Microbial Lactic Acid Production from Renewable Resources - Sustainable Biotechnology: Sources of Renewable Energy - page 215

by galactose-1-phosphate uridylyltransferase with concurrent production of
glucose-1-phosphate. The glucose-6-phosphate is formed from glucose-1-phosphate
by phosphoglucomutase catalysis, and then enters the homofermentative or hetero-
fermentative pathway to produce lactic acid, and ethanol and/or acetic acid.
Pentoses, such as xylose and arabinose, are mainly derived from the hemicel-
lulose of lignocellulosic biomass. In xylose-utilizing strains, xylose is reduced and
oxidized to xylulose by xylose reductase and xylitol dehydrogenase (Fig. 2, route 4)
[15]. Xylulose is subsequently phosphorylated to xylulose-5-phosphate and metab-
olized through the pentose phosphate pathway. Similarly, arabinose is converted
by arabinose-utilizing strains to the ribulose-5-phosphate and then enters the PK
pathway for producing lactic acid and acetic acid or ethanol consequently (Fig. 2,
route 3). LAB such as Lb. pentosus, Lb. brevis and Lc. lactis can ferment glucose
to lactic acid by homolactic fermentation and can also effectively convert xylose to
lactic acid and acetic acid by heterolactic fermentation [16, 17].
3 Materials and Methods
3.1 Pretreatment
When lignocellulosic biomass feedstock is used, a pretreatment process is required
prior to enzymatic hydrolysis and fermentation (Fig. 1). Physical (grinding and irra-
diation), chemical (dilute acid and alkali), and physicochemical (steam pretreatment
and autohydrolysis) processes have been performed to reduce the particle size and
crystalinity, to increase solubilization of hemicelluloses and lignin, and to enhance
the accessibility of cellulose to the enzyme in the following enzymatic hydrolysis
step.
Dilute acid pretreatment is performed by adding dilute acid (<4% H 2 SO 4 )at
elevated temperatures between 130 and 200 ◦ C for 2-80 min [18]. It is an inex-
pensive process with up to 90% hemicellulose and glucose yields during the
enzymatic hydrolysis [19]. During acid hydrolysis of lignocellulosic biomass, it
is important to select proper pretreatment conditions to maximize the solubiliza-
tion of hemicellulose and minimize the formation of inhibitors such as furfural and
hydroxymethylfurfural (HMF).
Alkaline pretreatment (NaOH or lime) can swell the pores of the cellulosic
biomass at temperatures ranging from 25 ◦ C (ambient temperature) to 85 ◦ C [20,
21]. Alkali pretreatment can reduce the degree of polymerization and crystallinity,
increase the surface area, and increase the solubilization of lignin and hemi-
celluloses [18]. The solid/liquid ratio of alkali treatment is about 10-20%. The
pretreatment time ranges from 1 to 30 h depending on the pretreatment temperature
and alkali loading. Alkali pretreatment is more effective on agricultural residues and
herbaceous energy crops than on wood materials. Aqueous ammonia treatment has
been successfully used as a pretreatment process of corn stover for lactic acid pro-
duction [17]. Aqueous ammonia treatment is conducted at temperatures between 70
and 90 ◦ C and pressures of 15-20 atm for less than 5 minutes. The liquid-to-solid