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nomically viable and continuousmanufacturing scheme capable of
producing monomer grade lactic acid with high productivity.2. Traditional production process
Traditionally lactic acid has been produced through either
chemical synthesis route or fermentation route. Chemical synthe-
sis route as shown in Fig. 1 uses by-products like lactonitrile from
other industries.Hydrogen cyanide (HCN) is added to acetaldehyde
(CH 3 CHO) in liquid phase in presence of a base catalyst under
high pressure when lactonitrile is produced. Crude lactonitrile is
then purified by distillation and subsequently hydrolyzed to lac-
tic acid by hydrochloric acid or sulphuric acid [2]. The process
is often dependent on other by-product industries and, consid-
ered expensive where petroleum based raw material is the major
cost-contributor. Moreover, chemical synthesis route produces a
mixture of L-lactic acid and D-lactic acid whereas in most of
the cases, L-lactic acid is the desirable product. The problems of
high cost of raw materials, impurity of the product and depen-
dence on other industries for raw materials could be overcome
in fermentation based process. Thus majority of the big lactic
acid manufacturing industries like Musahino chemical in Japan,CCA Biochemical BV of the Netherlands, Natureworks LLC etc.
have switched over to fermentation based technology as pre-
sented in Fig. 2. The scheme consists of a number of downstream
treatment schemes like precipitation, conventional filtration, acid-
ification, carbon adsorption, evaporation etc. Though possible
carbon sources may be numerous, mainly the cheap and renew-
able carbohydrates as shown in Table 1 are used as the source
of carbon and produces optically pure form of lactic acid [3]. The
substrates are chosen on the basis of their cost, levels of contam-
inants, fermentation rate, lactic acid yields, by-product formation,
and ability to be fermented with little or no pretreatment and
year round availability. Substrate cost is one of the major cur-
rent problems. Downstream purification needs are closely related
to the nature of these substrates, their degree of conversion and
the reagents or microbes involved in the conversion process along
with other factors. Themost widely used substrates for production
of lactic acid are glucose, sucrose and lactose which are soluble
carbohydrates [4]. Cucumber juice (CJ) is also a good substrate for
production of lactic acid as it contains malic acid with glucose and
fructose where malic acid is very rapidly converted to lactic acid.
Molar yield of lactic acid from CJ were found to be 11.2 [5]. Cheese
whey, a by-product from cheese manufacturing industry is also
used as substrate for production of lactic acid as it contains hugeamount of lactose. Composition of this whey permeate is 98–99%
total solids, 79–81%lactose, 8–10%ash and 2.5–4%protein[6]. Some
researchers used sugarcane bagasse cellulose for batch lactic acid
production and the yield was 83% but the productivity obtained
was very low(0.93 g/L/h) and concentration of lactic acidwas 67 g/L
[7].Molasseswhich is a byproduct in production of cane sugar from
sugar cane juice contains 40–60% sucrose have also been used as
a substrate. It has been reported [8] that lactic acid with yield of
0.88–0.96 and productivity of 4.3 g/L could be produced in a 40 h-
batch fermentation of molasses with initial sugar concentration of
190 g/L. As themolasses contains enough nitrogen source, need for
addition of yeast extract in such case, is eliminated. Timbuntam et
al. [9] used sugar cane juice containing 125 g/L sucrose, 8 g/L glu-
cose and 6 g/L fructose as substrate in a batch fermentation. The
highest yield of 96%was obtained and productivitywas 2.8 g/L/h at