Fermentation - Food and Industrial Bioproducts and Bioprocessing - page 191

fermentation usually involves two stages: start up and steady state. During the start-up
period, the fermentor is operated in batch mode until cells grow to the desired cell
concentration. Once the active culture is established, the substrate and nutrients are
continuously fed into the fermentor and products are removed at the same rate. The volume
inside the fermentor is kept constant at steady state operation. Cells can be either suspended
or immobilized in the fermentor. A recycle system, which is usually a membrane module, is
used to separate the cells in the product stream and recycle them back into the fermentor to
keep its cell mass concentration high. To ensure consistent performance, the feed rate should
be properly controlled, microorganism population in cell number and viability should be
maintained and contamination should be avoided.
The main advantage of the continuous scheme is its high productivity. However, the main
problems of the continuous scheme are the high possibility of contamination and wash out
of cells from the bioreactor at high flow rate. In addition, the final product concentration is
generally lower than in batch or fed-batch fermentations, which increases the cost of product
recovery.
7.6 FERMENTATION PRODUCTS
7.6.1 Acetone-Butanol-Ethanol (ABE) fermentation
One of the first processes to produce bulk chemicals through microbial fermentation was the
acetone-butanol-ethanol fermentation, or the ABE fermentation. In the early twentieth
century, butanol was desired for the production of synthetic rubber (Jones and Woods,
1986). This lead to several researchers pursuing the isolation of bacteria that could produce
butanol from starch, including Chaim Weizmann, who isolated the bacterium Clostridium
acetobutylicum that could produce a mixture of acetone, butanol, and ethanol from starch
under anaerobic conditions (Jones and Woods, 1986). Upon the outbreak of the First World
War, the British government was particularly interested in acetone, since it was needed for
the production of cordite, a component of smokeless gunpowder. As a result several
fermentation facilities were built in Britain, Canada, and the United States during the war to
produce acetone by fermentation using C. acetobutylicum (Jones and Woods, 1986 ).
Production of acetone and butanol by fermentation was mostly discontinued after the Second
World War, when petrochemical butanol production and the high cost of substrate made
butanol fermentation economically uncompetitive (Jones and Woods, 1986).
As stated earlier, interest in the ABE fermentation has recently increased due to the
desire to produce chemicals from renewable resources because of high petroleum costs and
greenhouse gas emissions from fossil fuel combustion. Research into the ABE fermentation
has generally focused on two species, C. acetobutylicum and Clostridium beijerinkii . Both
species are heterotrophs, which are organisms that derive energy from organic molecules.
They can utilize starch and simple sugars for ABE production. Native strains of these species
are not able to utilize cellulose due to their lack of cellulose degrading enzymes. In addition
to heterotrophs, some autotrophic bacteria have also been observed to produce butanol.
Clostridium carboxidivorans and Butyribacterium methylotrophicum both have been
observed to produce a mixture of butanol, ethanol, acetic acid, and butyric acid from carbon
monoxide (Datar et al ., 2004 ; Worden et al ., 1991 ).
All three of the primary products of ABE fermentation have industrial value. Acetone is
an effective solvent and is the principal ingredient in nail polish remover. Acetone also is