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3%, w/v sugar cane juice after 10 h of fermentation. Use of natural
substrates like starch and cellulose is not economically favourable
[10] due to requirement of pretreatment to release fermentable
sugars.
The most widely used homofermentative bacterial strains used
in such fermentation are Lactobacillus bulgaricus, Lactobacillus
Leichmanii, Lactobacillus delbrueckii, Lactobacillus amylophilus
and Lactobacillus plantarum. Fungal strains like Aspergillus niger
and Rhizopus species have also been used in several studies [11]
and have been claimed to be more selective in production of L-
lactic acid stereoisomer and less nutrient-demanding than their
bacterial counter parts. In general, fermentative process can selec-
tively produce the desirable L-lactic acid stereoisomer instead of
a mixture of L and D isomers using specific microbial strains [11]
but maintaining high-growth environment for these microbes is
difficult. With production and accumulation of acid, pH falls and
affects productivity of themicrobes. In the conventional fermenta-
tion process as shown in Fig. 2, pHof the batch reactor ismaintained
at around 5–6 by addition of calciumhydroxide or calciumcarbon-
ate and the ‘end-fermentation’ concentration of lactic acid is only
10wt% normally. Sometimes ammonium hydroxide is also used.
The problem of addition of lime in controlling pH is that it leads
to production of calcium lactate instead of lactic acid at high pH
as pKa value of lactic acid is 3.86 at 30 ◦C. Calcium lactate is then
separated fromthemicrobial cells by filtration and further purified
by activated carbon adsorption. The subsequent steps are evap-
oration and acidification by sulphuric acid to produce lactic acid
and insoluble calcium sulphate (gypsum) as by-product. Gypsum
by-productwhich remains associatedwith organicmass and is pro-
duced at the rate of 1 metric tonne per metric tonne of lactic acid
is a big environmental problem associated with conventional fer-
mentation process. Need for somany steps along with evaporation
involving phase change to get pure lactic acid naturally involves
high capital investment as well as high operating cost. The prob-
lemof low pH and hence low productivity can be largely overcome
if produced lactic acid is continuously removed from the fermen-
tor and this is possible if fermentation is carried out in a continuous
mode. Continuous removal of lactic acid from fermentation broth
can be done by adsorption [12–14], extraction [15,16] and mem-
brane separation. Adsorption and extraction based processes also
need quite a few steps as regeneration of adsorbent and recycling
of solvent is necessary. These processes themselves cannot ensure
separation of microbial cells for their recycle without additional
provision of cell separation and recycle. High cell concentration
in the fermenter is essential for high productivity. Moreover, the
extent of process intensification that can be achieved in a mem-
brane based process due to the associated advantages asmentioned
earlier cannot be expected from adsorption or extraction-based
processes. Thus the recent years havewitnessed extensive research
activities on solving the problems of traditional manufacturing
processes through membrane based separations. The subsequent3. Membrane based processes
For continuousmode operation of a fermentative process using
renewable carbohydrate sources for lactic acid production, the
components that need to be continuously separated from the
fermentation broth are microbial cells, proteins, nutrients (yeast
extract, salts of ammonium, potassium, phosphorus, etc.), uncon-
verted carbon sources,water and lactic acid as shown inTable 2. The