Tiny Nanofiber Sensors Turn Blue to Reveal When Cooking Oil Has Gone Bad

Every kitchen that relies on fried food faces the same invisible problem: cooking oil degrades as it is heated, and by the time the change is obvious to the eye or the nose, the oil may already be loaded with harmful oxidation products. A research team working across textile science and food chemistry in Egypt has now developed a strikingly simple answer to this problem, a thin mat of polymer nanofibers embedded with a dye that flips from red to blue as oil oxidizes. The work, published in npj Science of Food, describes how the researchers used a fast, low-cost fiber fabrication method called solution blowing to produce flexible sensors that can be suspended above a sample of soybean oil without ever touching it, and still report its state of deterioration within seconds.

The sensor is built from two components: polyacrylonitrile, a common polymer recovered in this study from textile industry waste, and Congo red, a dye well known for its sensitivity to changes in chemical environment. The team dissolved the polymer and the dye in dimethylformamide, added hydroxylamine hydrochloride to stabilize the formulation, and pushed the resulting solution through a fine nozzle using a high-speed stream of pressurized air. Unlike electrospinning, which requires high-voltage electric fields and produces fibers relatively slowly, solution blowing relies purely on aerodynamic force, making it cheaper, faster, and better suited to manufacturing the porous, flexible mats needed for practical sensing and smart food packaging.

What makes the study particularly interesting is that the researchers did not simply make one sensor. They made eight, systematically varying three parameters of the spinning process: the polymer concentration in the spinning solution, the air pressure used to stretch the jet, and the gauge of the needle through which the solution was extruded. Each combination produced a mat with a different average fiber diameter, ranging from roughly 231 nanometers in the finest mats to more than 400 nanometers in the coarsest. Scanning electron microscopy revealed how these processing choices translated directly into fiber morphology, and the team then asked a question with real practical weight: does the diameter of the fiber determine how well the sensor works?

The physics behind the diameter effect is straightforward but consequential. Finer fibers offer a higher surface area-to-volume ratio and a shorter diffusion pathway, so volatile oxidation products rising from heated oil can reach the embedded Congo red molecules more quickly and in greater numbers. The experiments bore this out. Mats produced with the smaller 21-gauge needle, which yielded thinner fibers, generally responded faster than their thicker counterparts. The fastest responses were recorded for the mats made from 12 percent polyacrylonitrile with the fine needle, while response times across all mats and all oxidation levels ranged from roughly 3 to 44 seconds. For a technology intended to sit in a bottle cap or a package lid and be read at a glance, that speed is a genuine advantage.

To validate the sensors, the team needed oil at known stages of degradation. They heated 500 milliliters of refined, bleached, and deodorized soybean oil in a shallow stainless-steel pan at 180 degrees Celsius for seven hours, drawing off samples every hour and freezing them for analysis. Standard chemical assays tracked the classic markers of lipid oxidation. The peroxide value, which measures primary oxidation products such as hydroperoxides, climbed steadily to 14.55 milliequivalents of oxygen per kilogram of oil after seven hours. The para-anisidine value, which captures secondary products such as aldehydes formed when hydroperoxides decompose, rose from 3.52 to 9.48. The acid value, an indicator of hydrolytic degradation and free fatty acid release, increased from 0.28 to 0.84 milligrams of potassium hydroxide per gram. Together these numbers charted a clear and progressive deterioration of the oil.

Against this chemical reference, the colorimetric response of the mats was measured using a portable colorimeter, quantifying the total color difference, known as delta E, between each mat before and after exposure to the oxidized oil. Every mat shifted visibly from red toward blue, confirming the sensing mechanism, but the magnitude of the shift varied widely, from a delta E of 10.97 for the weakest performer to 23.02 for the strongest. Intriguingly, the mats with the fastest response times were not always the ones with the largest color change. The team found that response speed and color-change magnitude represent distinct performance characteristics, and that the best formulation must balance both. When sensitivity and response stability were weighed together, the mat designated MAT VIII, produced with the 21-gauge needle, 15 percent polymer concentration, and 0.5 megapascals of air pressure, emerged as the most favorable.

Characterization by Fourier transform infrared spectroscopy and X-ray diffraction added depth to the story. The infrared spectra retained all the characteristic polyacrylonitrile absorption bands while showing new or intensified peaks associated with the sulfonate, carbon-nitrogen, and aromatic groups of Congo red, indicating that the dye was incorporated into the fiber matrix without chemically degrading either component. The team suggests the dye is held primarily by physical entrapment within the interconnected fiber network, possibly reinforced by non-covalent interactions such as hydrogen bonding. The X-ray analysis revealed that relative crystallinity varied dramatically among the mats, from just 1.2 percent to 46.8 percent, showing that spinning conditions shape not only fiber geometry but also the molecular ordering within each fiber, which in turn influences dye distribution and optical properties.

The broader context gives this work its urgency. Oxidized oils do not merely taste rancid; they lose nutritional value, lose sensory acceptance, and form toxic compounds associated with health problems in consumers. Yet the traditional methods for detecting oxidation, including titrations for peroxide and acid values, chromatography, mass spectrometry, and nuclear magnetic resonance, all demand laboratory equipment, chemical reagents, trained personnel, and time. A colorimetric mat that can be read with the naked eye, and that never needs to touch the oil it monitors, sidesteps nearly all of those barriers. The indirect-contact design is especially clever: a small sample of hot oil is sealed in a vial with the mat fixed in the cap, and volatile oxidation products do the rest of the work.

The researchers are candid about the limitations that remain before such sensors reach supermarket shelves or commercial kitchens. No quantitative dye-leaching study was performed, so the release of Congo red during prolonged contact with oxidized oil cannot yet be ruled out, and long-term storage stability and sensor shelf life were not systematically evaluated. The team recommends future studies of dye retention and sensor performance under controlled temperature and humidity, and further validation against established oxidation markers across a wider range of real food-use conditions. Congo red itself, a synthetic azo dye with a complicated regulatory history, may also prompt developers to consider alternative indicators in food-adjacent applications.

Even so, the study demonstrates a compelling principle: that the humble parameters of fiber fabrication, needle gauge, polymer concentration, and air pressure, can be tuned like the settings of an instrument to engineer a visual sensor with predictable speed and sensitivity. The fact that the polymer source was textile industry waste adds a sustainability angle that fits the growing field of smart packaging, where pH-sensitive pigments, anthocyanin extracts, and biodegradable films are already being explored for monitoring everything from shrimp freshness to olive oil quality. If the remaining validation hurdles can be cleared, a strip of red nanofibers in the cap of a frying-oil bottle could one day give every cook, professional or amateur, an instant and honest answer to the question of whether the oil is still fit to use.

Subject of Research: Colorimetric nanofiber sensors for detecting cooking oil oxidation

Article Title: Solution-blown PAN/Congo red nanofibrous sensors for colorimetric detection of soybean oil oxidation

Article References: Ibrahim, S. M., Hashim, A. F., El Said, A., & Hamouda, T. (2026). Solution-blown PAN/Congo red nanofibrous sensors for colorimetric detection of soybean oil oxidation. npj Science of Food, 10(1), Article 301. https://doi.org/10.1038/s41538-026-01157-z

Image Credits: AI Generated

DOI: 10.1038/s41538-026-01157-z

Keywords: nanofibers, solution blow spinning, Congo red, soybean oil, lipid oxidation, colorimetric sensor, food packaging, polyacrylonitrile, peroxide value, smart sensors, food safety, Solution-blown