A comprehensive new review published in Discover Green Chemistry argues that a quiet revolution is underway. Researchers led by Xinli Tang, Huayue Sun, and Jiankui Sun of North China University of Science and Technology systematically survey two decades of progress in chemically catalyzing the conversion of biomass, especially non-edible lignocellulosic material such as straw, wood, and agricultural waste, into lactic acid. Their analysis organizes the field into three competing routes: alkaline catalysis, acid catalysis, and an emerging family of photocatalytic and photothermal approaches that harness sunlight to drive the reaction at room temperature. Each route, the authors conclude, shares a common reaction network but differs in which step limits the overall rate, a unifying insight that could accelerate catalyst design across the entire field.

That shared network begins with sugars. Glucose, a six-carbon aldose, must first be isomerized into fructose, a transformation that Lewis acid sites catalyze through an intramolecular hydride shift known as the Lobry de Bruyn–van Ekenstein rearrangement. Fructose then undergoes retro-aldol cleavage, splitting into two three-carbon fragments, dihydroxyacetone and glyceraldehyde. These trioses dehydrate to pyruvaldehyde, which finally rearranges into lactic acid via a 1,2-hydride shift. Because fructose skips the isomerization step, it consistently outperforms glucose under identical conditions, while xylose, a five-carbon sugar, inevitably sacrifices part of its carbon skeleton to glycolic or formic acid, capping its lactic acid yield. The review reports yields exceeding 70 percent for glucose and up to 97 percent under optimized acid catalysis, but warns that raw lignocellulose typically delivers less than 50 percent because the recalcitrant lignin matrix blocks catalyst access and poisons active sites.

Alkaline catalysis, the oldest chemical route, exploits strong bases such as sodium and potassium hydroxide to cleave carbon-carbon bonds under hydrothermal conditions at or above 473 kelvin. Early work by Yan and colleagues showed that calcium and barium hydroxides form transition complexes with sugar intermediates, promoting selective C3–C4 bond cleavage, and that cellulose and starch could be converted directly to lactic acid in yields near 19 percent. More strikingly, Li’s group later achieved a 95.4 percent lactic acid yield from glucose at room temperature under anaerobic conditions, using barium hydroxide both as catalyst and as a reactant that traps the product as barium lactate. Yet the route carries a heavy price: high alkali concentrations generate salt waste, corrosion, and costly neutralization steps, and homogeneous bases cannot be recycled at all, making the economics unattractive for large-scale production.

Heterogeneous base catalysts attempt to resolve these problems. Layered double hydroxides of magnesium and aluminum, for example, enabled Albuquerque and colleagues to convert hydroxyacetone to lactic acid with 100 percent selectivity at just 40 degrees Celsius, using a recyclable solid base that eliminates neutralization entirely. Copper-based systems have proven particularly versatile: CuO supported on zirconia achieved complete glycerol conversion with 94.6 percent lactic acid selectivity, while copper oxide loaded on magnesia delivered a 70 percent yield from glucose at a relatively mild 393 kelvin. Glycerol itself, a cheap byproduct of biodiesel production, has emerged as a star feedstock, with noble-metal and copper catalysts converting it to lactic acid at yields of 80 to 96 percent, its simple C3 structure sidestepping the isomerization bottleneck that plagues six-carbon sugars.

Acid catalysis, however, is where the review places its strongest bet. Lewis acid zeolites, metal oxides, and rare-earth catalysts convert carbohydrates directly in water without the neutralization burden of alkaline chemistry. Tin-substituted beta zeolite, whose isolated tetrahedral Sn4+ sites act as water-tolerant Lewis acids, achieved a 67.1 percent lactic acid yield from glucose, while hierarchical zirconium zeolites reached 67.9 percent from xylose. Dealuminated ZSM-5 supported with erbium pushed yields to 69.1 percent by suppressing the formation of humins, the insoluble carbonaceous byproducts that plague sugar conversion. Rare-earth metals proved even more striking: erbium chloride delivered lactic acid from cellulose at yields up to 91 percent, and ytterbium chloride converted sugarcane bagasse to lactic acid within 15 minutes. Computational studies showed that heavier lanthanide ions lower the energy barrier for the critical C3–C4 bond cleavage, explaining their exceptional activity.

The most eye-catching numbers, though, come from the newest branch of the field: photocatalysis and its hybrid cousin, photothermal catalysis. Cao and colleagues developed a nitrogen-doped titanium dioxide catalyst that produced lactic acid from sugars with a 98.9 percent yield at just 60 degrees Celsius within 30 minutes under visible light. Huang’s team engineered a highly crystalline carbon nitride with structural oxygen that converted glucose at room temperature in 50 minutes, while Liu’s triazole-modified carbon nitride delivered yields of 85.5 to 98.3 percent from various sugars with 98.6 percent selectivity. Life cycle assessments attached to these systems are remarkable: the fluorine-doped carbon nitride route was calculated to generate only 0.7 kilograms of carbon dioxide equivalent per kilogram of lactic acid, roughly one-sixth of the petrochemical route, with an 87.8 percent reduction in fossil resource depletion.

Photocatalysis has historically been hobbled by poor selectivity. Mechanistic work by Zhang and colleagues revealed why: on pristine titanium dioxide, pyruvaldehyde preferentially follows low-barrier proton-coupled electron transfer pathways, producing unwanted C3 oxygenates, while the selective hydride shift to lactic acid faces a barrier of 1.22 electron volts. The solution proved elegant. By introducing oxygen vacancies that create Lewis acid sites and adding plasmonic gold nanoparticles that convert absorbed light into localized heat, the researchers steered the reaction toward the desired Cannizzaro-type pathway, achieving more than 90 percent lactic acid selectivity, a 3.4-fold improvement. Similar atomic-level heterojunctions, such as copper–sulfur moieties embedded in a cadmium zinc sulfide host, boosted glycerol conversion tenfold with selectivity above 95 percent, demonstrating that rational catalyst architecture can overcome the intrinsic kinetic limitations of light-driven chemistry.

Economics and durability remain the field’s stubborn obstacles. A landmark techno-economic assessment based on a 50,000-ton-per-annum plant suggested that erbium chloride-catalyzed glucose conversion could deliver an internal rate of return above 20 percent, but only if the expensive rare-earth catalyst is efficiently recovered and reused. Metal leaching from zeolites in hot water, carbonaceous fouling of oxide surfaces, and photocorrosion of semiconductors all erode catalyst lifetimes, and the review proposes a stability ranking that places zirconia and niobia at the top, followed by tin zeolites, carbon nitride photocatalysts, and layered double hydroxides. The authors argue that acid catalysis currently offers the best near-term balance of yield, feedstock flexibility, and practicality, while photocatalysis represents the most sustainable long-term option, pending breakthroughs in quantum efficiency and compatibility with real, untreated biomass.

What emerges from this sweeping analysis is a field in transition, moving from model sugars toward genuine waste streams, from precious metals toward abundant copper, zinc, and aluminum, and from brute-force heating toward sunlight-driven, carbon-negative chemistry. If researchers can marry the anti-leaching catalyst designs and standardized regeneration protocols the review calls for with the ambient-condition promise of photothermal systems, lactic acid could shift from a fermentation commodity to a cornerstone of the sustainable bioeconomy, and the biodegradable plastics built from it may finally compete with, and replace, the petrochemical polymers that now choke the planet.

Subject of Research: Chemo-catalytic conversion of biomass into lactic acid using alkaline, acid, and photocatalytic processes

Article Title: Research progress in the preparation of lactic acid from biomass by chemical catalytic process

Article References: Tang, X., Sun, H., Shi, Q., Zheng, D., Xie, J., & Sun, J. (2026). Research progress in the preparation of lactic acid from biomass by chemical catalytic process. Discover Green Chemistry, 1(1), Article 36. https://doi.org/10.1007/s44509-026-00038-8

Image Credits: AI Generated

DOI: 10.1007/s44509-026-00038-8

Keywords: lactic acid, biomass, catalysis, Lewis acid, photocatalysis, photothermal catalysis, polylactic acid, lignocellulose, zeolites, rare-earth catalysts, green chemistry, bioplastics

The appeal of waste apples as a fermentation feedstock lies in their biochemical makeup. Spoiled Jonagold apples supplied by Werder Frucht GmbH and prepared at ATB were pressed into a sugar-rich mash containing about 71.6 grams per liter of fructose, 21.5 grams per liter of glucose, and 7.5 grams per liter of sucrose, along with a high moisture content of 88.9 percent. Unlike apple pomace, the fibrous solid residue left after juice extraction that has dominated previous research, the mash retains readily fermentable simple sugars and requires minimal pretreatment. The solid fraction, composed mainly of peel and seeds, was excluded from the study. This compositional simplicity matters because lignocellulosic residues typically demand energy-intensive pretreatment to release their sugars and often generate inhibitory by-products that slow microbial growth.

Before fermentation could begin, the researchers needed to prepare the mash as a workable medium. They tested several commercial enzyme preparations, including Pectinase L40, Cellic CTec3 HS, Dextrozyme GA, and Viscoferm, at the mash’s natural pH of 4.5 and a temperature of 50 degrees Celsius. Surprisingly, none of the enzymes significantly increased the total reducing sugar content compared with an untreated control, a result the team attributes to the absence of starch in ripe apples and the low levels of cellulose and hemicellulose in the dry matter. However, Cellic CTec3 HS produced a marked reduction in viscosity, which is critical for fermentation because thick media impede mass transfer, create gradients of pH, temperature, and nutrients, and complicate downstream separation. The researchers ultimately selected a low-dose combination of Pectinase L40 and Cellic CTec3 HS, each at 0.5 milliliters of enzyme per kilogram of biomass, to liquefy the mash for subsequent microbial screening.

With a liquefied substrate in hand, the team screened microbial candidates for both target products. For lactic acid, five strains of Heyndrickxia coagulans, formerly known as Bacillus coagulans, were drawn from ATB’s internal collection of 700 isolates. Optical density measurements in apple mash medium, with and without yeast extract supplementation, identified strains A35, A138, and A203 as the strongest performers. For succinic acid, the researchers compared Actinobacillus succinogenes DSM 22257 against several strains of Basfia succiniproducens, confirming earlier reports that A. succinogenes converts both pentose and hexose sugars to succinic acid with superior yields. Subsequent bioreactor screening at 0.25 liters showed that all three H. coagulans strains produced lactic acid at similar concentrations, between 71.8 and 75.5 grams per liter, with yields of 0.91 to 0.94 grams per gram of sugar consumed. Strain A203, however, achieved the highest productivity at 3.5 grams per liter per hour and showed no lag phase, making it the clear choice for scale-up.

Nutrient availability emerged as a decisive factor in both fermentations. When yeast extract was omitted, lactic acid production by H. coagulans A203 collapsed to 32.3 grams per liter after 49 hours, with productivity falling to 0.7 grams per liter per hour. The effect was even more dramatic for A. succinogenes, whose succinic acid output dropped from 35.6 to 6.7 grams per liter without supplementation. These results indicate that apple mash, despite its abundant sugars, lacks sufficient nitrogen and growth factors to sustain industrial fermentation performance. The authors note that replacing yeast extract with cheaper agro-industrial nitrogen sources, such as wine lees or tomato pomace, represents a promising avenue for reducing production costs, since nutrient supply is one of the main economic burdens in biobased organic acid manufacturing.

The central innovation of the study lies in what the researchers removed from the process rather than what they added. Conventional bioprocesses typically separate the enzymatic hydrolysis and fermentation stages with centrifugation to remove solids and sterilization, usually at 121 degrees Celsius under pressure, to eliminate contaminants. Each of these steps adds capital cost, energy demand, and processing time. Instead, the team performed enzymatic liquefaction and fermentation sequentially in the same vessel. For lactic acid, no sterilization was needed at all because H. coagulans A203 is thermophilic, growing optimally at 50 degrees Celsius, a temperature that suppresses most contaminating microbes and shortens the fermentation window. For succinic acid, where A. succinogenes prefers a mesophilic 37 degrees Celsius, the researchers inserted a simplified thermal inactivation step, heating the mash to 80 to 85 degrees Celsius for 15 minutes, which denatures the proteins and disrupts the membranes of most spoilage organisms without the energy burden of full autoclaving.

After validating this simplified procedure at 1-liter laboratory scale, where it matched the performance of the conventional approach with no significant differences in concentration, yield, or productivity, the team moved to pilot scale. Lactic acid fermentation was carried out in a 30-liter working volume using 30 kilograms of apple mash in a Biostat UD bioreactor. The process delivered 73.8 grams per liter of lactic acid with a yield of 0.91 grams per gram of sugar consumed and a productivity of 2.7 grams per liter per hour. Remarkably, the final product exhibited an optical purity of 99.7 percent L-lactic acid, a strong indicator that contamination never took hold, since contaminating bacteria typically produce a mixture of L- and D-isomers. The overall product yield reached 0.78 grams of lactic acid per gram of apple mash on a dry basis, and the bacteria also consumed the malic acid naturally present in the fruit.

The succinic acid pilot run, conducted with 20 kilograms of apple mash in a 25-liter working volume, produced 36.8 grams per liter of succinic acid with a yield of 0.69 grams per gram and a productivity of 1.0 gram per liter per hour, alongside acetic and formic acids as natural by-products of A. succinogenes metabolism. Notably, the strain fixed carbon dioxide during succinate formation, an inherent environmental advantage of the process. These figures compare favorably with previous pilot-scale succinic acid fermentations using other food wastes: oat pomace with acid whey yielded only 19.6 grams per liter at 0.27 grams per liter per hour, while industrial candy waste reached 38.99 grams per liter. The lactic acid results similarly outperformed earlier work on apple pomace hydrolysate, which achieved just 40.72 grams per liter at a productivity of 0.58 grams per liter per hour, and mixed food waste, which produced 68.5 grams per liter at a yield of only 0.38 grams per gram of total solids.

Beyond the headline numbers, the study carries significant implications for how biorefineries handle agri-food waste. Retaining solids in the reactor eliminates an intermediate solid-liquid separation step, meaning only a single separation is needed after fermentation, when microbial biomass must be removed regardless of process configuration. Avoiding intermediate autoclaving also prevents heat-induced degradation of sugars and proteins, limiting the formation of unwanted by-products that complicate downstream purification, which is widely regarded as the economic bottleneck of large-scale bioproduction. Economically, waste apples serve as a zero- or low-cost feedstock that displaces commercial sugars and sidesteps the expensive pretreatment required for lignocellulosic biomass. Environmentally, the simplified process reduces energy consumption and carbon dioxide emissions, the high moisture content of the mash minimizes water input, and diverting spoiled fruit from landfill avoids methane generation.

The authors frame their work as a template for integrating agri-food residues into a circular bioeconomy, in line with the European Union’s Bioeconomy Strategy and Waste Framework Directive. They point toward future advances in intelligent biorefineries that use artificial intelligence and machine learning to adapt fermentation conditions to feedstock variability, as well as genetically engineered microbial strains designed to boost fermentation rates and product diversity. For now, the demonstration that a pilot-scale bioreactor can convert 30 kilograms of rotten apples into nearly 74 grams per liter of high-purity lactic acid marks a tangible step toward turning one of agriculture’s most familiar waste streams into a dependable industrial resource.

Subject of Research: Pilot-scale biotechnological production of lactic and succinic acids from waste apples

Article Title: Upcycling waste apples into platform chemicals: pilot-scale production of lactic and succinic acids

Article References: Portugal Rios da Costa Pereira, L., Schneider, R., Olszewska-Widdrat, A., Sturm, B., & Kaetzl, K. (2026). Upcycling waste apples into platform chemicals: pilot-scale production of lactic and succinic acids. Biotechnology for Biofuels and Bioproducts, 19(1), Article 67. https://doi.org/10.1186/s13068-026-02807-w

Image Credits: AI Generated

DOI: 10.1186/s13068-026-02807-w

Keywords: waste apples, lactic acid, succinic acid, fermentation, pilot scale, biorefinery, circular bioeconomy, Heyndrickxia coagulans, Actinobacillus succinogenes, agri-food waste, platform chemicals, bioplastics