2CO_2} \end{align*}

Pentose (xylose) fermentation​

One of the key challenges in the biochemical processing of lignocellulosic biomass is the presence of xylose that is formed from the broken down hemicellulose. Xylose, the most common 5 carbon sugar found in hemicellulose, cannot be fermented into ethanol using standard yeast strains and requires genetically modified strains to do so 1. Xylose is also susceptible to thermal degradation and acidic degradation into a class of organic compounds called furfural 2. Furfurals are natural fermentation inhibitors, which poses a challenge if xylose is not effectively separated from the cellulose after pretreatment and enters into the enzymatic or acid hydrolysis process 2. Besides the pretreatment process, the existence of xylose and how it can be effectively separated or fermented is the major challenge in the biochemical processing of lignocellulosic biomass and is one of the main areas of research into processes such as co-fermentation of xylose and glucose, which would offer large cost savings and increase ethanol yields 3.

Sources​

Footnotes​

  1. Naik, S. N., Goud, V. V., Rout, P. K., & Dalai, A. K. (2010). Production of first and second generation biofuels: A comprehensive review. Renewable and Sustainable Energy Reviews, 14(2), 578–597. https://doi.org/10.1016/j.rser.2009.10.003 ↩ ↩2

  2. International Renewable Energy Agency (IRENA). (2016). Innovation Outlook: Advanced Liquid Biofuels. IRENA. https://www.irena.org/publications/2016/Oct/Innovation-Outlook-Advanced-Liquid-Biofuels ↩ ↩2

  3. Balan, V. (2014). Current challenges in commercially producing biofuels from lignocellulosic biomass. ISRN Biotechnology, 2014, 463074. https://doi.org/10.1155/2014/463074 ↩