Biotechnological Applications of Hemicellulosic Derived Sugars: State-of-the-Art - Sustainable Biotechnology: Sources of Renewable Energy - page 75

conversion of hexose sugars into chemicals is well established; however, the ability
of these organisms to ferment pentose sugars is somewhat less so. The use-
ful exploitation of lignocellulosics by fermentation can be enhanced by efficient
utilization of the pentosanic fraction along with hexoses.
Yeasts that have been studied extensively for use in xylose fermentation include
Pachysolen tannophilus , Candida shehatae , Pichia stiptis, and Kluveromyces marxi-
anus [3]. The optimal performance of these microorganisms is usually controlled by
the air supply. Other yeasts investigated for their xylose-fermenting ability include
Brettanomyces , Clavispora , Schizosaccharomyces , several other species of Candida
viz. C. tenius , C. tropicalis , C. utilis , C. blankii , C. friedrichii , C. solani, and
C. parapsilosis , and species of Debaromyces viz. D. nepalensis and D. polymorpha .
Maleszka and Schneider [31] screened 15 yeast strains for their ability to utilize
D-xylose, D-xylulose, and xylitol for ethanol production under aerobic, microaero-
bic (low aeration), and anaerobic conditions using rich undefined or defined media.
In almost all cases, ethanol production by P. tannophilus and species belonging
to Candida and Pichia was better on rich media under microaerobic conditions
[3,4,31].
Several pentose-utilizing fungal species like Fusarium oxysporum , Rhizopus sp.,
Monilia sp. , Neurospora crassa , Paecilomyces sp., Mucor sp., Neurospora crassa,
and F. oxysporum and bacterial species like Bacillus macerans , B. polymyxa ,
Kiebsiella pneumoniae , Clostridium acetobutylicum , Aeromonas hydrophila ,
Aerobacter sp., Erwinia sp., Leuconostoc sp., Lactobacillus sp., Clostridium ther-
mocellum , C. thermohydrsulfurium , C. thermosaccharolyticum , and C. thermosul-
furogenes utilizing pentose, hexose, and lignocellulose hydrolysates for ethanol
production have been extensively reviewed [32].
Fermentation Methodologies
Researchers have performed all three fermentation processes (batch, fed-batch,
and continuous) for biomass conversion into ethanol. The most suitable fermen-
tation strategy depends upon the growth kinetics of the microorganism, the type of
hydrolysate, and the economics of the process. For ethanol production from lig-
nocellulosic biomass, batch fermentation has been extensively utilized in the past.
The batch process is a multivessel approach that allows flexible operation and easy
control in the bioconversion process [33]. In fed-batch fermentation, the micro-
bial cells can be acclimatized at low substrate concentrations that later assist in
accelerating the rate of ethanol formation during the entire course of the biocon-
version process. Fed-batch fermentation processes are ideal to obtain a high cell
density, which may help to achieve higher ethanol yields with greater productivity.
Higher cell density also helps to reduce the toxicity of lignocellulose hydrolysates,
particularly acid hydrolysates, to yeast cells. Continuous fermentation is another
state-of-the-art technology in which microorganisms work at a lower substrate con-
centration, maintaining higher ethanol concentration during the entire course of the
fermentation reaction [34]. Table 3 summarizes the fermentation profiles of different
microorganisms utilizing a variety of lignocellulose hydrolysates.