Biotechnological Applications of Hemicellulosic Derived Sugars: State-of-the-Art - Sustainable Biotechnology: Sources of Renewable Energy - page 80

3.3 2, 3-Butanediol
2, 3-BD is the 2R, 3R isomer of 1, 4-butanediol, a potential bulk chemical that can
be produced by a variety of microorganisms through microbial fermentation [55].
It has been utilized for the production of various chemical feedstocks and liquid
fuels, including the formation of the liquid fuel additive methyl ethyl ketone by
dehydration [56]. The esters of butanediol and suitable monobasic acids may find
uses as effective plasticizers for thermoplastic polymers, such as cellulose nitrate
and cellulose triacetates [55].
3.3.1 Microorganisms
Fermentation of xylose and glucose by Klebsiella oxytoca and Aerobacter aero-
genes yields 2, 3-BD as the major product [55]. Other microorganisms capable of
producing 2, 3-BD include Bacillus subtilis (Ford strain), Aeromonas hydrophilia,
and several Serratia sp. [55]. K. oxytoca is able to yield high concentrations of
2, 3-BD as mixtures of stereoisomers from monosaccharides, but is unable to uti-
lize polysaccharides. In comparison, B. polymyxa is able to ferment starch directly,
yielding 2, 3-butanediol and ethanol in almost equal amounts [55].
3.3.2 Fermentation Methodologies
The efficiency of 2, 3-BD fermentation can be judged by the product yield from
sugar, the final butanediol concentration, and the volumetric butanediol produc-
tion rate. The theoretical yield of 2, 3-BD from glucose is 0.50 g/g. Higher levels
of butanediol have been produced in fed-batch culture conditions that are main-
tained to minimize the effects of initial substrate inhibition and product inhibition.
A higher production rate of 2, 3-BD was reported in continuous reactors [55].
However, product inhibition and incomplete substrate utilization remain challeng-
ing issues. Immobilization of live cells on a supporting material, i.e., matrix, has
been attempted to increase the total yield of 2, 3-BD. In terms of overall perfor-
mance, a two-stage continuous immobilized live cell reactor was found to be the
most efficient for 2, 3-BD formation [55, 57].
The single greatest cost in most biomass conversion processes is the substrate
cost [1, 2]. Hence, an inexpensive carbohydrate substrate is essential to develop an
economical fermentation process for the production of 2, 3-BD. Different carbohy-
drate sources used by microorganisms producing 2, 3-BD under different culture
conditions were reviewed [55]. pH is a crucial parameter during 2, 3-BD formation.
A pH range from 5 to 6 was found to be optimal for accelerating the formation of
2, 3-BD by K. oxytoca [58]. In addition, a microbial growth temperature (i.e. 37 ◦ C)
at which the sugar uptake can be managed by increasing the rate of 2, 3-BD forma-
tion is absolutely necessary [55]. Another important variable that affects the yield
of 2, 3-BD and the productivity of the microorganisms is the rate of oxygen flow
in the fermentation reaction [55]. These factors significantly contribute to 2, 3-BD