Biomedical Engineering Reference
In-Depth Information
2,3-BD is a fermentation product of a number of bacteria such as Klebsiella sp.
(K. pneumoniae or K. oxytoca) and Panebacillus polymyxa, which are considered
of industrial importance in the production of 2,3-BD [ 66 , 67 ]. The advantage of
2,3-BD production using bacteria is that the species are easy to cultivate and they
grow rapidly in a simple medium and metabolize all of the major sugars present in
hemicellulose and cellulose hydrolyzates into 2,3-BD [ 68 , 69 ]. 2,3-BD fermen-
tations using hemicellulosic materials were reported as early as the 1980s [ 70 , 71 ].
Yu et al. [ 72 , 73 ] successfully developed the processes using both acid and
enzymatic hydrolyzed wood hemicellulose for 2,3-BD production by
K. pneumoniae. 2,3-BD yields of 0.4-0.5 g/g were obtained from hydrolyzed wood
hemicellulose. The authors proposed that such high yields were partially due to the
ability of K. pneumoniae to simultaneously ferment uronic acids, such as
D-glucuronic and D-galacturonic acid, present in the wood samples. Other com-
pounds in wood (such as furfural and lignin derivatives) were inhibitory to the
bacteria if the wood hydrolyzate was added at a high concentration [ 74 , 75 ].
A more efficient approach using woody biomass for 2,3-BD production was solid-
state fermentation (SSF) with addition of culture filtrates of Trichoderma
harzianum as a source of hydrolytic enzymes [ 76 , 77 ]. Corn fiber, a hemicellulose-
rich byproduct in corn processing, was also successfully used as a substrate for
2,3-BD production by Saha and Bothast [ 78 ]. They isolated a 2,3-BD-producing
Enterobacter cloacae from decaying wood/corn soil samples, which was found to
utilize sugars from acid plus enzyme-saccharified corn fiber and produced 2,3-BD
(0.35 g/g available sugars). The strain was also able to produce 2,3-BD from dilute
acid-pretreated corn fiber by SSF (0.34 g/g theoretical sugars).
In China, pure xylose, glucose-xylose mixtures, and hemicellulose-derived sugar
mixtures have been used and optimized for 2,3-BD fermentation [ 79 - 84 ]. Cheng
et al. [ 80 ] detoxified the acid hydrolyzate of corncob (hemicellulose components rich
stream) by sequentially boiling, over-liming, and adsorbing the hydrolyzate onto
activated charcoal, and then using the pentose-rich hydrolyzate as substrate for
2,3-BD production by K. oxytoca. Under optimum conditions, a maximal 2,3-BD
concentration of 35.7 g/L was obtained after 60 h of fed-batch fermentation, giving a
yield of 0.5 g/g reducing sugar and a productivity of 0.59 g/(L h). Corncob molasses,
a waste byproduct in xylitol production that contained high concentrations of
hemicellulose-derived arabinose and xylose, was recently reused as a substrate for
production of 2,3-BD using K. pneumoniae [ 82 ]. The maximum 2,3-BD concen-
tration was 78.9 g/L after 61 h of fed-batch fermentation. This approach, using
corncob molasses for 2,3-BD production, not only cut down the feedstock cost, but
also provided a method of exploiting xylitol industry byproducts, which could reduce
environmental pollution. Ji et al. [ 79 ] successfully developed an economical
industrial medium for glucose-xylose co-fermentation using a metabolic engineered
K. oxytoca [ 85 , 86 ]. However, when glucose and xylose were present at the same
time, xylose consumption generally did not commence until glucose was depleted,
i.e., sugars were sequentially consumed resulting in several exponential growth
phases that were separated by intermediate lag phases, developing so-called diauxic
growth [ 87 ]. This would hinder the possibility of utilizing the hemicellulose
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