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Lactic acid also has a long invention history with the first reported discovery by Scheele [16, 17] on 1780 as a milk component, later on, Lavoisier named this milk component “acid lactic” in 1789 and Pasteur in 1857 confirmed it as a fermented metabolite rather than milk component [18]. Lactic acid produced by microorganism fermentation or via a synthetic chemical pathway. The demand for lactic acid-based products increasing globally and estimated to be raised around 2,000 kilotons by 2020 [19]; the largest consumer markets in the world are the United States, followed by China and Western Europe [17]. Lactic acid consists of two optical isomers: L (+)-lactic acid and D(−)-lactic acid, which can be prepared as optically pure isomers, i.e., L(+)- or D(−)-lactic acid by microbial fermentation (Figure 4.4) of renewable resources with the correct choice of the microorganisms. Each isomer is advantageous over the other depending upon the application. Optically pure lactic acids are the best choice to make high molecular weight commercial grade bio-plastic rather than the plastics derived from the racemic mixture in the chemical synthesis method (Figure 4.4). Apart from those, other numerous bioresources are available for lactic acid production, like glycerol (a by-product of bio-diesel) and microalgae (harvesting can be possible anywhere with a concise harvesting cycle). Microorganisms producing lactic acid are classified into two groups: bacteria and fungi, and their use depend on the substrates to be fermented. However, lactic acid bacteria (LAB) is the most popular method over the fungal production in terms of production rate caused due to mass transfer limitation and by-products formation. LAB can be classified into two categories depending upon the end fermentation product, homo-fermentative. It converts glucose into lactic acid as the sole product, whereas in the case of hetero-fermentative predominating side products like CO₂ and ethanol are also formed along with the desired lactic acid. Several efforts have been given to optimizing lactic acid production through microorganism engineering, and in Table 4.1, a few of them are listed.

Figure 4.4 Overview of different manufacturing methods of lactic acid (a) chemical synthesis and (b) microbial fermentation [20].

Table 4.1 Reports in the literature about recent investigations on the biotechnological production of lactic acid from cheap raw materials.

Substrate	Microorganism	Fermentation method	Lactic acid	References
Process productivity g/(L.h)	Yield (g/g)
Sugarcane bagasse hemicellulose hydrolysate	Bacillus sp. 75 strain 17C5	Batch	0.8	0.93	Patel et al., 2004 [49]
Corn fiber hydrolysate	Bacillus coagulans MXL-9	Fed-batch	0.21	0.46	Bischoff et al., 2010 [50]
Biomass derived xylose	Bacillus coagulans NL01	Batch	1.04	0.75	Ouyang et al., 2012 [51]
Various carbohydrates	Enterococcus faecalis RKY1	Batch	5.1	0.96	Yun et al., 2003 [52]