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High Lactic Acid Production: Advances and Strategies
Screening and Breeding of High-Yielding Strains
The development of high-yielding strains for lactic acid production has seen significant advancements. A notable example is the ZY mutant strain of Lactobacillus casei subsp. rhamnosus, which was developed through composite mutation treatments with UV and diethyl sulfate (DES). This strain achieved a lactic acid content of 93.9%, with a sugar-to-acid transformation ratio of 96.3%. Such high-yielding strains are crucial for optimizing industrial lactic acid production.
Microorganisms and Substrate Sources
Lactic acid can be produced by various microorganisms, including lactic acid bacteria (LAB) like Lactobacillus spp., which are known for their efficient fermentation capabilities. Other microorganisms such as Corynebacterium glutamicum and E. coli have shown high productivity, especially after genetic modifications. Fungi like Rhizopus spp. and yeasts also contribute to lactic acid production, with the latter being improved through genetic manipulation to tolerate harsh environmental conditions. Substrates for fermentation include plant waste, dairy waste, and lignocellulosic materials, which provide cost-effective and sustainable options for lactic acid production.
High-Rate Continuous Production
Innovative bioreactor designs have been developed to enhance lactic acid productivity. For instance, a two-stage membrane cell-recycle bioreactor (MCRB) using Lactobacillus rhamnosus achieved a productivity of 21.6 g/L/h and a lactic acid concentration of 92 g/L when connected in series. This setup demonstrates the potential for high-rate continuous production, which is essential for industrial applications.
Enological Applications
In winemaking, the yeast Lachancea thermotolerans has been identified for its high lactic acid production, which can counteract the effects of climate change on wine acidity and ethanol content. A specific strain, P-HO1, produced 10.4 g/L of lactic acid in mixed fermentations with S. cerevisiae, highlighting its potential for biological acidification and ethanol reduction in wines.
Metabolic Engineering and Adaptive Evolution
Metabolic engineering coupled with adaptive evolution has led to the development of strains capable of producing high-quality L-lactic acid at elevated temperatures. For example, a strain of Lactobacillus paracasei developed through CRISPR-Cas9 and adaptive evolution produced 221 g/L of L-lactic acid with a productivity of 7.5 g/L/h and an optical purity exceeding 99.1%. Such advancements are pivotal for commercial production of polymer-grade lactic acid.
Fermentation from Food Waste
Utilizing food waste and waste activated sludge for lactic acid production offers a sustainable approach. High-rate lactic acid production was achieved by controlling pH and fermentation temperature, with significant yields observed under mesophilic and thermophilic conditions. This method not only reduces waste but also provides a cost-effective substrate for lactic acid production.
Mixed Cultures and Synergistic Effects
The use of mixed cultures in fermentation processes can enhance lactic acid production. For instance, combining Kluyveromyces marxianus with Lactobacillus delbrueckii ssp. bulgaricus and Lactobacillus helveticus resulted in higher lactic acid concentrations compared to individual cultures, demonstrating synergistic effects. This approach can be applied in various food fermentations to achieve high lactic acid yields.
Physiological and Transcriptional Responses
Understanding the physiological and transcriptional responses of microorganisms to high lactic acid concentrations is crucial for optimizing production. Studies on Saccharomyces cerevisiae revealed that high lactic acid concentrations induce iron homeostasis responses and upregulation of specific genes, which are essential for maintaining cell viability and productivity under stress conditions.
Lactic Acid from Lignocellulosic Materials
Lignocellulosic materials, such as corn stover, are attractive substrates for lactic acid production due to their abundance and low cost. Lactobacillus pentosus strain FL0421 produced high yields of lactic acid from NaOH-pretreated corn stover using a fed-batch simultaneous saccharification and fermentation process, achieving a titer of 92.3 g/L. This process highlights the feasibility of using agricultural residues for sustainable lactic acid production.
Conclusion
The advancements in high lactic acid production encompass the development of high-yielding strains, innovative bioreactor designs, metabolic engineering, and the utilization of sustainable substrates. These strategies not only enhance productivity and yield but also contribute to cost-effective and environmentally friendly lactic acid production, paving the way for its broader industrial application.
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