Historically, the use of dilute acid hydrolysis predated enzymic hydrolysis as a methodology used for cellulose processing beyond the laboratory stage of develop­ment (table 2.3). In the former Soviet Union, large-scale processes for single-cell protein[13] as animal feeds using acid-hydrolyzed woody materials were developed.49 More recently, highly engineered reactors have been devised and investigated for the efficient hydrolysis of lignocellulosic biomass with dilute sulfuric acid, including50

• Batch reactors operating at temperatures up to 220°C

• Plug-flow reactors, that is, flow-through reactors in which liquid and solid phases travel at the same velocity and reduce the residence time at high temperature (up to 230°C)

• Percolation reactors, including two-stage reverse-flow and countercur­rent geometries

Hydrolysis efficiencies can now rival those in enzymic (cellulase) hydrolyses with the advantages that none of the feedstock need be dedicated to support enzyme production and very low acid concentrations used at high temperatures may be economically competitive with enzyme-based approaches.

Two-stage processes employ mild hydrolysis conditions (e. g., 0.7% sulfuric acid, 190°C) to recover pentose sugars efficiently, whereas the more acid-resistant cellulose requires a second stage at higher temperature (e. g., 215°C); sugars are recovered from both stages for subsequent fermentation steps.51 Concentrated (30-70%) sulfuric acid hydrolysis can be performed at moderate temperature (40°C) and result in more than 90% recovery of glucose but the procedure is lengthy (2-6 hr) and requires efficient recovery of the acid posttreatment for economic feasibility.52

The major drawback remains that of the degradation of hexoses and pentoses to growth-inhibitory products: hydroxymethylfurfural (HMF) from glucose, furfural from xylose, together with acetic acid (figure 2.6). HMF is also known to break down in the presence of water to produce formic acid and other inhibitors of ethanol-producing organisms.53 In addition, all thermochemical methods of pretreatments suffer, to varying extents, from this problem; even total inhibition of ethanol production in a fermentation step subsequent to biomass presteaming has been observed (figure 2.7).

FIGURE 2.6 Chemical degradation of hemicellulose, xylose, and glucose during acid — catalyzed hydrolysis.

Two contrasting views have become apparent for dealing with this: either the growth — inhibitory aldehydes are removed by adsorption or they can be considered to be an additional coproduct stream capable of purification and resale.5455

Bioprocess engineering indicates that simply feeding a cellulosic hydrolysate with high concentrations of furfurals and acetic acid to yeast cells, rather than present­ing the full “load” of inhibitors in the batched medium, conditions the microorgan­ism to detoxify and/or metabolize the inhibitory products of sugar degradation.5657 A more proactive strategy is to remove the inhibitors by microbiological means, and a U. S. patent details a fungus (Coniochaeta lignaria) that can metabolize and detoxify furfural and HMF in agricultural biomass hydrolysates before their sac­charification and subsequent use for bioethanol production.58