Native Fruit and Vegetable Microbes Produce Lactic Acid Without Added Inoculum

A continuous bioprocess using the native bacterial populations naturally inhabiting fruit and vegetable residues can generate industrial concentrations of lactic acid without requiring thermal pretreatment, added commercial enzymes, or pure microbial starter cultures. In a laboratory investigation evaluating sustainable chemical synthesis from organic residues, an anaerobic continuous stirred tank reactor operated at twenty-five degrees Celsius and regulated to a persistent acidic level of pH 4.5 achieved a steady lactic acid accumulation of 33.2 grams per liter. This output corresponded to a bioconversion yield of 48.3 percent of the total incoming chemical oxygen demand and represented 87.7 percent of the total detected liquid metabolite pool. The research demonstrates that selecting appropriate operating parameters enables wild, unsterilized microbial consortia to redirect biological carbon flow selectively toward a single platform chemical, circumventing the complex sterility protocols and expensive synthetic nutrient media that typically constrain industrial fermentation schemes.

Lactic acid serves as a high-demand commodity across the chemical, food, pharmaceutical, and textile sectors, functioning as a primary monomeric precursor for bioplastics such as polylactic acid and polyhydroxyalkanoates. Commercial production has historically depended on pure cultures of lactic acid bacteria inoculated into refined sugar or starch feedstocks, including sugarcane, corn, and cassava. These traditional workflows utilize separate hydrolysis and fermentation or simultaneous saccharification and fermentation under strictly axenic conditions to prevent contamination by competing bacterial strains. However, relying on food-grade crops introduces ethical and economic conflicts over arable land and human nutrition. While non-food lignocellulosic biomass has been investigated as an alternative, its recalcitrant structural matrix of cellulose, hemicellulose, and lignin demands intensive thermochemical and enzymatic treatments to liberate fermentable monosaccharides, driving up processing complexity and operational energy budgets.

To overcome these limitations, organic residues from agricultural operations, municipal processing facilities, and retail food markets have attracted focus as sustainable carbon resources. Agroindustrial wastes, particularly surplus vegetables and fruits, possess elevated moisture content and abundant concentrations of readily biodegradable carbohydrates, making them suitable candidates for circular bioeconomic valorization. Conventional anaerobic treatment of these residues typically relies on mixed cultures inoculated with anaerobic sludge from wastewater treatment facilities. In those traditional digestions, primary intermediates such as lactic acid are rapidly consumed by secondary acidogenic and methanogenic microorganisms to yield short-chain fatty acids, carbon dioxide, methane, and hydrogen gas. The experimental design established in this study was devised to test whether the indigenous microbiota naturally populating vegetable skins could be enriched under continuous cultivation to arrest downstream metabolic degradation, maximizing lactic acid accumulation while suppressing secondary conversion pathways.

The experimental feedstock comprised agricultural market surpluses gathered in Spain, consisting of 25 percent cucumber, 25 percent zucchini, and 50 percent melon by wet weight. These commodities represent major horticultural outputs in the region and generate substantial waste volumes characterized by rapid biodegradability. The raw materials were homogenized with a mechanical blender without chemical pretreatment, external nutrient additions, or autoclaving. The homogenized substrate was loaded into a five-liter continuous stirred tank reactor with a three-liter liquid working volume and two liters of headspace. Agitation was sustained mechanically, while a water bath maintained the internal liquid temperature at twenty-five degrees Celsius. The digestion broth was held at pH 4.5 through automated dosing of three-molar sodium hydroxide. The reactor began with a three-day batch incubation period designed to promote the growth of epiphytic bacteria present on the waste surfaces, after which it shifted to continuous feeding at an organic loading rate of 3 grams of volatile solids per liter per day, corresponding to a hydraulic retention time of twenty days.

Process performance was tracked through comprehensive analytical profiling across three hydraulic retention times to establish steady-state operations. Effluent samples collected twice weekly were monitored for total and soluble chemical oxygen demand, total solids, volatile solids, ammonium nitrogen, and individual fermentation metabolites via high-performance liquid chromatography. Carbohydrate quantification of raw influent identified substantial pools of glucose and fructose, alongside structural polysaccharides and residual proteins. At steady state, the system maintained a lactic acid production efficiency of 0.75 grams of lactic acid per gram of soluble sugars available in the feedstock. Volatile solids removal reached 36.6 percent, reflecting robust organic solubilization and carbohydrate breakdown. Acidification efficiency reached 68.2 percent, while overall chemical oxygen demand removal remained low at 13.8 percent. Gaseous emissions of methane and hydrogen remained beneath analytical detection thresholds, confirming that carbon mass remained preserved within the liquid effluent rather than being lost to off-gas production.

Detailed stereochemical evaluation of the accumulated lactic acid revealed an enantiomeric distribution of 61.3 percent D-lactic acid and 38.7 percent L-lactic acid. While pure L-lactic acid is typically favored for specific food and pharmaceutical applications, mixed chiral pools represent versatile intermediate feedstocks for synthesizing specialized polylactate polymers and formulating microbial growth substrates for polyhydroxyalkanoate biopolymer production and microbial chain elongation, where enantiomeric purity is noncritical. Crucially, avoiding sewage-derived anaerobic sludge eliminates significant biosafety and regulatory hurdles. Conventional municipal sludge carries risks of heavy metal contamination, fecal pathogens, and residual endotoxins. By restricting the microbial inoculum solely to the epiphytic flora of fresh agricultural surpluses, the resulting bioproduct aligns closely with European regulatory standards governing food additives such as E270 under Regulation EC 1333/2008, food contact hygiene under Regulation EC 852/2004, cosmetic ingredient safety under Regulation EC 1223/2009, and packaging materials governed by Regulations EC 1935/2004 and 2023/2006.

Microbial community succession was characterized by sequencing the hypervariable V3 and V4 regions of 16S ribosomal RNA genes extracted after the three-day batch phase and again once continuous steady-state conditions were verified. Sequencing revealed an initial bacterial community comprising 151 operational taxonomic units with a Shannon diversity index of 2.736. This initial consortium was dominated by the phyla Firmicutes at 68.7 percent and Proteobacteria at 29.1 percent. Genus-level taxonomy showed that the epiphytic flora on the fresh vegetable surfaces was predominantly composed of Pediococcus at 45.8 percent, alongside members of the family Enterobacteriaceae at 21.7 percent, Leuconostoc at 13.2 percent, and Erwinia at 6.7 percent. These pioneer taxa reflect the typical ecology of fresh produce surfaces, where fast-growing Proteobacteria assist in the primary softening and breakdown of plant cellular tissues, releasing enclosed mono- and disaccharides that support the subsequent expansion of fermentative lactic acid bacteria.

Upon reaching continuous steady-state conditions, the operating environment of pH 4.5 and twenty-five degrees Celsius drove a major phylogenetic restructuring. Total richness remained consistent at 160 operational taxonomic units, but the Shannon diversity index declined to 1.747, indicating the development of a highly specialized and functionally focused microbiome. Firmicutes expanded to represent 93.0 percent of total relative sequence abundance, whereas Proteobacteria contracted sharply to 6.4 percent. Within Firmicutes, the family Lactobacillaceae accounted for 92.7 percent of the phylum, with the genus Lactobacillus alone comprising 36.3 percent of the total microbial community. Enterobacteriaceae and other acid-sensitive taxa were largely suppressed by the persistent acidity, effectively halting the emergence of short-chain fatty acid pathways and eliminating methane-producing archaea that typically consume organic acids in conventional anaerobic digestion systems.

Metabolic pathway modeling based on metabolite yields demonstrated that the enriched consortium operated predominantly through the homofermentative Embden-Meyerhof-Parnas glycolytic pathway. Homofermentative lactic acid bacteria convert hexose sugars into two moles of lactic acid per mole of glucose or fructose, whereas obligate heterofermentative bacteria utilize the phosphoketolase pathway, yielding an equimolar blend of lactic acid, ethanol, and acetic acid alongside carbon dioxide. In the steady-state reactor, the molar ratio of lactic acid at 368.6 millimoles per liter to secondary by-products—consisting of 2.5 grams per liter of ethanol and 2.1 grams per liter of acetic acid totaling 89.3 millimoles per liter—stood at 4.1 to 1. Biochemical stoichiometric analysis indicated that roughly 76 percent of the total lactic acid was synthesized via the Embden-Meyerhof-Parnas pathway, while the phosphoketolase pathway contributed approximately 24 percent.

The persistence of secondary metabolites was partly driven by the presence of Acetobacter, which accounted for 5.8 percent of sequence abundance at steady state. Acetobacter species tolerate low-pH environments and frequently establish mutualistic cross-feeding relationships with lactic acid bacteria. In these mixed systems, Acetobacter can oxidize a portion of the microbially produced ethanol into acetic acid, slightly modulating the acid balance. Concurrently, the controlled pH of 4.5 remained safely above the lactic acid dissociation constant of 3.86. At this level, the concentration of undissociated organic acid remained low enough to avert severe acid-stress responses in the bacterial cells, preventing the metabolic shift toward full solventogenesis that typically triggers excess ethanol production under extreme acidification.

The study also recorded an ammonium nitrogen effluent concentration of only 0.17 grams of nitrogen per liter. Low ammonium generation confirms that proteolytic degradation of residual plant proteins was substantially restricted under the mildly acidic operating regime. Because lactic acid synthesis relies on carbohydrate fermentation, suppressing proteolytic deamination channels cellular energy directly into sugar breakdown, preventing the contamination of downstream liquors with complex nitrogenous degradation products. Comparisons with previous digestion trials highlight the critical synergy between low operating pH and the selective advantage of indigenous flora. Earlier trials applying conventional anaerobic sewage sludge to food residues yielded lower lactic acid concentrations of 10 to 22.6 grams per liter, accompanied by fluctuating yields and competing microbial guilds, whereas running vegetable self-fermentation at neutral pH values resulted almost exclusively in short-chain fatty acids rather than lactic acid.

Overall, the experimental outcomes establish that unsterilized agricultural residues harbor robust microbial consortia capable of autonomous specialization under continuous ambient-temperature processing. By stabilizing conditions at pH 4.5 and twenty-five degrees Celsius without adding synthetic inocula or chemical hydrolysates, the system matched the volumetric production metrics of conventional pure-culture systems while consuming less energy and eliminating sterile containment requirements. The process demonstrates a mechanically straightforward and ecologically sustainable pathway for transforming perishable agricultural surpluses into purified organic platform chemicals, providing a practical framework for industrial-scale waste biorefineries operating within circular economic models.

Subject of Research: Valorization of agroindustrial residues into lactic acid via open-mixed fermentative culture

Article Title: Open-mixed fermentative culture as a feasible strategy to valorize agroindustrial residues into lactic acid

Article References: Martorell-Múgica, A., Sánchez-Mata, D., González-Fernández, C., & Greses, S. (2026). Open-mixed fermentative culture as a feasible strategy to valorize agroindustrial residues into lactic acid. AMB Express, 16(1), Article 89. https://doi.org/10.1186/s13568-026-02081-x

Image Credits: AI Generated

DOI: 10.1186/s13568-026-02081-x

Keywords: lactic acid, agroindustrial waste, open-mixed culture, anaerobic fermentation, indigenous microbiota, Lactobacillaceae, Lactobacillus, bioreactor, waste valorization, fermentative acidification, chiral distribution, circular bioeconomy, Embden-Meyerhof-Parnas pathway, Pediococcus, biochemical conversion, continuous stirred tank reactor