Microbial Lactic Acid Production from Renewable Resources - Sustainable Biotechnology: Sources of Renewable Energy - page 213

There are two optical isomers of lactic acid: L (+) -lactic acid and D (-) -lactic
acid. Pure L (+) and D (-) lactic acid can be obtained by microbial fermentation
of renewable sources when an appropriate microorganism is selected. Racemic
DL-lactic acid is always produced via chemical synthesis from petrochemical
resources [3, 5]. While homopolymers form regular structures and develop a crys-
talline phase, copolymerization with D- or L- lactic acid leads to the interruption
of the regular structures and the formation of amorphous materials. In order to
achieve the desired polymer properties, high purity D- or L-lactic acid monomers
are necessary [6].
2 Background Research
More than 100 different lactic acid bacteria (LAB) and filamentous fungi have been
used for the microbial lactic acid production from renewable resources. However,
Lactobacillus (Lb.) and Lactococcus (Lc.) species were most frequently studied in
the past years for lactic acid production. LAB can be classified into two groups:
(1) homofermentative and (2) heterofermentative. Homofermentative LAB convert
sugars exclusively into lactic acid, while heterofermentative LAB produce other
byproducts such as acetic acid, ethanol, and/or carbon dioxide along with lactic
acid [5].
The two major pathways for assimilation of hexoses (glucose and galac-
tose) and pentoses (xylose and arabinose) in the lactic acid bacteria are the
Embden-Meyerhof-Parnas (EMP) pathway (Fig. 2, route c) and the pentose phos-
phoketolase (PK) pathway (Fig. 2, route b). Under conditions of excess glucose
and limited oxygen, homofermentative LAB such as Lc. lactis [7], Lb. del-
bruecki [8], and Lb. helveticus [9] catabolize one mole of glucose in the EMP
pathway to yield two moles of pyruvate. Intracellular redox balance is main-
tained through the oxidation of NADH, concomitant with pyruvate reduction to
lactic acid. This process yields two moles lactic acid per mole of glucose con-
sumed. Heterofermentation metabolizes sugars through the pentose phosphate
pathway in some microoganisms. For example, Lb. brevis is a heterofermenta-
tive lactic acid bacterium, which produces lactic acid, carbon dioxide and ethanol
from hexoses [10]. One mole glucose-6-phosphate is initially dehydrogenated to
6-phosphogluconate and subsequently decarboxylated to yield one mole of CO 2 .
The resulting ribulose-5-phosphate is cleaved to one mole glyceraldehyde phos-
phate and one mole of acetyl-phosphate. The glyceraldehyde phosphate is further
metabolized to lactic acid, while the acetyl phosphate is reduced to ethanol via
acetyl-CoA and acetaldehyde intermediates. Lb. sunfruncisco is another hetero-
fermentative LAB which converts glucose to glyceraldehyde phosphate, then to
lactic and acetic acid by catalyzing the acetyl- phosphate [11]. Lactic acid and
other substances (typically acetic acid and/or ethanol) are the main end products
and their compositions and ratios vary with the microorganisms and fermentation
conditions [12].
Lactic acid bacteria also ferment other sugars such as lactose or galactose via
different pathways [13, 14]. The catabolism of the disaccharide lactose involves the