Green lactic acid production using low-cost renewable sources and potential applications

Abstract Lactic acid is one of the most commonly used organic acids in various industries due to its versatile applications. To meet the market demand its production by various methods, such as chemically and microbiologically, is gaining significant interest. Chemically synthesized lactic acid (hydrolysis of acetonitrile) is generally a racemic mixture of D- and L-lactic acid which further needs purification for use in pharma and food industries. On the other hand, lactic acid produced by microbial fermentation is in an optically pure form of D- or L-lactic acid and does not require any further purification. Microbes used for lactic acid fermentation, such as bacteria, fungi, yeast, and algae, have the potential to use lignocellulosic biomass, organic agrowaste, kitchen waste, dairy, and petroleum industry waste as a feedstock. Therefore recycling of different organic waste for the production of lactic acid not only resolves the issues of waste management but may also help in reducing the production cost of lactic acid. These microorganisms ferment both pentose and hexose sugars to the desired product by two different pathways, i.e., homolactic and heterolactic fermentation pathways. In the course of these pathways, other than lactic acid there are a few other economically important platform chemicals (acetic acid, ethanol, butanol, etc.) also produced. Different microbes play a specific role at different stages of organic waste recycling to lactic acid. Briefly, lignocellulolytic and different glucoamylases-producing microbes (fungi and bacteria) are the main players in the initial steps of pretreatment and saccharification of biomass. After saccharification, when monomeric or dimeric sugar is released from the biomass, the fermentative lactic acid bacteria converts these sugars to the isomeric form of lactic acid. The obtained lactic acid is purified further by various mechanisms and the pure lactic acid then can be used in pharmaceutical, cosmetic, textile, and food industries.