3230 Chapter 26 



pulp. After a period of growth, the broth is filtered to give the enzyme-contain- 

 ing solution used in hydrolysis. Hydrolysis generally is carried out at pH 4.8 and 

 50°C on ball-milled cellulosic substrates. Pulverizaton by ball milling is neces- 

 sary to break down cellulose crystallinity. Yields depend on the substrate and 

 range from 16-77 percent. Pure cellulose pulp and shredded paper give respect- 

 able yields of up to 77 percent. Wood waste, on the other hand, yielded only 16 

 percent reducing sugars. On pulped wood chips the yield was 57 percent (Man- 

 dels et al. 1978; BUss and Blake 1977). 



While these enzymes may lead to a non-corrosive process (as opposed to acid 

 hydrolysis), they require pretreatment of the wood cellulose because they are 

 greatly inhibited by the cellulose crystallinity and the ligno-cellulosic complex. 

 The best possibility for overcoming the low reactivity of wood is to develop a 

 cost-effective pretreatment that makes lignocellulosics react to enzymatic hy- 

 drolysis (Millett et al. 1975). One promising avenue is steam explosion of chips 

 to produce a fiberous material favorable to the enzymes. 



The steam explosion process for wood chips yields fiber that is more reactive 

 to enzymes than normal wood yet does not require the mechanical energy of ball 

 milling or twin-screw extrusion, nor addition of chemical to dissolve lignin. Use 

 of such fiber with selected or genetically engineered enzymes may lead to 

 economic enzymatic hydrolysis. Goldstein (1980b) found that in the 1970's use 

 of hypercellulolytic mutants of Trichoderma fungus have resulted in a 30-fold 

 increase in enzyme concentration and a 24-fold increase in enzyme productivity. 



lotech Corp., Ottawa, Canada, is a pioneer in developing this process where- 

 by hardwood chips (preferably Populus sp.) are treated with saturated steam and 

 a catalyst at 240-300°C and 500-1 ,000 psi in a vessel (gun reactor) for 5 to 300 

 seconds and then exploded to atmospheric pressure to yield fibers and fine 

 particles. These particles are subjected to enzymatic attack and resulting 5- and 

 6-carbon sugars are conventionally fermented into ethanol. Tests have recorded 

 sugar concentrations of 10 to 12 percent and alcohol concentrations of 5 to 6 

 percent before distillation. Lignin can be recovered at the end of the process by 

 filtration. The enzymes needed for the process reportedly can be grown directly 

 on the exploded fiber, and need not be grown on purer cellulose substrates such 

 as cotton-gin trash, as is the usual practice (Chemical Engineering 1981). 



Readers needing additional information on enzymatic hydrolysis will find 

 useful reviews by Goldstein (1980b) and Hajny (1981). Jones et al. (1979) 

 provided an economic analysis of the Natick enzymatic hydrolysis process used 

 to produce ethanol from wood. 



26-8 PREHYDROLYSIS 



An alternative to whole-wood hydrolysis is to remove the hemicelluloses for 

 chemical or other use in a mild prehydrolysis step, and then to use the remaining 

 fibrous cellulose residue for pulp, particleboard, fiber, or for fuel (fig. 26-40, 

 Herrick and Hergert 1977). 



