BIOCHEMICAL CONVERSION: BIOFUELS BY INDUSTRIAL FERMENTATION - Biomass as a Sustainable Energy Source for the Future - page 430

In most oleaginous microorganisms, the stored lipids consist of 80 90% triacylgly-

cerols (TAG) with a fatty acid composition similar to that of vegetable oils. It has been
demonstrated that such microbial oils, also called single-cell oils, can be used as feed-
stock for biodiesel production. Under conditions of nitrogen limitation, accumulation
has been reported to increase up to 70

−

− 80% of the dry cell mass (Subramaniam

et al., 2010).
After fermentation, the cells need to be harvested from the medium. Due to the lipid
content, the density difference with the medium is low, and separation by, e.g., cen-
trifugation is not efficient. Cross-flow filtration has been used as an alternative, but
fouling of the filtration membrane by the cells and other medium components results
in low flux, and thus, large membrane areas are required to achieve satisfactory
throughputs.
Following cell harvest, the oil has to be removed from inside the cells. This is
usually done by a combination of mechanical cell disruption and solvent extraction
or by direct contact of the cells with a solvent, which results in membrane permeabil-
ization and cell rupture. In either case, the extracted oil is then further refined and
transesterified into biodiesel.
Clearly, the intracellular production has disadvantages with respect to reaction and
product recovery:

￿ The cell density becomes considerable lower, affecting mixing in the reactor and

the use of separation equipment like centrifuges.

￿ The need for cell disruption for product recovery limits the applicability of

reactor intensification techniques such as cell recycle.

Additional disadvantages of this route include the low genetic accessibility of the
microorganisms, hampering improvements in the achievable product concentrations
and the type of product obtained, and the need for additional conversion steps for
direct use as biofuel. Because of this, biodiesel production from oleaginous micro-
organisms has not yet reached industrial implementation. Nevertheless, the lipids
produced by these microorganisms are still receiving attention for higher-value niches
in the food and pharmaceutical sectors.

13.5.2 Routes Using Engineered Microorganisms

The metabolism of well-studied industrial microorganisms (mostly Escherichia coli
and S. cerevisiae ) and photosynthetic organisms (e.g., cyanobacteria) is being directed
toward the production of biodiesel and diesel and jet fuel replacements. Much of this
research is being carried out in biotechnology companies (e.g., REG Life Sciences,
Amyris, Joule Unlimited). The emphasis has been on the production of FAAE, isopre-
noid compounds such as farnesene (C 15 H 24 ), and fatty acid-derived alkanes such as pen-
tadecane (C 15 H 32 ). FAAE can be used in the same way as biodiesel. Farnesene is highly
unsaturated, resulting in a low cetane number and low oxidative stability. Therefore, it
requires an additional hydrogenation step for fuel use. This chemical hydrogenation