Lactic acid, an organic acid produced by numerous microorganisms, has many applications in the food, chemical, pharmaceutical, and polymer industries, with novel applications being developed particularly in the chemical industry. The lactic acid market has steadily grown with the introduction of novel and environmentally friendly products. Therefore, developing novel technologies for lactic acid production, with improved yield and reduced production costs, has become a major research goal. Utilization of inexpensive and renewable biomass is a major strategy for economically producing lactic acid. However, most lactic acid bacteria cannot directly utilize lignocellulosic biomass and require hydrolysis, which is costly. Immobilization techniques for large-scale production can enhance fermentation yield. Moreover, novel techniques, such as cell recycling systems and simultaneous saccharification, will enable microorganisms to produce lactic acid with increased concentration, yield, and productivity. Recently, genetic and metabolic engineering methods have been used for key solving problems, such as product inhibition, by-product formation, and suboptimal culture conditions, and for the effective use of cheap substrates. This review presents comprehensive insights into the most recent advances in the biological production of lactic acid from different substrates, bioprocess techniques for yield improvement, lactic acid purification, and applications of lactic acid for human welfare.

Stevia rebaudiana (Bertoni) leaves consist of dietetically important diterpene steviol glycosides (SGs): stevioside (ST) and rebaudioside-A (Reb-A). ST and Reb-A are key sweetening compounds exhibiting a sweetening potential of 100 to 300 times more intense than that of table sucrose. Ultrasound-assisted extraction (UAE) of SGs was optimized by effective process optimization techniques, such as response surface methodology (RSM) and artificial neural network (ANN) modeling coupled with genetic algorithm (GA) as a function of ethanol concentration (X₁: 0–100%), sonication time (X₂: 10–54 min), and leaf–solvent ratio (X₃: 0.148–0.313 g·mL⁻¹). The maximum target responses were obtained at optimum UAE conditions of 75% (X₁), 43 min (X₂), and 0.28 g·mL⁻¹ (X₃). ANN-GA as a potential alternative indicated superiority to RSM. UAE as a green technology proved superior to conventional maceration extraction (CME) with reduced resource consumption. Moreover, UAE resulted in a higher total extract yield (TEY) and SGs including Reb-A and ST yields as compared to those that were obtained by CME with a marked reduction in resource consumption and CO₂ emission. The findings of the present study evidenced the significance of UAE as an ecofriendly extraction method for extracting SGs, and UAE scale-up could be employed for effectiveness on an industrial scale. These findings evidenced that the UAE is a high-efficiency extraction method with an improved statistical approach.