Perspectives on Bioenergy and Biofuels - Sustainable Biotechnology: Sources of Renewable Energy - page 185

from hexoses [11, 12]. This would not only lead to loss of fermentable sugars
(lowering possible ethanol yields) but that these degradation compounds themselves
may prove inhibitory in the fermentation process [13, 14] itself, again leading to
lowering the efficiency of the process.
1.10 (Ligno)Cellulose Hydrolysis
Sulphuric acid can also be used for the hydrolysis conversion of the carbohy-
drates to free fermentable sugars, however in order to minimise acid costs as
well as generating waste streams containing large amounts of inorganic waste
recycling and reuse of the sulphuric acid. Such processes have attracted a lot
of attention, with the Arkenol concentrated sulphuric acid process already being
operated by NEDO/JGC in Izumi, Japan and BlueFire Ethanol Inc. Using their
patented technology [15] all components of the biomass have found applica-
tion and auxiliary reagents such as the sulphuric acid may be reused (via ion
exchange) and residual acid converted to gypsum. Other methods using sulphuric
acid are being explored [16]. The sulphuric acid recycling focuses on the use on
anion selective membranes [17] and anaerobic treatment [18] - The Biosulfurol
Process.
It is also possible to use enzymes (cellulases) in the hydrolysis process step.
There has been a large amount of discussion as to the actual cost contribution of the
cellulases per litre (or gallon) ethanol with early studies showing very high costs.
Thus development in this area was required to improve enzyme production and
efficiency to improve the costs of cellulase use. Strategies to help reduce cellulase
production costs include strain improvement, using mutagenesis for example, or iso-
lation of overproducing strains. The two major companies in the field, Novozymes
and Genencor, have undertaken significant effort in cellulase development and this
has resulted in cost reductions. In 2004 Novozymes announced a twelvefold reduc-
tion in the enzyme cost contribution in bioethanol production from >$5.00 to <$0.50
per gallon bioethanol [19].
1.11 Fermentation of Sugars
Traditional bakers yeast ( Saccharomyces cerevisiae ) allows ready conversion of
glucose to ethanol. However in order maximise ethanol production, utilisation of
pentoses (such as xylose and arabinose) that are released during pretreatment and
hydrolysis should also be converted. Approaches include genetically engineered
Saccharomyces cerevisiae yeasts that can ferment both xylose and glucose have
seen development [20-22] and modified Zymomonas, E. coli and Klebsiella bacte-
ria with improved pentose metabolism. Such progress in the field has culminated
in announcements such as those from Danisco and DONG Energy to open a
demonstration plant.