The theoretical journey from cellulose to glucose, a crucial step in ethanol production, is a nuanced process influenced by various factors. The envisioned ethanol yields from different lignocellulosic sources, as researched by Sassner and team in 2008, present promising possibilities yet are hampered by practical inefficiencies.
Theoretical ethanol yields, measured in liters per dry metric ton, vary across lignocellulose sources:
- Hardwood: 345 (hexose fermentation) and 121 (pentose fermentation)
- Softwood: 426 (hexose fermentation) and 59 (pentose fermentation)
- Corn stover: 302 (hexose fermentation) and 191 (pentose fermentation)
While these figures hint at potential success, the actual conversion processes face significant hurdles, impeding the efficiency of cellulose-to-ethanol conversion. Lignocellulosic biomass, derived from diverse sources such as woody wastes, corn cobs/stover, switchgrass, spent grains, paper waste, and municipal solid waste, undergoes pre-treatment and hydrolysis. This process yields fermentable sugars alongside chemical inhibitors, including furfural, hydroxymethyl furfural, organic acids (acetic acid, formic acid, levulinic acid), and lignin-degradation products (phenolic compounds like ferulic and coumaric acids).
The presence of these inhibitors, acting as suppressors of yeast and bacterial activities, poses a significant challenge in converting hydrolysate sugars to ethanol. Moreover, the heterogeneous C5 and C6 sugar slurries derived from lignocellulosic feedstocks are not readily fermented by yeast, compounding the complexity of the process.
Various systems, such as batch, fed-batch, simultaneous saccharification and fermentation (SSF), simultaneous saccharification and co-fermentation (SSCF), separate hydrolysis and fermentation (SHF), consolidated bioprocessing (CBP), drop-add, and continuous cascades, are employed for processing and fermenting lignocellulosic hydrolysates. These processes involve enzymatic or microbial hydrolysis, producing cellulases and hemicellulases, and subsequent fermentation of hexose and pentose sugars.
Overcoming the challenge posed by pentose sugars, which conventional yeasts like Saccharomyces cerevisiae struggle to metabolize effectively, demands innovative approaches. Strategies include:
- Pentose-Fermenting Yeasts: Utilizing yeasts like Pichia stipitis, Candida shehatae, Kluyveromyces marxianus, which exhibit pentose-fermenting abilities.
- Genetic Engineering: Modifying S. cerevisiae to ferment xylose, achieved through successful cloning of xylose isomerase genes from various sources.
- Genetically Engineered Bacteria: Exploring bacteria like E. coli, Zymononas, Klebsiella oxytoca, Thermoanaerobacetrium, Geobacillus, equipped with xylose-utilizing genes.
Despite these advancements, challenges persist, especially in the engineering of yeast strains for lignocellulosic hydrolysates. Industrial-scale lignocellulosic bioethanol plants, including Mascoma, Poet, Range Fuels, Verenium, Celunol, DuPont in the US, Iogen in Canada, and DONG (Denmark), TMO (UK, The Netherlands) in Europe, signify progress but highlight the ongoing complexity of the task.
Some bacterial processes, particularly those operating at high temperatures, present advantages in utilizing all C5 and C6 sugars efficiently. However, ethanol tolerance, especially in levels exceeding 8% v/v, remains a concern for certain thermophilic bacteria.
Addressing the toxicity of chemicals in lignocellulosic hydrolysates to fermentative microorganisms involves strategic methods like “steam stripping,” nanofiltration membranes, and polymeric adsorbent materials. These approaches selectively remove inhibitors from soluble sugar fractions, optimizing conditions for fermentation.
While scientific advancements continually propel lignocellulosic-to-ethanol bioconversions, it’s essential to acknowledge the longstanding industrial applications of wood hydrolysate fermentations in Europe and Siberia. These examples underscore the intricate blend of historical practices and modern scientific innovations in the pursuit of sustainable and efficient ethanol production.