The study, published in the journal Results in Chemistry, was led by researchers at the Instituto Politécnico Nacional, working with material collected from a juice processing facility in Álamo, Veracruz, a state that produces nearly half of Mexico’s oranges. Their technique, hydrothermal carbonization, or HTC, subjects wet biomass to temperatures between 180 and 350 degrees Celsius under the pressure generated naturally by the sealed reactor. Unlike pyrolysis or gasification, HTC requires almost no energy to dry the feedstock, which makes it especially well suited to soggy agricultural residues like citrus peel. Water plays a double role during the process, acting simultaneously as solvent and catalyst, breaking apart polysaccharides, depolymerizing lignin, and stripping carbon dioxide from the molecular framework of the biomass.

The team dried and ground Valencia orange peels, pre-extracted them with ethanol, and then loaded ten grams of the prepared biomass into a one-liter stainless steel reactor with 100 milliliters of distilled water. After purging the vessel with nitrogen to create an inert atmosphere, they heated the mixture at a controlled rate of five degrees Celsius per minute to target temperatures ranging from 200 to 300 degrees Celsius, holding each run for 120 minutes under constant stirring. Autogenous pressures climbed from roughly 300 pounds per square inch at the low end to 1,300 at the highest temperature. The experiments were performed in duplicate, and every run yielded three products: a solid carbon-rich hydrochar, a gas composed mainly of carbon dioxide, and the prize the researchers were after, a liquid fraction they call light bio-oil.

Temperature proved decisive. Bio-oil yields climbed steadily from 8.9 percent of the dry feedstock mass at 200 degrees to a peak of 18.7 percent at 250 degrees, then fell back to 10.2 percent at 300 degrees. The pattern reflects a delicate chemical balancing act. At lower temperatures, hydrolysis of the lignocellulosic matrix remains limited, leaving cellulose and lignin largely intact. At 250 degrees, several productive reactions converge: polysaccharides break down into sugars that dehydrate into furan compounds, ether bonds within lignin cleave to release phenolic monomers such as guaiacol and vanillin derivatives, and cell walls rupture, liberating the flavonoids and phenolic acids native to citrus peel. Push the temperature higher, however, and secondary reactions take over. Reactive intermediates condense and repolymerize into heavy molecules that lock themselves into the hydrochar, while heat-sensitive flavonoids begin to degrade.

The resulting bio-oil, designated LBO-250, was a dark brown, viscous liquid with a density of 1.08 grams per milliliter, a pH of 4.2, and an unmistakable citrus aroma hinting at terpenes carried over from the peel’s essential oils. The acidity is typical of HTC products and stems from carboxylic acids formed during carbohydrate decomposition. To understand what the liquid actually contained, the researchers deployed a formidable analytical arsenal: scanning electron microscopy to track how the peel’s fibrous architecture collapsed during processing, ultraviolet-visible spectroscopy to detect the conjugated systems characteristic of flavonoids, thermogravimetric analysis to map the thermal stability of the solid residues, and infrared spectroscopy to identify the functional groups suspended in the oil.

The spectroscopic evidence converged on a consistent picture. Infrared spectra of LBO-250 showed a broad hydroxyl band near 3,400 wavenumbers, signatures of phenolic compounds, alongside carbonyl peaks at 1,736 wavenumbers pointing to carboxylic acids and esters, and a strong band at 1,636 wavenumbers indicating aromatic carbon-carbon double bonds. Nuclear magnetic resonance confirmed oxygenated carbons between 62 and 68 parts per million, aromatic and olefinic carbons between 120 and 155, and carboxyl carbons near 178, all hallmarks of lignocellulosic degradation products. Gas chromatography coupled with mass spectrometry then named the individual players: phenol, guaiacol, syringol, 4-ethylguaiacol, vanillin, guaiacylacetone, and cis-2-methoxycinnamic acid, a roster of well-documented chain-breaking antioxidants.

Quantitatively, the bio-oil contained 13.18 milligrams of gallic acid equivalents per gram in total phenolics, 71.54 milligrams of chlorogenic acid equivalents per gram of hydroxycinnamic acids, and 5.71 milligrams of quercetin equivalents per gram of flavonoids. Those numbers fall well short of Oligopin, a standardized commercial extract from French maritime pine bark, which delivered 443.25 milligrams of gallic acid equivalents per gram under the same assays. The gap is unsurprising. HTC operates well above the temperatures at which many plant polyphenols degrade, and orange peel contains only about 2 percent lignin, the principal renewable precursor of the aromatic phenols that HTC releases. The team’s earlier work on avocado peel, which is considerably richer in lignin, produced a bio-oil with far higher phenolic density under identical conditions.

The antioxidant testing revealed something more interesting than raw potency: radical selectivity. Against peroxyl radicals, the main chain-propagating species in lipid oxidation, LBO-250 achieved 25 percent inhibition at a concentration of 1,757.86 micrograms per milliliter, statistically significant but roughly five times weaker than Oligopin. Against hydroxyl and nitric oxide radicals, however, the bio-oil showed no measurable activity at all. The researchers attribute this not to a general weakness but to molecular architecture. Effective hydroxyl radical scavenging typically requires catechol or galloyl motifs capable of chelating transition metals, structures that were scarce among the methoxy-substituted phenols dominating the oil. Nitric oxide trapping, meanwhile, favors amino-bearing compounds or specific glycosylated flavonoids that were likewise absent. Peroxyl scavenging, by contrast, depends simply on hydrogen donation from phenolic hydroxyl groups, which the oil’s constituents supply readily.

That selectivity, the authors argue, defines rather than diminishes the product’s application space. Inhibiting lipid peroxidation is the dominant antioxidant function in food preservation, active packaging, and cosmetic formulations designed to protect against ultraviolet-induced oxidation, and a reproducible, radical-selective agent fits those niches well. Meanwhile, the co-produced hydrochar is far from waste: orange-peel-derived hydrochar has already been demonstrated as a precursor for activated carbons that capture carbon dioxide, and hydrochars generally serve as adsorbents, catalyst supports, and soil amendments. Continuous pilot-scale HTC of orange peel has shown the process can be scaled. The Mexican team, funded by national research programs, frames the work within the United Nations Sustainable Development Goals on energy, responsible production, and climate action, and calls for future optimization of temperature, pressure, and residence time to broaden the oil’s radical coverage. What began as a landfill problem may yet end up on the ingredient label of a cosmetic cream, or sealed inside a carbon filter, a small but tangible example of chemistry turning agricultural refuse into resource.

Subject of Research: Hydrothermal carbonization of orange peel waste to produce antioxidant bio-oil fractions

Article Title: Hydrothermal carbonization of orange peel waste: physicochemical characterization and antioxidant activity evaluation of bio-oil fractions

Article References: Rodríguez-Castillo, L. J., Méndez, F. J., García-Macedo, J. A., Calzada, L. A., Albíter, E., García-Pérez, M. E., Avalos-Viveros, M., de los Reyes, J., Santolalla-Vargas, C. E., Gómez, E., & Santes, V. (2026). Hydrothermal carbonization of orange peel waste: physicochemical characterization and antioxidant activity evaluation of bio-oil fractions. Results in Chemistry, 31, Article 103928. https://doi.org/10.1016/j.rechem.2026.103928

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103928

Keywords: hydrothermal carbonization, orange peel waste, bio-oil, antioxidants, phenolic compounds, flavonoids, hydrochar, biomass valorization, circular economy, green chemistry, Citrus sinensis, radical scavenging