Biofuels in Perspective

Enzymatic Production of Biodiesel 147
With cell-surface-displayed ProROL, substrate molecules could easily access ProROL
and no treatment was needed to catalyze methanolysis reaction. Since the initial reaction
rate of FSProROL-displaying cells was as high as that of soluble ROL, the displayed
FSProROL may have the same accessibility to substrates as free enzyme. For the industrial
bioconversion process, lipases immobilized on the cell-surface are more cost effective and
convenient, since these whole-cell biocatalysts can be prepared simply by cultivation and
recovered easily.
8.6 Conclusions and Future Prospects
In recent years, biodiesel has become more attractive as an alternative fuel for diesel engines
because of its environmental benefits and the fact that it is made from renewable resources.
Used oils can also be utilized for making biodiesel fuel, thus helping to reduce the cost
of wastewater treatment in sewerage systems and generally assisting in the recycling of
resources. 80
For the production of biodiesel fuel, an alkali-catalysis process has been established
that gives high conversion levels of oils to MEs, and at present this is the method that
is generally employed in actual biodiesel production. However, it has several drawbacks,
including the difficulty of recycling glycerol and the need for either removal of the catalyst
or wastewater treatment. In particular, several steps such as the evaporation of methanol,
removal of saponified products, neutralization, and concentration, are needed to recover
glycerol as a by-product.
To overcome these drawbacks, which may limit the availability of biodiesel fuel, enzymatic
processes using lipase have recently been developed. Since the cost of lipase
production is the main hurdle to commercialization of the lipase-catalyzed process, the use
of intracellular lipase or cell-surface-displayed lipase as a whole-cell biocatalyst 47,48,75 is
an effective way to lower the lipase production cost. Unlike in the case of extracellular
lipase, these whole-cell biocatalysts can be prepared by simple cultivation and recovered
easily.
However, to utilize these whole-cell biocatalysts for industrial application, a high ME
content of 90–95 % in many repeated methanolysis reaction cycles is required. One potential
solution is the use of a whole-cell biocatalyst possessing a non-specific lipase from a source
such as C. antarctica 33 or P. cepacia 36 within the cell or on the cell-surface, since these
lipases realize ME content of more than 95 %. Such a system could offer a promising
prospect of realizing industrial biodiesel fuel production.
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