Microbial Lactic Acid Production from Renewable Resources - Sustainable Biotechnology: Sources of Renewable Energy - page 216

ratio is generally between 10 and 30%. The alkali treated materials are washed with
water until the pH value reaches around 7.0.
Steam pretreatment is performed with high-pressure saturated steam at a tem-
perature between 160 and 240 ◦ C for a specific time ranging from several seconds
to a few minutes. The severity of steam treatment can be described by R o ( R o =

t
exp ( T - 100)/14.75) which is a function of treatment time ( t , min) and tempera-
ture ( T , ◦ C) [22]. Previous work has shown that SO 2 -catalyzed steam treatment is
an effective pretreatment for lignocellulosic biomass [23]. Pretreatment using water
at a controlled pH (5.0-7.0) and temperature (150-200 ◦ C) for 10-30 min has also
been proven to be effective for corn stover [18].

3.2 Enzymatic Hydrolysis and Fermentation

Although pure sugars or disaccharides, including lactose, are good feedstocks for
lactic acid production, feedstocks such as starchy and lignocellulosic biomass are
cheaper and more abundant. Usually, starch-based materials and cellulosic biomass
feedstocks used for fermentation have to be hydrolyzed to glucose before they
can be fermented to lactic acid. Conventionally, gelatinization and liquefaction of
starch is carried out enzymaticaly at high temperatures of 90-130 ◦ C for 10-30 min
with -amylase followed by enzymatic saccharification to glucose at 50-60 ◦ Cfor

2-4 h with glucoamylase [24]. The optimum pH values for the liquefaction and
saccharification steps are 6.0 and 4.5, respectively [25].
Pretreatment of lignocellulosic biomass is required before enzymatic hydroly-
sis. When enzymatic hydrolysis and fermentation are performed sequentially, it is
referred to as separate hydrolysis and fermentation (SHF). However, the cellubiose
and glucose produced during lignocellulose saccharification strongly inhibit the cel-
lulase activities. This two-step process involving consecutive enzymatic hydrolysis
and microbial fermentation also prolongs the total processing time. Simultaneous
saccharification and fermentation (SSF) is an effective process in which enzymatic
hydrolysis and fermentation of lignocellulosic materials are conducted in the same
reactor which can eliminate the substrate inhibition [26]. However, the different
optimal temperatures and pH required for saccharification and fermentation are the
main problems for SSF [27]. The optimal conditions for enzymatic hydrolysis are
50 ◦ C and a pH value below 5.0, while the optimum conditions for lactic acid fer-
mentation are 37-42 ◦ C and a pH value of 5.5-6.5. Some compromise is needed for
the SSF process in order to obtain high overall lactic acid yield and productivity.
Acid hydrolysis of starchy biomass, an alternative approach to enzymatic hydrol-
ysis, is carried out with 1-5% H 2 SO 4 with a liquid-to-solid ratio of approximately
8-10:1 (w/w) at temperatures ranging from 100 to 130 ◦ C for 1-3 h. After cooling
to room temperature, the acid hydrolysate is neutralized to pH 6.5 with an alkali or
CaCO 3 [16, 27].
Direct fermentation, which couples the enzymatic hydrolysis of carbohydrate
substrates and microbial fermentation into a single step by utilizing the amylolytic
lactic acid bacteria (ALAB), has received increasing interest [28]. A few amylolytic

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