Lactic acid fermentation using Rhizopus spp.: current insights and future prospects

Lactic acid (LA) is a versatile organic acid widely used in food, chemical, pharmaceutical, and cosmetics industries. Its demand has significantly increased due to its role in producing biodegradable and biocompatible polylactic acid (PLA) polymers. Fungal species from the Rhizopus genus offer several advantages over bacteria when producing lactic acid through fermentation of renewable substrates, including amylolytic capabilities, minimal nutrient requirements, and valuable fungal biomass as a by-product. This review highlights recent advancements in the metabolic and enzymatic pathways, fermentation substrates, modes, and methods utilized in LA production by Rhizopus species. It explores critical bioprocess parameters such as nutrient composition, pH, and fungal morphology, which are examined for their roles in optimizing production. Furthermore, developments in high cell-density fermentation and improved downstream processes for lactic acid recovery and purification are discussed. The challenges and opportunities for scaling up LA production from various substrates are critically analyzed, along with future strategies for improving fungal fermentation systems. Finally, the techno-economic feasibility of fungal-based LA production is also discussed.