Numerous treatments have been developed to enhance the enzymatic digestibility of lignocellulosic biomass including: physical (e. g., ball milling, two-roll milling), chemical (e. g., dilute-acid hydrolysis, alkali), physico-chemical (e. g., steam explosion, Ammonia Fiber Explosion), and biological (e. g., white-rot fungi) (6). For the MixAlco Process, alkaline treatment is selected because the acids produced in the fermentor will neutralize the alkali, thus allowing recovery of the treatment agent. Of the various alkalis that are effective (e. g., sodium hydroxide, ammonia), lime was selected because of its low cost and compatibility with other process steps.

Compared to other alkalis, the literature on lime treatments is relatively sparse. Most of the studies have been performed by animal scientists seeking simple, room-temperature treatments to enhance ruminant digestibility. Because the treatment temperature was low, their results were poor; the general consensus is that lime is not as effective as other alkalis. However, by optimizing the reaction temperature and other conditions, lime is a very effective treatment agent.

Table I summarizes the results of some recent lime treatment studies using extracellular enzymes to hydrolyze the biomass. Compared to untreated biomass, lime-treated biomass has an enzymatic digestibility roughly ten fold larger. Because of its low lignin content, herbaceous biomass requires only lime treatment. However, because of its high lignin content, woody biomass requires the addition of oxygen to partially oxidize the lignin and remove it from the biomass. In addition, woody biomass requires more severe time and temperature.

Table П shows that lime treatment roughly doubles the ruminant digestibility of biomass. Comparing the digestibilities reported in Table I versus Table П, the digestibility within the ruminant is greater than that achieved with extracellular enzymes. This suggests that an industrial process based on a mixed culture of microorganisms may have advantages over one based upon extracellular enzymes.