Fuels and Chemicals from Hemicellulose Sugars - Biotechnology in China: Biofuels and Bioenergy - page 208

hansenii [ 50 ], Pachysolen tannophilus [ 51 ], Candida tropicalis [ 52 ], recombinant
E. coli [ 53 ], recombinant S. cerevisiae [ 54 ], and recombinant Corynebacterium
glutamicum [ 55 ]. Among these microorganisms, C. tropicalis ferments xylose to
xylitol with good yield and productivity (Table 5 ).
In China, the xylitol bioconversion process using hemicellulose sugars as
substrates has been successfully developed [ 44 , 56 - 60 ]. Fang et al. [ 56 ] investi-
gated the influences of xylose, glucose, arabinose, fructose and the inhibitors from
corncob hemicellulosic hydrolyzate on xylitol production by Candida sp. It was
found that the optimum initial xylose concentration in the culture media was about
100 g/L and the inhibitions of acetic acid and furfural in the hydrolyzate increased
gradually when their concentrations exceeded 1 g/L. Ding et al. [ 57 ] studied the
effect of different aeration conditions on xylitol production from corncob hemi-
cellulose hydrolyzate by Candida sp. ZU04, and found that the two-phase aeration
process was more effective than one-phase aeration for xylitol production. Cheng
et al. [ 58 ] studied the effects of glucose and acetate concentrations in the hydro-
lyzate on xylitol production by C. tropicalis W103. It was found that glucose in
corncob hydrolyzate promoted the growth of C. tropicalis while acetate at high
concentration was inhibitory. In another study, C. tropicalis cells were immobi-
lized in Ca-alginate beads and used for xylitol production from corncob
hydrolyzates [ 59 ]. In 10 repeated batches (30 days) fermentation, the average
xylitol yield was 73.7%. The immobilized cells have high density, steady fer-
mentability and good resistance to inhibitors in the hydrolyzates. Deng et al. [ 44 ]
employed the ammonia steeping strategy to pretreat the rice straw, and converted
the hydrolyzate to xylitol by C. tropicalis directly. Experimental results showed
that the content of toxic compounds created in the hydrolysis process, such as
acetic acid and phenolic compounds, was greatly reduced and the fermentation of
the hydrolyzate was enhanced. Xylitol fermentation was investigated in flasks and
a 2-L bioreactor. The xylitol yield and volumetric productivity were 0.746 g/g and
0.686 g/(L h) in the lab-flask fermentation. The corresponding results conducted in
bioreactor fermentation were 0.689 g/g and 0.697 g/(L h), respectively.
3.3 2,3-Butanediol, Methyl ethyl ketone, and 2-Butanol
When evaluating the benefits of 2,3-butanediol (2,3-BD) production, consideration
must be given to the following facts. 2,3-BD is less toxic to microbial systems than
other alcohol products, and its energy content is comparable with other microbial
liquid fuels. An equimolar mixture of ethanol and 2,3-BD has an energy content of
27,700 kJ/kg, while pure ethanol, 2,3-BD, and methanol contain 29,100, 27,200,
and 22,100 kJ/kg respectively [ 61 ]. Therefore, 2,3-BD can be used as an alter-
native to traditional ethanol. Additionally, 2,3-BD may serve as a feedstock for the
production of numerous valuable chemicals, including printing inks, perfumes,
fumigants, moistening and softening agents, explosives, plasticizers, foods, and
pharmaceuticals [ 62 - 65 ].