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