INTRODUCTION:

Lactic acid (LA) or 2-hydroxypropionic acid is the simplest hydroxycarboxylic acid. This hydroxycarboxylic acid is perhaps the most widely occurring in nature. LA is a chiral molecule with two optical active isomeric forms which are the dextrorotatory form L (+)-lactic acid or (S)-lactic acid and the levorotatory form, D (–)-lactic acid or (R)- lactic acid¹_.LA are often produced via chemical or microbial fermentation routes.

A racemic mixture of D-LA and L-LA is obtained via chemical synthesis, which is not desirable for food, beverage, and pharmaceutical applications. The fermentative route however offers several advantages over the chemical synthesis such as simple operation, utilization of low-cost renewable feed stocks, mild pressure and temperature conditions and reduced risk of contamination, better environmental performance and low energy consumption². Biodegradable synthetic polyesters have recently found widespread application in biomedical and engineering fields, such as tissue engineering, therapeutic delivery and bioimaging. Currently, many applications involving lactic acid based polyesters are being explored with polymers derived from monomers that are endogenous to the human metabolism³. Poly (lactic acid) (PLA) polymer which belongs to the family of aliphatic polyesters commonly made from α-hydroxyl acids has been being used and studied increasingly in huge number of applications such as biodegradable plastic, cardiovascular applications or drug delivery system^4,5.

Around 90% of the total lactic acid production worldwide is mainly from microbial fermentation⁶. It has attracted interest because of its various advantages compared with chemical synthesis, such as production of pure isomers and the use of renewable resources as fermentation substrates. The lactic acid molecule is found naturally in animals, plants and microorganisms and may even be produced by the fermentation of carbohydrates or by chemical synthesis from natural gas, coal and petroleum products. Industrially, lactic acid may be produced by chemical synthesis or by fermentation. The two main processes of lactic acid production are shown in Fig.1.

Fig 1. Lactic Acid production from Petrochemical resources and Renewable resources⁷.

An optically pure L (+) - or D (-) lactic acid can be obtained by microbial fermentation of renewable resources when the appropriate microorganism is selected⁷.Currently, the most noteworthy use of lactic acid is the production of polylactic acid to replace plastics from the petrochemical industry. Poly (lactic acid) (PLA) has become one of the most concerned polymer as biodegradable and biocompatible material⁸. The ability to produce a high purity isomer has important ramifications in the chemistry and ultimate process/property relationships achievable in the polymers produced from LA.

Petrochemical LA production generates a racemic LA mixture that, once polymerized will result in an amorphous PLA (Figure 2), with a low real use for the industry. However, the PLA industry can build polymers that can replace almost any known polymer derived from the petrochemical industry, apart from producing biodegradable plastics for daily use. Presently LA is produced from carbohydrates (glucose) but due to the high cost of sugar it makes it unfeasible and uneconomical for use. However, waste products from food industries, agricultural industries, sugarcane mills, and biomasses are often used, which is advantageous from an environmental and economic standpoint.

LA production becomes even more relevant when considering its production from renewable raw materials, especially second-generation (2G) substrates, like lignocellulosic biomass. This reduces the human dependence on oil for both energy and fuel production, also as for the assembly of plastics and other chemicals since LA remains one of the most relevant building block chemicals. Currently it is possible to produce lactic acid from the most diverse 2G substrates available around the world. Thus, LA production by fermentation of 2G-sugars are often related to several existing bio refinery models, like for biofuels and chemicals production.

The fermentation pathways of lactic acid bacteria (LAB) have high conversion rates, which means large amounts of sugar are fermented and not much sugar is converted for cell biomass production. It leads to high yields of LA and makes them industrially interesting⁹.A further interesting ability is that many LAB are able to utilize various hexoses and pentoses⁷, making them perfect to exploit lignocellulosic sugars to produce second-generation (2G) LA. Taking all the above facts into consideration the main achievements of the 2G Lactic Acid research is presented in this review.

Fermentative 2G lactic acid production from sugarcane bagasse:

Sugar industry is the second largest agro industry in India and a large amount of bagasse is obtained as a waste material^10,11,12. In sugar mills, sugarcane bagasse (SCB) is the by-products of the sugar industry with a well-established supply chain, one of the surplus agro- industrial wastes. This makes sugarcane bagasse one of the cheapest source for the production of 2G lactic acid because it is available over the year that makes the sugar cane industry feasible, sustainable and economically independent. But the lignocellulosic biomasses need some pretreatment methods to alter the structure for greater enzyme accessibility for conversion of cellulose into glucose units. Hence, a suitable pretreatment method will remove structural barriers that limit the conversion of these materials to fuels and chemicals¹³. Fig 3 below shows world production of sugarcane bagasse (thousand metric tons) in 2020.

Table 1: World production of sugarcane bagasse (thousand metric tons) in 2020¹⁴.

S. No	Country	Sugarcane Bagasse in Thousand MT
1.	Brazil	31.16
2.	India	15.14
3.	China	6.54
4.	Thailand	5.69
5.	USA	4.38
6.	Pakistan	3.47
7.	Mexico	3.41
8.	Russia	2.96
9.	France	2.67
10.	Germany	2.62

As the sugarcane bagasse contains 40-50% cellulose, 20-35% hemi-cellulose and 20-35% lignin^15,16,17. It is commonly used for steam and electricity generation (co-generation) using high-pressure boilers and turbo – alternators. However, there are limitations to the use of SCB for co-generation due to diminishing market price of electricity. Hence it is important to look for alternative products from SCB such as ethanol, lactic acid, biogas, dye adsorbents¹⁸, cellulose nanofibers¹⁹etc. For a sustainable lignocellulosic biomass (LCB) based bio refinery, the researchers and engineers must emphasize on the production of platform and commodity chemicals at higher titers from hydrolyzed and fermentable sugars. However, it can only be accomplished when enzymatic liquefaction is performed at a high solid loading of pre-treated lignocellulosic biomass with uncompromised and concentrated sugar yields²⁰. Some recent studies of 2G-LA production from sugarcane bagasse are presented in Table 2.

Sugar beet pulp (SBP):

Sugar beet provides 16% of the sugar produced worldwide²⁶.The top most country by sugar beet production in the world is the Russian Federation. In 2017, the European regulation for the quotes and minimum sugar prices was abolished which dramatically increased the competition and caused a decline in the prices²⁷. Considering this scenario, it is expected that sugar producers will look for new business models to diversify the sugar beet industry revenue²⁸.

With increasing biotechnological advancements, sugar beet pulp (SBP) represented a potential low cost biomass which can be valorized to produce different value added ligno cellulosic platform chemicals such as lactic acid, biogas, ethanol etc. of economic importance. The process of production of sucrose and ethanol has been known for a long-time in the sugar beet industry. However, the concept of bio refinery to aggregate value to its residues has just recently been highlighted ^28,29. The main residue from this industry, with a large potential to be used in the bio refinery, is the pulp remaining after the juice removal. Sugar beet pulp (SBP) is normally used as an animal feed ^30,31,32,33. Now, there is a new possibility to produce second-generation (2G) lactic acid from SBP. Lactic acid is an organic acid with widespread applications mainly in pharmaceutical, cosmetic, chemical, as well as food industries³⁴. SBP is a promising substrate for lactic acid production, approaching a sustainable bio refinery production mode. As a substrate to second-generation lactic acid production, it can reconfigure the sugar beet industry, inserting this crop in the modern bio refinery concept of product valorization and sustainability. As per a recent study, using Bacillus coagulans in a continuous fermentation, 2781.01 g of lactic acid was produced from 3916.91 g of sugars from hydrolyzed sugar beet pulp, with a maximum productivity of 18.06 g L−1h−1³⁵. So the use sugar beet pulp as a 2G substrate has a potential to overcome those needs since it is a complex organic material instead of pure processed sugar.

Sweet sorghum bagasse:

Sweet sorghum bagasse represents a potential low-cost biomass which can be valorized to produce different value-added lignocellulosic platform chemicals of economic importance. Lignocellulosic biomass is the only suitable raw material as a sustainable renewable resource in the production of biofuels³⁶. Sweet sorghum is a bioenergy crop that produces sizable amounts of soluble sugars in its stems (3–7 Mg ha−1) and produce large amounts of bagasse (15–20 Mg ha−1) as a lignocellulosic feedstock. These sugars can not only be fermented to biofuels but also to bio-based chemicals. Lignocellulose comprises mainly of cellulose, along with hemicellulose and lignin in smaller quantities that needs pre-treatment before it is valorized to different value-added products. Cellulose which is a major constituent of the lignocellulosic biomass is used as a raw material for pulp and paper production and for many other value-added products³⁷. Moreover, cellulose available in various powder forms ( micro fine cellulose and microcrystalline cellulose) and its derivatives (sodium carboxymethyl cellulose) finds variety of uses in plastics, coatings, films, suspension agents, and composites in different application areas, viz., biomedical,, food aerospace, sports, marine industry, automobile construction industries, packaging, etc.³⁸.

As per a recent research, Carbohydrates in the sorghum bagasse were also fermented after pretreatment with 0.5% phosphoric acid at 90deg C for 5min. Simultaneous saccharification and co-fermentation of all the sugars (SScF) by B. coagulans resulted in a conversion of 80 % of available carbohydrates to optically pure lactic acid³⁹. Thus B. coagulans is an effective biocatalyst for fermentation of all the sugars present in sweet sorghum juice and bagasse to optically pure lactic acid at high titer and productivity as feedstock for bio-based plastics. All the sugars in the unsterilized sweet sorghum juice can also be fermented to D- or L-lactic acid at concentrations of 1.4 and 1.7 M, respectively, representing a yield of 90–100 %. Optically pure lactic acid which is a feedstock for biodegradable plastics and can be easily derived from the carbohydrates and sugars in sweet sorghum as a sustainable alternative to petroleum-derived plastics. Sweet sorghum, an annual bioenergy crop with a short life cycle (4 months), offers the potential for multiple crops per year in warm climates. These crops can also be produced on low productivity lands.

Other fermentative feedstock for 2G lactic acid production:

Lactic acid use on industrial scale is still limited due to relatively high cost production, mainly due to the cost of carbon source. Most of the lactic acid is currently fermented from refined food carbohydrates showing the importance of exploring alternative cheaper carbon sources for LA production^40. In the last few decades, with new uses and products based on LA, the production of lactic acid has increased substantially. Global lactic acid demand is estimated to be 1220 Kt in 2016 to 1960 Kt by 2025⁴¹. According to⁴², 2G-products are those produced from non-food materials, such as agricultural residues, wood, and energy crops typically high in lignocellulose, among others.

Many studies propose 2G lactic acid production as a viable scenario after pretreatment of agricultural biomass or lignocellulogic biomass to obtain fermentable sugar. These sugars derived from agricultural residues, wood, and energy crops are the 2G feedstock which have high amounts of lignocellulose. Although the major challenge is to obtain quality sugars of high concentration in a cost effective way. Considering this definition of 2G-products, some recent studies of 2G-LA production are presented in Table 3.

The viability of 2G-LA production broadly varies according to the combination of several factors, such as substrate, microbial strain, and operational conditions. In order to make the process economical, feasible and sustainable, it is important to consider the stable availability of the substrate over the year, as well as its transportation, storage, and (logistics) detailed planning of the operation.

After considering these factors, attention is focused on the utilization and availability of a cheap biomass in several region of the globe, which makes it possible to produce lactic acid, bio ethanol and other chemicals in a great diversity of places around the world.

Finally, some of the major challenges in LA production using 2G-substrates can be solved by new genetic engineering tools in development for LAB, such as substrate recalcitrance, carbon catabolite repression, byproduct formation, optical purity, LA and pH inhibition, sensibility to inhibitors released in the biomass pretreatment ^43,44.

Table 3: 2G Lactic acid production evaluated from lignocellulosic biomass

2G Substrate	Strain	˚C	Titer g/L	Productivity g/L/n	Yield	Isomer	Reference
Bagasse Sulfite pulp	B. coagulans CC-17	50	110.0	0.55	0.72	L	45
Brewers’ spent grains	L. rhamnosus ATCC 7469	37	48.0	0.96	0.87	L	46
Coffee pulp	B. Coagulans	52	-	4.02	0.78	L	47
Corn stover	L. pentosus FL0421	37	92.3	1.92	0.66		48
Corn stover	B. coagulans NBRC 12714	50	92.0	13.8	0.91	L	39
Corncob residue	B. coagulans H-1	50	68.0	-	0.85	L	49
	B. coagulans LA204	50	123.0	1.37	0.77		50
	B. coagulans IPE22	52	53.5	2.97	0.92	L	51
Dried distiller’s grains	L. Coryniformis torquens DSM 200004	37	38.1	0.80	0.35	D	52
Food waste	L. casei Shirota	37	94.0	2.61	0.94	-	53
Food waste	Streptococcus sp. A620	35	58.0	2.16	0.81	L	54
Jackfruit seed powder	Streptococcus sp. A620	37	109.0	-		L	55
Oil palm empty fruit bunch	B. coagulans J112	50	120.0	4.30	0.49	L	56
Orange peel	L. delbrueckii delbrueckii CECT 236	37	-	6.72	0.88	D	57
Orange peel	L casei 2246	37	209.6 g/kg	-	0.88	-	58
Paper sludge	L. rhamnosus ATCC 7469	37	108.2	-	0.62	-	59
S. flavescens residues and food waste	L casei CICC 6106	37	48.4	0.73	0.90	-	60

Genetic engineered microorganisms for 2G-lactic acid production:

Metabolic engineering of lactic acid bacteria also presents a novel approach for re-routing metabolic reactions to produce desired compounds in higher amounts, such as LA, flavor compounds, sweeteners, exopolysaccharide, vitamins, among others ^9,61. Genetic engineering of 2G-LA production has concentrated on improving the LA fermentation parameters, enhancing the acid tolerance of production organisms and their abilities to utilize a broad range of substrates, including fermentable 2G-sugars⁴⁴. Substrates incur for the major costs in the LA production chain⁴⁴, one of the reasons why genetic engineering is expected to enhance and be a tool enabling 2G-LA production from lignocellulosic biomass.

CONCLUSION:

Second generation lactic acid production still has to overcome a few barriers before becoming a world reality. However, the current scenario shows a promising prospective for 2G-lactic acid production to reach industrial-scale production, especially considering the bio refinery concept. The use of waste and / or lignocellulose material remaining from already consolidated industrial processes, especially in food and chemical production, opensthe possibility of expanding sustainable and renewable processes to produce 2G-LA. The above process provides an interesting scenario for replacing petrochemical derivates and for 2G-LA to be produced from already available material by not only a more economical method but by a method which has a high social impact due to the growing consciences that it is important to reduce the world petrochemical dependence and change the current production model to a more sustainable one.

CONFLICT OF INTEREST:

The authors declare no conflict of interest.

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Received on 18.10.2024 Revised on 14.02.2025

Accepted on 04.05.2025 Published on 01.10.2025

Available online from October 04, 2025

Research J. Pharmacy and Technology. 2025;18(10):5035-5040.

DOI: 10.52711/0974-360X.2025.00727

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