ETHANOL FROM RENEWABLE CARBON SOURCES

Researcher: Jim Kastner, jkastner@engr.uga.edu

Ethanol production from lignocellulosic feedstocks requires the development of methods (environmental controls, or feeding strategies) to optimize product formation from multiple substrates (mixed sugar streams).

Lignocellulosics, such as corn stover, wood waste, sugar cane baggase, are examples of large, but untapped renewable carbon sources. The predominant polymer in many renewable feedstocks is cellulose, which generates glucose upon hydrolysis. However, depending on the feedstock, large fractions of other six and five carbon sources are released when hemicellusoe is hydrolyzed; e.g., xylose. This necessitates a mixed substrate fermentation. Mixed sugar fermentations are more complex than standard pure substrate processes. Regulatory processes such as transport competition or inhibition, induction, repression, and catabolite inactivation can increase fermentation times due to diauxic growth and lag, and reduce product yields from the secondary substrates. However, most fermentation research has focused on optimizing product formation from single substrates.

Thus, research is required to develop methods (environmental controls, or feeding strategies) to optimize product formation from multiple substrates. Potential fermentation methods to induce simultaneous substrate utilization and generate high product yields may include:

• Environmental manipulation (e.g., pH, media composition, substrate ratios, etc.),
• Pre-induction before large scale fermentations,
• Identification and feeding of metabolic inducers,
• Novel reactor configurations, such as a two-phase fed batch processes (e.g., aerobic growth on the inducer at low concentrations and generation of high cell densities followed by controlled feeding of the mixed sugars for product formation).

Regardless of the method used, a quantitative technique is required to measure the impact of environmental manipulation (or genetic manipulation) on substrate utilization rates and patterns. We propose to use flux balance analysis (FBA) potentially coupled with other methods (e.g., c₁₃ NMR analysis) to elucidate carbon flow during mixed substrate fermentations and optimize fermentation product yields.