Lipid oxidation is one of the chief enemies of food quality. When unsaturated fatty acids react with oxygen, they form hydroperoxides, aldehydes, and a cascade of secondary products that ruin flavor, erode nutritional value, and raise safety concerns. Two measurements dominate the monitoring of this decay: the peroxide value, which tracks the early stage of peroxidation, and malondialdehyde, or MDA, a hallmark of advanced oxidative chain-scission. Photooxidation, in which light energizes the process, is among the most damaging pathways, yet the precise role of heme proteins in accelerating it has remained surprisingly underexplored.

The research team, led by Mahdi Hajimohammadi of Kharazmi University in collaboration with colleagues in Iran and Iraq, set out to close that gap. Rather than isolating purified hemoglobin, they used intact erythrocyte suspensions derived from commercially supplied sheep blood, preserving the protein’s native structure and redox environment. This choice made the system biologically realistic: hemoglobin remained embedded in the matrix it normally occupies, at the boundary between an aqueous phase and the oil droplets of a commercial canola oil emulsion. The mixture was illuminated by a solar simulator, an array of 276 LED lamps spanning 380 to 780 nanometers and delivering an intensity of roughly 59,660 lux, closely matching the spectrum of natural sunlight.

The results were striking. After 2.5 hours of irradiation in the presence of air, the peroxide value of the oil climbed to 9.7 milliequivalents of oxygen per kilogram, while control experiments kept in the dark, lacking hemoglobin, or starved of oxygen showed only trace oxidation. The conclusion was unambiguous: hemoglobin, oxygen, and light must all be present simultaneously for the reaction to proceed efficiently. Under true sunlight, the peroxide value reached an even higher 10.5, confirming that the laboratory simulator faithfully reproduces the outdoor process. Spectroscopic monitoring of the heme’s characteristic Soret absorption band at 407 nanometers revealed a 69.8 percent decline over extended irradiation, evidence that the photosensitizer itself is progressively degraded as it works.

To dissect the mechanism, the researchers deployed chemical scavengers as molecular interrogators. Adding butylated hydroxytoluene, a potent free-radical quencher, cut the peroxide value to 3.8, implicating carbon-centered radicals generated by hydrogen abstraction. Adding sodium azide, a classic singlet oxygen scavenger, suppressed oxidation even further, to 3.1, demonstrating that singlet oxygen, an excited and highly reactive form of molecular oxygen, is a major player. Both pathways operate at once, which helps explain why hemoglobin outperformed two benchmark oxidants: the dye Rose Bengal, a selective singlet oxygen generator that achieved a peroxide value of only 6.8, and potassium permanganate, a classical chemical oxidant that managed just 2.1 under comparable conditions.

Proton nuclear magnetic resonance spectroscopy provided product-level confirmation of the damage. Signals from vinylic protons on the oil’s carbon-carbon double bonds, at chemical shifts of 5.2 to 5.4 parts per million, fell by 31.2 percent, consistent with singlet oxygen attacking double bonds through ene reactions. Bis-allylic protons, at 2.7 to 2.8 parts per million, declined even more sharply, by 36.9 percent, a signature of radical-mediated hydrogen abstraction at the most vulnerable positions of polyunsaturated fatty acids. A new aldehydic resonance near 9.5 parts per million announced the formation of MDA, the definitive marker of advanced peroxidation and chain scission. Perturbations in the glycerol proton region around 4.1 parts per million suggested that radical-driven beta-scission was even cleaving the ester bonds of the triglyceride backbone, fragmenting the oil into smaller, more polar molecules.

An independent probe sealed the case for singlet oxygen. Anthracene, a compound that reacts selectively with singlet oxygen to form a colorless endoperoxide, lost roughly 33 percent of its absorbance at 375 nanometers after four hours of irradiation in the hemoglobin-oil system, and sodium azide largely halted the bleaching. The solvent experiments added a subtle layer of physical chemistry: oxidation efficiency followed the order acetonitrile, ethanol, acetone, methanol, then dimethyl sulfoxide, mirroring the known lifetimes of singlet oxygen in these media, which range from about 65 microseconds in acetonitrile down to 19 microseconds in DMSO. Highly coordinating solvents like DMSO may also interfere with the formation of ferryl heme intermediates, the high-valent iron-oxo species suspected of driving the radical pathway.

That ferryl pathway remains the tentative half of the mechanism. The authors are careful to note that they did not directly observe the ferryl species, hemoglobin iron in the +4 oxidation state bound to oxygen. Its involvement is inferred from indirect evidence: the suppressive effect of the radical scavenger BHT, the preferential depletion of bis-allylic protons, and the damping effect of DMSO. Previous work on related metalloporphyrin and hemoglobin systems has detected ferryl intermediates spectroscopically, lending plausibility to the interpretation, but direct confirmation in this food system awaits future study. Even so, the combined evidence supports a dual oxidative engine: singlet oxygen generated by energy transfer from the light-excited heme, working in parallel with a heme-iron radical chemistry that abstracts hydrogen from the lipid chains.

The implications stretch well beyond the shelf life of bottled oil. Hemoglobin is abundant, biodegradable, and essentially free, recovered as a by-product of meat processing that would otherwise be discarded. The study’s preliminary results suggest that this waste protein could be harnessed as a low-cost photosensitizer for sunlight-driven degradation of lipid-rich effluents from edible oil processing, replacing ozonation and Fenton treatments that demand harsh chemical oxidants and energy-intensive conditions. The authors caution that real industrial wastewater, long-term protein stability under continuous illumination, light penetration, oxygen transfer, and reactor scale-up all remain to be tested. Still, the vision is compelling: a circular food system in which blood from the slaughterhouse becomes a solar-powered catalyst for cleaning the oil industry’s dirtiest water, while simultaneously teaching food scientists exactly how to keep light and heme apart to protect the products on our shelves.

Subject of Research: Hemoglobin-mediated sunlight-induced lipid oxidation in edible oils and its potential for sustainable wastewater treatment

Article Title: Sunlight‐Induced Lipid Oxidation and Degradation of Edible Oil Mediated by Hemoglobin: Implications for Food System Sustainability

Article References: Hajimohammadi, M., Faraj, F. H., Boghdachi, M., Alwasiti, A. A., & Shnain, Z. Y. (2026). Sunlight‐Induced Lipid Oxidation and Degradation of Edible Oil Mediated by Hemoglobin: Implications for Food System Sustainability. Food Science & Nutrition, 14(10), Article e72423. https://doi.org/10.1002/fsn3.72423

Image Credits: AI Generated

DOI: 10.1002/fsn3.72423

Keywords: hemoglobin, lipid oxidation, singlet oxygen, edible oil, photosensitizer, photooxidation, malondialdehyde, ferryl heme, food science, wastewater treatment, circular economy, sunlight

The scale of the problem is staggering. Global plastic production now exceeds 400 million tons per year, and less than 10 percent is recycled. Microplastics—defined as particles smaller than 5 millimeters, or down to the nanometer scale under the European Chemicals Agency’s broader definition—are generated in two ways. Primary microplastics are manufactured small, such as exfoliating beads or fibers shed from synthetic textiles, while secondary microplastics arise when larger items like bottles, mulch films, and packaging degrade in place. Photooxidation initiated by solar ultraviolet radiation, followed by mechanical breakup, is the dominant pathway producing secondary particles, and understanding it is essential to predicting where plastic pollution will accumulate and how it will harm ecosystems.

At the molecular level, the chemistry follows a well-established three-stage sequence. Chain initiation begins when a polymer absorbs ultraviolet photons, either through intrinsic chromophores—aromatic rings in polystyrene, polycarbonate, and polyethylene terephthalate absorb light readily—or through impurities, additives, and structural defects in polymers like polyethylene and polypropylene that lack natural light absorbers. In propagation, the resulting polymer radicals react with oxygen to form peroxyl radicals, which strip hydrogen atoms from neighboring chains and generate hydroperoxide groups. Because the oxygen-oxygen bond in these groups is weak—its dissociation energy is roughly 176 kilojoules per mole, far lower than adjacent bonds—it cleaves easily under ultraviolet light, spawning alkoxy and hydroxyl radicals that sustain an autocatalytic cycle of oxidation. Termination eventually occurs when radicals combine into inert products, but not before crosslinking, branching, and the formation of carbonyl groups have reshaped the material.

The consequences for particle integrity are profound. Chain scission cuts polymer backbones, reducing molecular weight, while oxidation introduces carbonyl, hydroxyl, and carboxyl groups that embrittle the surface. Long-term experiments show the effects clearly: polypropylene photooxidized for 48 weeks increased in crystallinity and became progressively more prone to cracking, while simulated sunlight studies found cracks on polyethylene and polypropylene surfaces within 60 days and reported polystyrene fragmenting most aggressively of all, releasing micron-scale particles from surface fissures after a year of irradiation. One study of polystyrene in seawater recorded particles shrinking from an average of 12 micrometers to 2 micrometers, accompanied by a drop in surface contact angle from 130 to 60 degrees—evidence of both physical breakdown and a chemical shift toward a more hydrophilic, oxidized surface.

Water, counterintuitively, modifies this process in both directions. Photooxidation proceeds more slowly in rivers and oceans than in air because water attenuates ultraviolet radiation, cools the reaction, and limits oxygen availability. Yet lower surface oxidation does not mean less fragmentation. When polyethylene films weathered in air, oxidation stiffened and bent the surface layer, suppressing crack formation; in water, however, molecules infiltrated oxidation-induced voids and coalesced them into macrocracks that propagated perpendicular to the extrusion direction, yielding elongated fragments. Aquatic aging also releases dissolved transformation products: polystyrene irradiated for 150 days shed oxidized, low-molecular-weight compounds that altered colloidal stability and mobility, meaning water can retard chemical oxidation while accelerating physical failure and chemical leakage simultaneously.

The surrounding chemical milieu adds further complexity. Dissolved ions such as chloride, bromide, carbonate, and nitrate can generate reactive species—including hydroxyl radicals, carbonate radicals, and reactive chlorine species—that attack particle surfaces. Dissolved organic matter plays a double role: at low concentrations, fulvic acid adsorbs onto microplastics and produces triplet excited states and reactive oxygen species that accelerate aging, but at higher concentrations humic and fulvic acids act as light filters and radical scavengers, significantly inhibiting photooxidation of polypropylene in lake water. Additives embedded during manufacturing further modulate the outcome. Plasticizers can migrate out and leave polymers brittle; ultraviolet stabilizers and antioxidants delay degradation, with additive-containing polypropylene retaining tensile strength for roughly 88 days versus 22 days for additive-free material—though after 176 days both formulations fragmented into on the order of one hundred million particles per square centimeter. Protection, in other words, postpones fragmentation but does not prevent it.

Measuring all of this demands a battery of techniques, and the review is emphatic that no single method suffices. Fourier-transform infrared spectroscopy remains the workhorse, tracking carbonyl, hydroxyl, and vinyl groups through indices normalized to reference carbon-hydrogen bands. But carbonyl-index calculations vary wildly across studies—researchers use different carbonyl and reference wavenumbers—making cross-study comparison unreliable and prompting the authors to recommend a standardized area-based method for polyethylene and polypropylene. X-ray photoelectron spectroscopy quantifies surface oxygen content; gel permeation chromatography documents chain scission; scanning electron microscopy visualizes cracks and debris; atomic force microscopy resolves nanoscale roughness and stiffness, and AFM-IR achieves chemical mapping at 50-to-100-nanometer resolution. Pyrolysis gas chromatography-mass spectrometry enables mass-based polymer identification, though the review warns that oxidative aging and natural organic matter can distort its signals, producing false positives or biased quantification in weathered environmental samples.

Location shapes everything. Synthesizing carbonyl-index data from 48 field studies across four environmental compartments, the review finds that urban microplastics—transported quickly through runoff and wastewater systems, often shaded—show modest oxidative aging, with median carbonyl indices around 0.17 to 0.19 for polyethylene and polypropylene. Agricultural soils present intermediate signals, where sunlight strikes exposed mulch films but burial by tillage shifts aging toward non-photochemical processes. Beaches and estuaries tell a stronger story: repeated wetting-drying cycles, prolonged sunlight, and sediment abrasion produce median carbonyl indices as high as 0.82 for polyethylene, roughly four times the urban median. Offshore surface waters yield the highest values for polypropylene, with a median of 0.79 and maxima exceeding 2, consistent with cumulative solar exposure during long oceanic transport—though the review cautions that net sampling preferentially collects larger floating particles, so these size patterns cannot be read as direct measures of fragmentation intensity.

The deeper message is that fragmentation is a coupled chemical-mechanical phenomenon, not a simple function of oxidation. In semi-crystalline polymers, degradation attacks amorphous regions between crystalline lamellae first; when enough tie molecules connecting crystalline domains are severed, lamellae separate as micro- and nanoplastic fragments. Crystalline morphology even dictates fragment shape—polystyrene’s spherulitic structure yields small, non-elongated pieces, while polyethylene cracks along predictable directions to form elongated shards. The authors argue that future research must standardize aging metrics, harmonize sampling and detection limits, distinguish accelerated laboratory exposures from environmentally realistic conditions, and integrate photooxidation, fragmentation, and ecotoxicological assessment. As aged microplastics display altered contaminant adsorption and biological responses compared with pristine particles, understanding how sunlight writes its chemical signature into plastic surfaces is becoming central to evaluating the true risk of the pollution already circling the globe.

Subject of Research: The mechanisms by which ultraviolet-induced photooxidation degrades and fragments microplastics across environmental compartments

Article Title: Impact of Photooxidation on the Fragmentation of Microplastics: A Review

Article References: Ji, W., Dhulia, A., Boufadel, M. C., Redman, A. D., Davis, C. W., Chen, H., & Galgani, F. (2026). Impact of Photooxidation on the Fragmentation of Microplastics: A Review. Environmental Challenges, Article 101650. https://doi.org/10.1016/j.envc.2026.101650

Image Credits: AI Generated

DOI: 10.1016/j.envc.2026.101650

Keywords: microplastics, photooxidation, ultraviolet degradation, carbonyl index, polyethylene, polypropylene, polystyrene, fragmentation, nanoplastics, plastic pollution, FTIR spectroscopy, environmental aging