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Lactic acid fermentation

Lactic acid fermentation is a biological process in which certain microorganisms, primarily lactic acid bacteria (LAB), convert carbohydrates such as glucose into lactic acid as the primary end product, thereby lowering the pH and preserving food while enhancing flavor and nutritional value.^[1] This anaerobic metabolic pathway occurs in the absence of oxygen and is fundamental to the production of fermented foods like yogurt, cheese, sauerkraut, and kimchi, where the accumulation of lactic acid inhibits spoilage organisms and pathogenic bacteria.^[1] The process typically achieves an acidity of 1.7–2.3% in products like sauerkraut after 20 days at 18°C, resulting in a pH range of 3.5–4.5 that ensures microbial stability.^[1] The key microorganisms involved in lactic acid fermentation belong to genera such as Lactobacillus, Leuconostoc, Pediococcus, and Streptococcus, which thrive in nutrient-rich environments like plant materials, milk, or meat.^[1] These bacteria initiate fermentation by hydrolyzing complex carbohydrates into simple sugars, followed by glycolysis to produce pyruvate, which is then reduced to lactic acid via lactate dehydrogenase, regenerating NAD⁺ for continued metabolism.^[2] The chemical reaction for homolactic fermentation, the most common type, is represented as C₆H₁₂O₆ → 2 CH₃CH(OH)COOH (glucose to two molecules of lactic acid).^[2] Lactic acid fermentation is classified into homolactic and heterolactic pathways based on the products formed.^[1] In homolactic fermentation, species like Lactobacillus plantarum produce almost exclusively lactic acid (over 90%), maximizing acid yield for preservation.^[1] Heterolactic fermentation, carried out by bacteria such as Leuconostoc mesenteroides, yields lactic acid along with carbon dioxide, ethanol, and other compounds via the phosphoketolase pathway, contributing to texture and flavor in products like sourdough bread or idli.^[1] Beyond food, the process has industrial significance; as of 2025, global lactic acid production stands at approximately 1.9 million tons, driven by applications in biodegradable plastics like polylactic acid (PLA).^[3] Historically rooted in ancient practices—such as the fermentation of pulque by the Aztecs or sourdough leavening—lactic acid fermentation remains essential for food safety and diversity, while modern challenges include optimizing yields from sustainable feedstocks like agricultural waste to reduce costs, which account for 40–70% of production expenses.^[1]^[2] Additional antimicrobial compounds produced by LAB, such as hydrogen peroxide, diacetyl, or bacteriocins like nisin, further enhance preservation without synthetic additives.^[1]

Overview

Definition and basic process

Lactic acid fermentation is an anaerobic metabolic process in which microorganisms, primarily lactic acid bacteria, convert carbohydrates such as glucose or lactose into lactic acid, generating energy in the form of ATP without the need for oxygen.^[4] This process serves as a form of anaerobic respiration, allowing cells to continue glycolysis when oxygen is unavailable, in contrast to aerobic metabolism, which fully oxidizes glucose through the citric acid cycle and oxidative phosphorylation to yield up to 32 ATP molecules per glucose, compared to only 2 ATP from anaerobic glycolysis.^[5] The basic chemical reaction for homolactic fermentation, the predominant pathway, can be represented as:

\text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2 \text{CH}_3\text{CH(OH)COOH}

This equation illustrates the conversion of one molecule of glucose into two molecules of lactic acid, with no net production of carbon dioxide.^[1] The process begins with glycolysis, where glucose is broken down into two molecules of pyruvate, producing 2 ATP and reducing NAD⁺ to NADH. Under anaerobic conditions, pyruvate is then reduced to lactate by the enzyme lactate dehydrogenase (LDH), which uses NADH as a cofactor to transfer electrons and regenerate NAD⁺, enabling the continuation of glycolysis. LDH exists in multiple isozymes and requires the cofactor NADH for its reversible reaction: pyruvate + NADH + H⁺ ⇌ lactate + NAD⁺, ensuring efficient energy production in oxygen-limited environments.^[5]^[4]

Significance in nature and industry

Lactic acid fermentation plays a crucial role in natural ecosystems by facilitating food preservation through the production of antimicrobial lactic acid, which lowers pH and inhibits the growth of spoilage and pathogenic microorganisms.^[6] This process occurs spontaneously in various environments, such as during the ensiling of forage crops where lactic acid bacteria (LAB) dominate the microbial community, rapidly acidifying the material to prevent aerobic deterioration and nutrient loss in agriculture.^[7] In animal physiology, lactic acid fermentation is integral to gut microbiomes, where LAB metabolize carbohydrates into lactate, supporting microbial balance, modulating immune responses, and aiding digestion, particularly in humans.^[8] Industrially, lactic acid production via fermentation has scaled to approximately 1.8 million tons annually as of 2024, driven by demand for its applications as a food preservative (designated E270 in the European Union) to extend shelf life and enhance flavor, and as a monomer for polylactic acid (PLA), a biodegradable bioplastic used in packaging and medical devices.^[9] This bio-based production leverages renewable feedstocks like corn or sugarcane, contrasting with synthetic alternatives. The process contributes significantly to the global fermented foods market, valued at over $578 billion in 2023, encompassing probiotics, beverages, and bio-based chemicals that bolster health and sustainable manufacturing sectors.^[10] Environmentally, fermentation-derived lactic acid offers a sustainable alternative to petrochemical routes, achieving up to 86% reductions in non-renewable energy use and 187% reductions in greenhouse gas emissions through closed-loop carbon cycles from biomass.^[11] This shift mitigates reliance on fossil fuels, lowering overall CO2 footprints in chemical production. Evolutionarily, lactic acid fermentation represents an ancient anaerobic metabolic pathway, predating the rise of oxygen-rich atmospheres over 2 billion years ago, enabling early life forms to generate energy in low-oxygen conditions and persisting as a foundational process in microbial and eukaryotic metabolism.^[12]

Biochemical Pathways

Homofermentative pathway

The homofermentative pathway in lactic acid fermentation represents the primary biochemical route by which certain lactic acid bacteria convert glucose almost exclusively into lactic acid under anaerobic conditions, utilizing the Embden-Meyerhof-Parnas (EMP) glycolytic pathway.^[13] This process achieves yields of 90-95% lactic acid from glucose, with the theoretical maximum being two moles of lactic acid per mole of glucose.^[14] Unlike the heterofermentative pathway, it produces no significant CO₂ or ethanol byproducts, emphasizing its efficiency for lactic acid production.^[13] The pathway begins with the phosphorylation of glucose to glucose-6-phosphate by hexokinase, consuming one ATP molecule.^[13] This is followed by isomerization to fructose-6-phosphate via phosphoglucose isomerase, and then phosphorylation to fructose-1,6-bisphosphate by phosphofructokinase (PFK), which commits the substrate to glycolysis and also consumes ATP.^[13] Cleavage by aldolase yields dihydroxyacetone phosphate and glyceraldehyde-3-phosphate; the former is isomerized to the latter by triose phosphate isomerase.^[13] Glyceraldehyde-3-phosphate is then oxidized to 1,3-bisphosphoglycerate by glyceraldehyde-3-phosphate dehydrogenase (GAPDH), generating NADH.^[13] Subsequent steps include transfer of phosphate to ADP by phosphoglycerate kinase to form 3-phosphoglycerate and ATP, mutase conversion to 2-phosphoglycerate, dehydration to phosphoenolpyruvate by enolase, and finally, pyruvate kinase-mediated transfer to ADP yielding pyruvate and ATP.^[13] Pyruvate is reduced to lactate by lactate dehydrogenase (LDH), regenerating NAD⁺ from NADH to sustain glycolysis.^[13] The overall balanced equation for the homofermentative pathway is:

\text{C}_6\text{H}_{12}\text{O}_6 + 2 \text{ADP} + 2 \text{P}_\text{i} \rightarrow 2 \text{CH}_3\text{CH(OH)COOH} + 2 \text{ATP} + 2 \text{H}_2\text{O}

This reaction yields a net gain of 2 ATP per glucose molecule through substrate-level phosphorylation, providing the energy for the bacteria.^[13] In homofermentative lactic acid bacteria, such as those in the genus Lactobacillus, the stereochemistry of the product is predominantly the L(+)-lactic acid isomer, determined by the specificity of the LDH enzyme.^[15] The pathway is tightly regulated to match cellular energy needs, with PFK serving as a key control point through allosteric inhibition by high ATP levels and activation by ADP, ensuring glycolytic flux responds to the ATP/ADP ratio.^[16] This regulation prevents overproduction of lactate when energy is abundant.^[16]

Heterofermentative pathway

The heterofermentative pathway, also known as the phosphoketolase pathway, is an alternative metabolic route used by certain lactic acid bacteria to ferment glucose, yielding approximately 50% lactic acid along with ethanol, carbon dioxide, and occasionally acetate as byproducts.^[17]^[18] This branched metabolism contrasts with more efficient linear pathways by diverting carbon flux through the pentose phosphate pathway, enabling the production of diverse end products essential for the bacteria's energy generation under anaerobic conditions.^[19] The pathway begins with the phosphorylation of glucose to glucose-6-phosphate, consuming one ATP molecule. Glucose-6-phosphate is then oxidized in the pentose phosphate pathway to 6-phosphogluconate, which undergoes decarboxylation to ribulose-5-phosphate, releasing one CO₂; ribulose-5-phosphate is isomerized to xylulose-5-phosphate. The key enzyme, xylulose-5-phosphate phosphoketolase, cleaves xylulose-5-phosphate into glyceraldehyde-3-phosphate (GAP) and acetyl phosphate. GAP is further metabolized through the lower branch of glycolysis: GAP is dehydrogenated to 1,3-bisphosphoglycerate (producing NADH), converted to 3-phosphoglycerate, then to phosphoenolpyruvate, and finally to pyruvate via pyruvate kinase (yielding one ATP); pyruvate is reduced to lactate by lactate dehydrogenase, regenerating NAD⁺. Meanwhile, acetyl phosphate is hydrolyzed to acetate (producing one ATP) or converted to acetyl-CoA, which is then reduced to acetaldehyde and ethanol via alcohol dehydrogenase, utilizing NADH.^[17]^[18]^[19] The overall balanced equation for the simplified heterofermentative fermentation of glucose is:

\mathrm{C_6H_{12}O_6 \rightarrow CH_3CH(OH)COOH + C_2H_5OH + CO_2}

This process results in a net energy yield of one ATP per glucose molecule, as two ATP are generated in the lower glycolytic steps from glyceraldehyde-3-phosphate to lactate, with no ATP from the acetyl branch when reduced to ethanol, but one is expended in initial phosphorylation.^[18]^[19] The lower efficiency compared to homofermentative routes supports survival in nutrient-limited environments but limits lactic acid yield.^[17] The byproducts play key roles in food applications: CO₂ contributes to leavening and texture in fermented products like sourdough and sauerkraut, while ethanol imparts flavor in beverages such as wine and certain dairy ferments.^[18]^[1] Heterofermentative bacteria, such as those in the genus Leuconostoc, primarily produce the D(-)-lactic acid isomer.^[20]

Bifidobacterium pathway

The Bifidobacterium pathway, also known as the bifid shunt or fructose-6-phosphate phosphoketolase pathway, represents a distinctive variant of lactic acid fermentation employed by Bifidobacterium species. This pathway diverges from typical glycolytic routes by utilizing phosphoketolase enzymes to cleave intermediates, enabling efficient ATP generation without the production of ethanol or carbon dioxide. It processes hexoses like glucose into a mixture of lactate and acetate, supporting the energy needs of these anaerobic, saccharolytic bacteria predominant in the human gut.^[21] The pathway begins with the uptake and phosphorylation of glucose to form glucose-6-phosphate, which is isomerized to fructose-6-phosphate. The key initial step involves fructose-6-phosphate phosphoketolase (F6PPK, EC 4.1.2.22), which splits fructose-6-phosphate into erythrose-4-phosphate and acetyl-phosphate in a thiamine diphosphate-dependent reaction. Acetyl-phosphate is then converted to acetate and ATP by acetate kinase (AckA, EC 2.7.2.1). The erythrose-4-phosphate is recycled through non-oxidative pentose phosphate pathway enzymes, including transaldolase and transketolase, which, in conjunction with additional fructose-6-phosphate, generate glyceraldehyde-3-phosphate. This intermediate proceeds via lower glycolysis to 3-phosphoglycerate, pyruvate, and ultimately lactate through lactate dehydrogenase. To balance the stoichiometry, the pathway is often described for two moles of glucose, but per mole of glucose, it yields 1.5 moles of acetate and 1 mole of lactate. A second phosphoketolase, xylulose-5-phosphate phosphoketolase (X5PPK or XP, EC 4.1.2.9), provides flexibility by cleaving xylulose-5-phosphate (derived from pentoses or hexoses) into glyceraldehyde-3-phosphate and acetyl-phosphate, enhancing adaptability to diverse carbohydrates.^[21]^[22]^[23] The overall reaction for the pathway can be approximated as:

\text{Glucose} + 2.5 \text{[ADP](/page/ADP)} + 2.5 \text{P}_\text{i} \rightarrow 1.5 \text{ [acetate](/page/Acetate)} + 1 \text{ [lactate](/page/Lactate)} + 2.5 \text{ATP}

This yields 2.5 moles of ATP per mole of glucose, surpassing the 2 moles from homofermentative lactic acid fermentation and the 1 mole from standard heterofermentative routes, thereby conferring a metabolic advantage in nutrient-limited environments like the gut.^[21]^[23] This pathway occurs primarily in Bifidobacterium species, such as B. longum and B. bifidum, which are adapted for fermenting complex oligosaccharides, including those from human milk, facilitating their dominance in the infant intestinal microbiome. The presence of both F6PPK and X5PPK enzymes underscores the pathway's uniqueness, as these thiamine diphosphate-dependent phosphoketolases are rare outside bifidobacteria and certain other actinobacteria, enabling specialized carbohydrate catabolism.^[21]^[22]

Microorganisms Involved

Lactic acid bacteria genera

Lactic acid bacteria (LAB) are a group of Gram-positive, non-spore-forming, catalase-negative bacteria primarily belonging to the phylum Firmicutes and the order Lactobacillales, which produce lactic acid as the major end product of carbohydrate fermentation.^[24] These bacteria are aerotolerant anaerobes, capable of growth in the presence or absence of oxygen, though they do not respire.^[25] They exhibit rod-shaped (bacilli) or spherical (cocci) morphologies and thrive optimally at pH levels between 4 and 6, while demonstrating tolerance to acidic environments down to pH 3.5.^[1] In 2020, a comprehensive phylogenomic analysis led to a major taxonomic reorganization of the Lactobacillus genus, which previously encompassed over 260 diverse species; this revision reclassified them into 23 novel genera alongside an emended Lactobacillus, resulting in more than 25 genera within the unified family Lactobacillaceae (merging former Lactobacillaceae and Leuconostocaceae).^[26] This restructuring was based on core genome phylogeny, average amino acid identity, and ecological adaptations, enhancing the understanding of their functional diversity.^[27] Key genera involved in lactic acid fermentation include Lactobacillus (now narrowed to 38 species, such as L. delbrueckii),^[26]^[28] Leuconostoc (e.g., L. mesenteroides), Pediococcus (e.g., P. acidilactici), and Streptococcus (e.g., S. thermophilus in the family Streptococcaceae).^[26] Post-reclassification, former Lactobacillus species were redistributed into genera like Limosilactobacillus (e.g., L. fermentum, heterofermentative), Lacticaseibacillus (e.g., L. casei, homofermentative), and Ligilactobacillus (e.g., L. salivarius).^[27] LAB genera exhibit species-specific fermentation modes, broadly categorized as homofermentative or heterofermentative. Homofermentative species, such as those in Pediococcus, Streptococcus thermophilus, and certain Lactobacillus subgroups (e.g., L. delbrueckii subsp. bulgaricus), convert glucose primarily to lactic acid via the Embden-Meyerhof-Parnas pathway, yielding approximately 90% lactic acid.^[29] In contrast, heterofermentative genera like Leuconostoc and some Lactobacillus species (e.g., L. brevis) utilize the phosphoketolase pathway, producing lactic acid along with ethanol, acetic acid, and CO₂ in roughly equimolar ratios.^[29] Genetically, LAB genomes often harbor plasmids that encode traits advantageous for fermentation environments, including bacteriocin production for competitive inhibition of rival microbes; for instance, pediocin PA-1 in Pediococcus acidilactici is plasmid-borne.^[30] Additionally, many LAB species possess CRISPR-Cas systems, which provide adaptive immunity against bacteriophages by incorporating viral DNA spacers, a feature highlighted in recent genomic surveys of Lactobacillus and related genera for enhanced phage resistance in industrial settings.^[31] These genetic elements contribute to the resilience and adaptability of LAB in diverse ecological niches.^[26]

Other contributing microbes

In mixed fermentations, yeasts such as Saccharomyces cerevisiae contribute by producing carbon dioxide, which complements the lactate generated by lactic acid bacteria to facilitate processes like dough leavening.^[32] These interactions occur naturally in environments like sourdough, where yeast metabolism supports overall fermentation dynamics without directly producing significant lactate.^[33] Among other bacteria, Acetobacter species play a transitional role in vinegar production, succeeding lactic acid bacteria by oxidizing residual ethanol to acetic acid after initial lactate accumulation lowers the pH.^[34] Bifidobacterium species, while utilizing a distinct fermentation pathway, contribute ecologically in microbial consortia by cross-feeding metabolites like acetate, which can influence lactic acid bacteria activity in gut or dairy environments.^[35] Fungi and molds, such as Rhizopus oryzae, occasionally participate in silage fermentation, producing limited amounts of lactic acid (up to 10 mg/g fresh matter) alongside lactic acid bacteria, though their overall contribution to lactate is minimal and often supplementary through substrate degradation.^[36] In consortia like sourdough, metabolic interactions between lactic acid bacteria and yeasts enhance acidification, as yeasts supply amino acids and vitamins that promote bacterial lactate production and pH reduction.^[37] Emerging research highlights probiotic strains such as Akkermansia muciniphila, which indirectly support lactic acid bacteria in the gut microbiome by co-modulating lactate-influenced pathways like tryptophan metabolism to 5-hydroxytryptamine.^[38] These microbes typically function in secondary capacities, aiding rather than dominating lactate production in lactic acid fermentation ecosystems dominated by lactic acid bacteria genera.^[39]

Historical Development

Early discoveries

Lactic acid fermentation has roots in ancient civilizations, where it played a crucial role in food preservation and nutrition. In the Middle East, the earliest archaeological evidence for the processing of milk, possibly including early forms of fermented dairy products like yogurt, dates to approximately 6400 BCE in the Fertile Crescent, based on lipid residue analysis of pottery vessels from sites like Tell Sabi Abyad in Upper Mesopotamia, indicating the use of goat, sheep, and cattle milk.^[40] Proteomic studies of pottery from Çatalhöyük in Central Anatolia further confirm milk proteins from dairy processing, suggesting possible fermentation, around 5900–5800 BCE, indicating widespread adoption during the Neolithic period.^[41] In China, while the earliest fermentation evidence from around 7000 BCE primarily involves alcoholic beverages from rice, honey, and fruit in Neolithic pottery, lactic acid fermentation in traditional foods, such as fermented vegetables and soy products, is documented from around the 1st millennium BCE onward, with applications to meats as part of preservation techniques.^[42]^[43] During the 17th and 18th centuries, early microscopic observations began to reveal the microbial basis of lactic acid fermentation. In 1674, Dutch microscopist Antonie van Leeuwenhoek examined sour milk and described "globules" and small moving particles, which he termed "animalcules," using his handmade lenses, marking one of the first sightings of microorganisms involved in milk souring. This observation preceded the chemical identification of the process; in 1780, Swedish chemist Carl Wilhelm Scheele isolated lactic acid from sour milk as an impure syrup, naming it after its milk origin and establishing it as a distinct organic acid produced during fermentation.^[44] The 19th century brought pivotal scientific advancements linking microbes directly to lactic acid fermentation. In 1857, Louis Pasteur conducted experiments on milk spoilage, demonstrating that specific microorganisms caused lactic fermentation, distinct from alcoholic fermentation by yeast, through microscopic examination and controlled cultures that produced lactic acid without alcohol.^[45] Building on this, British surgeon Joseph Lister achieved the first pure culture of the bacterium responsible in 1878, isolating Bacterium lactis (now classified as Lactococcus lactis) from milk and showing it uniquely induced lactic acid production, a foundational step in microbiology that emphasized sterile techniques.^[46] Prior to these discoveries, lactic acid fermentation served essential pre-scientific roles in food preservation across cultures, enabling storage without refrigeration by lowering pH and inhibiting pathogens through acid production, as seen in ancient dairy and vegetable ferments that extended shelf life in warm climates.^[1]

Modern research and applications

In the early 20th century, Danish microbiologist Søren Orla-Jensen established the foundational classification of lactic acid bacteria (LAB) in his 1919 monograph The Lactic Acid Bacteria, categorizing them based on cellular morphology, glucose fermentation modes, and other physiological traits, which laid the groundwork for subsequent taxonomic frameworks.^[47] This system emphasized the diversity within LAB genera, influencing industrial applications by enabling targeted strain selection for fermentation processes. Concurrently, the 1920s marked the scaling of industrial yogurt production, with commercial facilities in Europe and the United States adopting controlled LAB cultures to standardize lactic acid fermentation, transitioning from artisanal methods to large-scale manufacturing that boosted output and consistency.^[48] In the early 1900s, Élie Metchnikoff proposed that lactic acid bacteria in fermented milk contribute to longevity by modulating gut health, laying the foundation for probiotic research.^[49] Post-World War II advancements focused on enhancing LAB through bacteriocin production, exemplified by nisin, a lantibiotic isolated from Lactococcus lactis and first commercialized in 1953 as a natural food preservative to inhibit spoilage bacteria during fermentation.^[50] Following the isolation of nisin, mid-20th century research employed classical strain selection and mutagenesis to improve LAB production of antimicrobials, enhancing fermentation safety and yield without synthetic additives. By the 1980s, research identified exopolysaccharides (EPS) secreted by LAB like Streptococcus thermophilus and Lactobacillus delbrueckii, which contribute to improved texture and viscosity in fermented dairy products, prompting their exploitation as natural thickeners.^[51] The elucidation of the glycolytic pathway in the 1930s–1940s confirmed a net production of two ATP molecules per glucose via this route in lactic acid fermentation, clarifying energy efficiency limits and guiding strain optimization.^[52] Entering the 21st century, metagenomic approaches revolutionized understanding of LAB roles in human microbiomes, with the Human Microbiome Project launched in 2007 sequencing microbial communities and revealing LAB such as Lactobacillus species as key modulators of gut fermentation, immune responses, and pathogen resistance.^[53]^[8] This project highlighted LAB's contributions to lactate-mediated interspecies interactions, informing probiotic development. In the 2010s, CRISPR-Cas9 genome editing emerged as a transformative tool for LAB, enabling precise modifications like enhanced acid tolerance and bacteriocin expression in strains such as Lactobacillus reuteri, with early applications demonstrated in 2014 for recombineering and functional gene insertions.^[54] Taxonomic refinements continued, including the 2011 reclassification of species like Lactobacillus catenaformis into novel genera based on 16S rRNA phylogeny and chemotaxonomy, refining LAB diversity for targeted research.^[55] Recent 2020s research addresses sustainability challenges, with advancements in lactic acid production for polylactic acid (PLA) bioplastics using agricultural waste feedstocks fermented by engineered LAB, achieving higher yields and reducing reliance on petrochemicals through optimized strains like Lactobacillus delbrueckii.^[56]^[57] Parallel efforts develop climate-resilient LAB inoculants for silage fermentation, selecting thermotolerant strains from forage crops to maintain efficacy under elevated temperatures and variable moisture, enhancing feed preservation amid climate change.^[58] These innovations underscore LAB's evolving role in eco-friendly bioprocessing and agricultural resilience.

Applications

Food and beverage production

Lactic acid fermentation plays a central role in producing a wide array of fermented foods and beverages, where lactic acid bacteria (LAB) convert sugars into lactic acid, lowering pH to preserve products, enhance flavors, and improve textures.^[59] This process is essential in traditional and commercial production, relying on specific microbial consortia to achieve consistent quality and safety.^[60] In dairy production, yogurt is made by fermenting milk with a symbiotic culture of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus, which convert lactose to lactic acid over 4-6 hours at 40-45°C, resulting in coagulation and a tangy flavor.^[61]^[62] Cheese production involves initial lactic acid fermentation by LAB such as Lactococcus lactis to acidify milk and form curds, followed by secondary ripening with Propionibacterium freudenreichii in varieties like Swiss cheese, where it produces carbon dioxide for characteristic holes and propionic acid for nutty flavors.^[63]^[64] Vegetable fermentations, such as sauerkraut and kimchi, begin with Leuconostoc mesenteroides initiating the process in salted cabbage, followed by Lactobacillus plantarum dominating to produce lactic acid, dropping the pH to around 3.5 over several weeks at ambient temperatures, which preserves the product and develops sour, umami notes.^[65] Pickles are similarly produced by immersing cucumbers in a 5-8% brine, where naturally occurring or added LAB like Lactobacillus plantarum ferment sugars into lactic acid over 1-4 weeks at 18-24°C, creating a crisp, acidic preserve.^[66]^[67] Beverages like kefir result from a symbiotic fermentation of milk by LAB (Lactobacillus kefiranofaciens) and yeasts (Kluyveromyces marxianus), producing lactic acid, ethanol, and carbon dioxide over 24 hours at 20-25°C, yielding a probiotic-rich, effervescent drink.^[68] Sour beers, such as lambic, undergo mixed fermentation where LAB (Lactobacillus and Pediococcus spp.) alongside Brettanomyces yeast slowly acidify wort over 1-3 years in oak barrels, generating complex sour and fruity profiles through lactic acid accumulation.^[69]^[70] In meat and fish products, fermented sausages employ Pediococcus acidilactici and Lactobacillus sakei as starter cultures to rapidly lower pH via lactic acid production during 2-3 days at 20-25°C, enhancing flavor through volatile compounds and ensuring safety by inhibiting pathogens.^[71] Surströmming, a Swedish fermented herring, involves high-salt (10-15%) brining followed by LAB such as mesophilic lactobacilli and Tetragenococcus spp. driving lactic acid fermentation over 2-3 months at cool temperatures, resulting in a pungent, preserved product.^[72]^[73] Process controls in lactic acid fermentation include the use of defined starter cultures to standardize microbial activity, alongside monitoring temperature (typically 15-45°C depending on product), salt levels (2-10%), and pH (targeting 3.5-4.6) to promote LAB growth while preventing over-acidification or spoilage by unwanted microbes.^[74] Global variations highlight regional adaptations, such as African fermented porridges like ogi, where Lactobacillus plantarum and Leuconostoc mesenteroides spontaneously ferment soaked maize over 48-72 hours at 25-30°C, reducing anti-nutritional factors and yielding a sour, digestible weaning food.^[75] These practices underscore LAB genera like Lactobacillus and Leuconostoc as key players across diverse culinary traditions.^[59]

Industrial and pharmaceutical uses

Lactic acid is primarily produced industrially through microbial fermentation using lactic acid bacteria (LAB) such as Lactobacillus species or fungi like Rhizopus oryzae, which convert renewable carbohydrate sources into optically pure L-lactic acid with yields exceeding 95% optical purity.^[76]^[77] This high-purity lactic acid serves as the key monomer for polylactic acid (PLA) bioplastics, which are biodegradable alternatives to petroleum-based polymers used in packaging and medical devices.^[78] The global lactic acid market, driven by PLA demand, reached approximately 1,550 thousand tons in 2025, reflecting sustained growth from bio-based applications.^[79] Industrial fermentation processes often employ fed-batch strategies, where glucose or other sugars are incrementally added to maintain optimal substrate levels and avoid inhibition, achieving productivities up to 10 g/L/h.^[80] Downstream purification typically involves esterification of crude lactic acid with alcohols like ethanol to form lactates, followed by hydrolysis and distillation to remove impurities such as residual sugars and proteins, yielding food- or polymer-grade acid with over 99% purity.^[81]^[82] In the 2020s, sustainability efforts have shifted toward waste biomass substrates like agricultural residues and food waste, which reduce reliance on costly virgin sugars and lower overall production costs through integrated biorefinery approaches.^[56]^[83] These feedstocks, pretreated via hydrolysis, enable efficient fermentation while minimizing environmental impacts from waste disposal.^[84] Key challenges in large-scale production include contamination by unwanted microbes, which can reduce yields and necessitate stringent sterilization, and the separation of D- and L-isomers for specific applications, often requiring chiral chromatography or selective crystallization.^[85]^[86] In pharmaceuticals, lactic acid fermentation supports probiotic production, with strains like Lactobacillus rhamnosus GG demonstrating efficacy in alleviating irritable bowel syndrome (IBS) symptoms such as abdominal pain and bloating through gut barrier modulation.^[87] Additionally, bacteriocins like nisin, produced by Lactococcus lactis via lactic fermentation, act as natural antimicrobials and were granted GRAS status by the FDA in 1988 for use in processed foods and pharma formulations.^[88]^[89] Beyond these, lactic acid fermentation yields products for silage additives, where LAB inoculants enhance forage preservation by accelerating pH drop and inhibiting spoilage organisms in livestock feed.^[90] In cosmetics, fermented lactic acid is used in chemical peels at 30-50% concentrations to promote exfoliation and collagen synthesis for skin rejuvenation.^[91] For textiles, PLA derived from fermented lactic acid forms bio-based fibers via melt spinning, offering sustainable alternatives to synthetic yarns with comparable tensile strength.^[92]

Physiological Roles

In human microbiomes

Lactic acid fermentation is integral to the human gut microbiome, particularly during infancy when Bifidobacterium species rapidly colonize the intestine, often comprising 60-90% of the fecal microbiota in breastfed newborns within days of birth. These early colonizers utilize human milk oligosaccharides as substrates, fermenting them into lactic and acetic acids via the unique bifid shunt pathway, which supports microbial dominance and provides energy for the developing host.^[93]^[94] As infants transition to solid foods and adulthood, the gut microbiota diversifies significantly, with Bifidobacterium abundance declining to less than 10% while Lactobacillus species become more established, maintaining lower but consistent levels of lactic acid production amid a broader microbial community.^[95] In the adult gut, lactic acid bacteria (LAB) like Lactobacillus and residual Bifidobacterium perform key ecological functions through fermentation. They act as precursors for short-chain fatty acids (SCFAs) by producing lactate, which other microbes such as Faecalibacterium prausnitzii convert to butyrate, supporting epithelial integrity and immune modulation. LAB also lower luminal pH via lactic acid accumulation, inhibiting pathogens; for example, this acidification suppresses Clostridium difficile toxin production (tcdA and tcdB) and spore germination, reducing infection risk in dysbiotic states.^[96]^[97] Dysbiosis in conditions like inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS) is characterized by reduced LAB abundance, with Lactobacillus and Bifidobacterium populations often significantly decreased compared to healthy controls, correlating with elevated inflammation and impaired barrier function. Fecal microbiota transplantation (FMT) addresses this by restoring lactate producers; clinical trials from the 2010s, including randomized studies in ulcerative colitis patients, showed FMT increased Bifidobacterium and Lactobacillus engraftment, leading to remission rates of up to 40% and normalized lactate metabolism.^[98]^[99] Dietary interventions further modulate these communities, as prebiotics like inulin selectively boost Bifidobacterium fermentation in human trials, increasing fecal lactate concentrations and enhancing SCFA yields without altering overall microbial diversity.^[100] Lactic acid fermentation extends to other human microbiomes, notably the vaginal tract, where L. crispatus dominates in healthy individuals (up to 70% of the community), producing D-lactic acid to sustain a pH of 3.5-4.5 and prevent pathogen adhesion, thereby lowering risks of bacterial vaginosis and yeast infections. In the oral microbiome, LAB such as Lactobacillus species contribute similarly by acidifying plaque biofilms, modulating Streptococcus growth to maintain ecological balance. Emerging 2023 research positions microbial lactate as a key signaling molecule in the gut-brain axis, where it crosses the blood-brain barrier to activate neuronal receptors like HCAR1, influencing mood regulation and neuroinflammation in preclinical models.^[101]^[102]^[103]

Impact on muscle metabolism

During high-intensity exercise, skeletal muscle relies on anaerobic glycolysis to produce ATP rapidly when oxygen supply is insufficient, leading to the accumulation of lactate as a byproduct of pyruvate reduction. This process, often termed lactic acid fermentation in muscle, occurs under conditions of oxygen debt, where glucose is broken down to pyruvate via glycolysis, and then pyruvate is converted to lactate to regenerate NAD+ for continued glycolytic flux.^[104] The reaction is catalyzed by lactate dehydrogenase (LDH), specifically the M4 isozyme predominant in skeletal muscle, which favors the pyruvate-to-lactate direction under anaerobic conditions:

\text{Pyruvate} + \text{NADH} + \text{H}^+ \xrightarrow{\text{LDH-M4}} \text{Lactate} + \text{NAD}^+

This mechanism allows muscle fibers, particularly fast-twitch types, to sustain short bursts of power output.^[105] Blood lactate levels typically remain low (1-2 mmol/L) at rest or during moderate aerobic exercise but rise sharply beyond the lactate threshold, around 4 mmol/L, marking the point where production exceeds clearance. In maximal sprints or intense efforts, concentrations can peak above 20 mmol/L, reflecting the scale of anaerobic metabolism.^[106] Contrary to longstanding myths, accumulated lactate does not primarily cause muscle cramps or fatigue; instead, research from the 2020s emphasizes ionic imbalances, such as potassium efflux and calcium dysregulation, as key contributors to these sensations. Lactate actually buffers fatigue by facilitating NAD+ regeneration, enabling prolonged glycolysis, though associated acidosis from H+ ions can indirectly impair contractility.^[107]^[108] Post-exercise, lactate clearance primarily occurs via the Cori cycle, where the liver takes up circulating lactate, converts it back to pyruvate, and then to glucose through gluconeogenesis for release into the bloodstream. This recycled glucose supports muscle recovery, serving as a major source of energy during the initial recovery phase.^[109] Endurance training adaptations, such as increased mitochondrial density and oxidative enzyme activity in skeletal muscle, enhance aerobic capacity, thereby reducing reliance on lactate production during submaximal efforts and improving overall lactate tolerance.^[106]

Health benefits and risks

Lactic acid fermentation contributes to health benefits primarily through probiotics derived from lactic acid bacteria, which have been shown to reduce the risk of antibiotic-associated diarrhea by approximately 50% in meta-analyses of clinical trials.^[110] These probiotics also promote immunomodulation by inducing the production of anti-inflammatory cytokines such as interleukin-10 (IL-10), which helps regulate immune responses and mitigate inflammation in the gut.^[111] For individuals with lactose intolerance, affecting about 65% of the global adult population, fermented dairy products like yogurt offer a viable alternative as fermentation reduces lactose content significantly—typically to 4-5 grams per 125-gram serving in plain yogurt compared to approximately 6 grams in an equivalent 125 ml serving of milk—allowing better tolerance without full digestion impairment.^[112]^[113] Emerging evidence highlights additional protective effects, including anti-carcinogenic properties where lactate from lactic acid bacteria inhibits tumor cell proliferation and enhances anti-tumor immunity by boosting CD8+ T cell stemness in preclinical models.^[114] In cardiovascular health, certain lactic acid bacteria strains, such as Lactobacillus plantarum, lower plasma levels of trimethylamine N-oxide (TMAO), a gut-derived metabolite linked to atherosclerosis, thereby potentially reducing cardiovascular risk in high-risk patients.^[115] Post-2020 research has also uncovered links to mental health, with gut-derived lactate promoting the conversion of tryptophan to serotonin, which may alleviate depressive symptoms via the microbiota-gut-brain axis.^[38] Despite these benefits, risks exist, particularly D-lactic acidosis in patients with short bowel syndrome, where excessive heterofermentative lactic acid bacteria ferment undigested carbohydrates, leading to elevated D-lactate levels and neurological symptoms like encephalopathy.^[116]^[117] Additionally, rare infections from lactic acid bacteria, such as bacteremia or endocarditis, can occur in immunocompromised individuals, though these are infrequent and often linked to underlying conditions rather than routine probiotic use.^[118] Guidelines from the World Gastroenterology Organisation, building on FAO/WHO expert consultations, emphasize that probiotics must be strain-specific, safe, and administered in adequate amounts to confer benefits, with viable cell counts verified through clinical evidence.^[119] Recommended daily intake for general health support is typically 10^9 colony-forming units (CFU) or more, though higher doses up to 5 × 10^9 CFU may be advised for specific conditions like diarrhea prevention.^[120]

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