6-22: Forbs as alternative feedstocks for bioethanol production

Choosing the best feedstock for biofuel production is a complicated decision involving many factors. Harvesting feedstocks from abandoned fields or restored prairie offers benefits like low agronomic inputs and high biodiversity. However, a major component of such landscapes is forbs (herbaceous dicotyledons). The potential and obstacles of using forbs compared to grasses and woody plants are poorly understood. We are, therefore, analyzing the factors that contribute to the recalcitrance of forbs, such as: low cellulose content, resistance to leading pretreatments, cell wall composition, poor compatibility with commercial enzymes, and inhibitors of enzymes and/or fermentation microorganisms. We chose four representative forbs: lamb’s quarter (Chenopodium album), goldenrod (Solidago canadensis), milkweed (Asclepias syriaca), and Queen Anne’s lace (Daucus carota). These plants represent taxonomic diversity, are wildly distributed, and are weedy but not invasive.

The cell walls of forbs were found to have lower cellulose and xylan content than corn stover (CS), but percent yields of Glc and Xyl were comparable to CS. Enzymatic hydrolysis of alkaline hydrogen peroxide (AHP) pretreated lamb’s quarter resulted in the conversion of 60% of cellulose into glucose as opposed to 70% in AHP-pretreated CS. After washing, a notable increase in Glc yield was observed in almost all cases, leading to the conclusion that forbs contain significant levels of enzyme inhibitors. It was also found that forbs required a different enzyme combination than CS. Accordingly, the forbs which are readily available and do not require time- and money-consuming efforts to grow, represent an interesting potential alternative feedstock for bioethanol.