Microbial Lactic Acid Production from Renewable Resources - Sustainable Biotechnology: Sources of Renewable Energy - page 217

LAB strains have been isolated, such as Lb. amylophilus JCM 1125 [29], Lb. mani-
hotivorans LMG 18010T [30], Lb. amylophilus GV6 [31], and Streptococcus bovis
148 [32].
According to Hofvendahl and Hahn-Hagerdal [5], the optimum pH for lactic
acid production by microorganisms varies between 5.0 and 7.0, depending on the
microorganism species. Too-low or too-high pH levels would inhibit the strains'
growth and lactic acid production. When fermentation experiments are carried out
in orbital shakers, CaCO 3 (10-30 g/l) is generally added to neutralize the lactic
acid produced and maintain the pH at the optimal range. A 4-10 M NaOH or
NH 4 OH solution is generally used to maintain the pH at the designated value in
the fermentation reactor.
The influence of temperature on lactic acid production is related to the growth
kinetic parameters of the organism, lactic acid production, and substrate consump-
tion. Among the LAB, most lactic acid productivity studies have been conducted
at a temperature of 35-39 o C. Agitation influences the mixing and mass transfer
rates. Agitation speed of 150-300 rpm is generally used for lactic acid production
in fermentation reactors. The medium is generally supplemented with the desired
salt or nutrient concentrations such as yeast extract 0.1-0.5%, K 2 HPO 4 0.1%, and
MgSO 4 · 7H 2 O, 0.5% etc.

Lactic acid production can be performed as a batch, fed-batch or continuous
fermentation process [33-35]. For batch processes, the low level of nutrient that
can be tolerated by the cells limits the final cell and product concentration. It is
unable to attain and sustain high cell concentrations with the resources available in a
typical growth medium. To overcome nutrient limitations, fed-batch processes have
been practiced for lactic acid production with different substrates. For fed-batch
fermentation, the microorganisms are grown in batch medium for 12-24 h, and then
fresh media is continuously added into the reactor at a constant feeding rate, until
the working volume of the reactor is reached. For repeated fed-batch fermentation,
after 24 h of fermentation, half of the fermentation broth is withdrawn from the
fermentor and an equal volume of fresh medium is added into the fermentor at a
constant feeding rate [36].
Cell immobilization on a solid carrier is another method which has been widely
studied for lactic acid production. A high cell density can be achieved in immobi-
lization culturing which can be easily re-used for lactic acid production over a long
period, especially for a fed-batch or continuous fermentation process [37].

3.3 Separation

In the traditional separation process, the fermentation broth is first neutralized by
C a CO 3 and then filtered to remove cells, carbon treated, evaporated and acidified
with H 2 SO 4 to get lactic acid and C a SO 4 . The insoluble C a SO 4 is removed by filtra-
tion. Pure lactic acid is further obtained by hydrolysis, esterification, and distillation.
This process yields calcium salt as a by-product, resulting in high chemical cost and
waste generation.