Microbial Lactic Acid Production from Renewable Resources - Sustainable Biotechnology: Sources of Renewable Energy - page 218

Other alternative lactic acid separation processes such as adsorption [38], reac-
tive distillation [39], electrodialysis [40], and nanofiltration [41, 42] have also been
studied for lactic acid separation and purification. Advances in membrane technolo-
gies have improved their use in the fields of separation and purification. There has
been a shift toward membrane separation processes because they are often more
capital and energy efficient when compared with chemical separation processes.
Membrane processes have advantages such as no energy-intensive phase changes or
potentially expensive solvents or adsorbents as well as the potential for simultaneous
separation and concentration of lactic acid.
The lactic acid fermentation broth contains lactate, lactose residues, cells, and
other organic and inorganic fermentation residues. The cells and large molecular
weight residues can be removed by microfiltration or ultrafiltration. Nanofitration
membranes, with membrane molecular weight cut-off of 100-400, can retain
97-100% of lactose to obtain a permeate containing only lactic acid and water
[41-43]. With reverse osmosis separation, lactic acid in the nanofiltration perme-
ate can be further concentrated which can substantially reduce the energy cost of
the subsequent evaporation process [44].
Electrodialysis is used to remove ions from the fermentation broth under the driv-
ing force of an electrical field generated by stacking cation- and anion-exchange
membranes. For a two-stage electrodialysis, desalting electrodialysis is applied
first to recover the lactate, and the water splitting eletrodialysis is applied for
acidification of lactate using biopolar membranes which have both anion- and
cation-exchange layers. One-stage water splitting eletrodialysis [45] and combined
nanofitration and water splitting electrodialysis [46] have also been studied for the
lactic acid separation and purification.

4 Results and Discussion

All lactic acid bacteria could ferment pure sugars such as lactose, glucose, xylose,
and arabinose to lactic acid via EMP pathway and/or the pentose PK pathway. The
performances of main strains with glucose as substrate are summarized in Table 1.
High lactic acid yield and productivity were obtained with most studied microor-
ganisms when lactose and glucose were fermented through the EMP pathway [21].
Some LAB such as Lb. Brevis and Lb. Pentosus can ferment pentose (xylose, arabi-
nose) using the PK pathway, but with a much lower lactic acid yield and productivity
as acetic acid and/or ethanol is also produced along with lactic acid.
Currently, batch fermentation is still the most commonly used method in indus-
trial lactic acid production. Lactic acid yield of 0.74 g/g glucose and productivity
of 4.4 g/l

h were obtained with Lb. casei NRRL B-441 from glucose [52]. The lac-
tic acid yield or productivity is limited by the inhibition of substrate (glucose) and
product (lactic acid) in batch fermentation [61]. Coupled fermentation and separa-
tion with cell and sugar recycling have been practiced for lactic acid production to
prevent nutrient depletion, prolong the growth phase, and increase the lactic acid

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