At present the two principal substrates that have been used commercially to produce ethanol are sugar (sucrose) and starch. There are problems with both these substrates in terms of can these crops supply sufficient ethanol, and in doing so will they com­promise the supply of food crops. There are other abundant, inexpensive, non-food substrates and the most obvious is lignocellulose.

Lignocellulose is the most abundant potential source for bioethanol production with a potential yield of 442 billion l (Balat et al., 2007). For countries where biofuel

118

Glucose-6-phosphate

6-Phospho-gluconolactone

I
6-Phospho-gluconate

1

2-Oxo-3-deoxy-6-phosphogluconate

Carbon dioxide

Acetaldehyde

Fig. 6.9. The production of ethanol via the Entner-Doudoroff pathway.

crops are difficult to grow, lignocellulose is an attractive option. Lignocellulose can be obtained from trees species, wood residues, sawdust chips, construction residues, municipal wastes, paper, sewage sludge, maize stover, straws and grasses such as Miscanthus, switchgrass, sorghum, reed canary grass, bagasse, sugarbeet pulp, soft­wood, wheat straw, rice straw, pulp and paper mill residue, forest thinnings, munici­pal solid waste, winter cereals, and recycled paper.

Unfortunately, there are no ethanol-producing yeasts, other than genetically modified (GM) strains, that are capable of metabolizing starch and lignocellulose and only a few bacteria are capable of metabolizing lignocellulose. To make lignocellulose and starch suitable for fermentation both need to be converted into sugars in an inexpensive process.