As with corn stover, bagasse is the residue from sugarcane juice extraction and, as such, is an obligatory waste product. Development programs for bioethanol produc­tion from bagasse started in Brazil in the 1990s, and Dedini S/A Industrias de Base (www. dedini. com. br) now operates a pilot plant facility with the capacity of produc­ing 5,000 l/day of ethanol from bagasse in Sao Paolo state; up to 109 l of hydrated alcohol can be produced per tonne of wet bagasse, and this could be increased to 180 l/tonne with full utilization of hemicellulose sugars.

The commercial Brazilian process uses organic-solvent-treated bagasse. Other reports from Cuba, Denmark, Sweden, Japan, Austria, Brazil, and the United States describe alternative processes, all with some merits:

• Steam explosion — impregnation with SO2 before steam explosion gives high yields of pentose sugars with no additional formation of toxic inhibi­tors when compared with the absence of any acid catalyst84-86

• Liquid hot water pretreatment — probably the cheapest method (requiring no catalyst or chemical) and, when operated at less than 230°C, is effec­tive at solubilizing hemicellulose and lignin while leaving cellulose as an insoluble residue for further processing87,88

• Peracetic acid — alkaline pre-pretreatment followed by the use of peracetic acid gives synergistic enhancements of cellulose digestibility89,90

• Ammonia-water mixtures — vacuum-dried material from the alkaline treatment could be used for enzymatic digestion of cellulose without wash­ing or other chemical procedure34

• Dilute acid — this has not yet been fully tested for ethanol production, but as a method for preparing xylose as a substrate for xylitol production, it is capable of yielding hydrolysates with high concentrations of free xylose91,92

• Wet oxidation — alkaline wet oxidation at 195°C for 15 minutes produces solid material that is 70% cellulase; approximately 93% of the hemicellu — loses and 50% of the lignin is solubilized, and the cellulose can be enzymi — cally processed to glucose with 75% efficiency93

Three different species of yeasts have been demonstrated to ferment pentoses and/or hexoses from chemical hydrolysates of sugarcane bagasse: S. cerevisiae, C. shehatae, Pichia stipitis, and Pachysolen tannophilus.94-97 Acid hydrolysates are

best detoxified by ion exchange materials or activated charcoal; laccase and high pH precipitation methods are less effective.95 The most recent of the reports attempted lime treatment to neutralize the acid but found that the novel technique of electrodi­alysis (migration of ions through membranes under a direct electric field) removed the sulfuric acid and also the acetic acid generated during acid hydrolysis of hemicellu — loses so effectively that the reutilization of the sulfuric acid could be contemplated.[35]97 Recombinant xylose-utilizing yeast has been desensitized to hydrolysates containing increasing concentrations of phenolic compounds, furfuraldehydes, and carboxylic acids without loss of the xylose-consuming capacity and while retaining the ability to form ethanol rather than xylitol.96 Acetic acid and furfural at concentrations similar to those measured in sugarcane bagasse hydrolysates adversely affect both “laboratory” and “industrial” strains (see chapter 3, section 3.2.4) of S. cerevisiae.98 One highly practical solution is that the predominantly pentose-containing hydrolysates from bagasse pretreatments can also be used to dilute the sugarcane juice-based medium for sugar ethanol fermentation while maintaining an equivalent sugar concentration and utilizing a pentose-consuming P. stipitis to coferment the sugar mixture.99

Sugarcane is, however, not entirely without its industrial biohazards. Bagassosis, caused by airborne cells (or fragments) of Thermoactinomyces sacchari, was once very prevalent in workforces exposed to bagasse dust. In the United States and also in Japan (where outbreaks occurred in sugar refineries and lacquerware factories), the disease is thought to have been mostly eradicated during the 1970s by improved product handling and safety practices.100,101