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

A new study published in Food Chemistry: X offers a surprisingly elegant solution, and it comes from a part of the peanut that manufacturers routinely throw away. A research team led by Yuan Gao and Mingqing Wang of Shandong Academy of Agricultural Sciences turned its attention to the peanut embryonic axis, sometimes called the peanut heart. This tiny structure, which contains the hypocotyl, radicle, and plumule and represents only about 2.5 percent of the total kernel mass, is often removed during commercial processing because of its bitter taste. While peanut skins and hulls have attracted considerable attention as sources of valuable phenolics and proanthocyanidins, the embryonic axis has remained largely unexplored territory, despite earlier reports that it contains lower levels of the major allergen mRNAs Ara h 1 and Ara h 3 and higher concentrations of minerals such as magnesium, phosphorus, potassium, calcium, iron, and zinc compared with the cotyledon.

To understand what makes the embryonic axis chemically distinctive, the researchers conducted a comprehensive comparison between the axis and the cotyledon, the fleshy tissue that constitutes the bulk of the peanut kernel and the main raw material for peanut butter. Proximate analysis revealed meaningful compositional differences. The cotyledon contained 46.90 percent fat, whereas the embryonic axis held only 39.35 percent, a reduction of more than seven percentage points. The axis also carried more crude fiber, at 8.07 percent versus 6.31 percent in the cotyledon, and more carbohydrate, at 22.7 percent versus 16.4 percent. Protein content, by contrast, showed no significant difference between the two tissues. This profile of lower fat and higher fiber immediately suggested that the axis could serve as a functional ingredient capable of improving both the nutritional profile and the physical behavior of peanut butter.

The most striking findings emerged from untargeted metabolomics performed with ultra-performance liquid chromatography coupled to tandem mass spectrometry. Using a Thermo UHPLC-Q Exactive system operating in both positive and negative electrospray ionization modes, the team identified 419 metabolites that were significantly up-regulated and 201 that were down-regulated in the embryonic axis relative to the cotyledon. Among the plant secondary metabolites driving this separation, terpenoids dominated, accounting for more than half of the differential compounds, with triterpenoids forming the largest subgroup. Of the top 25 metabolites distinguishing the two tissues, 20 were triterpenoids, and all were markedly more abundant in the embryonic axis. These included compounds with well-documented biological activities, such as beta-elemonic acid, momordicinin, and maslinic acid, which have been associated with anti-cancer and anti-inflammatory effects, along with ginsenoside I, ginsenoside Rh7, and several soyasaponins known for antioxidant and antimicrobial properties.

Armed with this chemical portrait, the researchers formulated peanut butter in which roasted, peeled peanut kernels were partially replaced by embryonic axis at substitution levels of 2.5, 5, 7.5, and 10 percent by weight. The control spread was prepared from 100 percent roasted kernels with no added salt, sugar, oil, or stabilizers, ensuring that any observed effects could be attributed to the axis itself. Proximate analysis of the finished products showed that fat content declined progressively from 51.71 percent in the control to 49.23 percent at the 10 percent substitution level, while crude fiber rose significantly from 6.73 percent to 7.43 percent and carbohydrate climbed to 16.0 percent at the highest inclusion. Protein remained essentially unchanged across all formulations, hovering between 23.7 and 23.9 percent. Because lipid oxidation begins with the substrate itself, reducing the pool of oxidizable fat inherently slows the initiation and propagation of peroxidation reactions.

Texture and rheology measurements revealed that the embryonic axis did more than simply dilute the fat. Hardness increased from 0.79 newtons in the control to 0.94 newtons at 10 percent substitution, while adhesiveness rose from 0.96 to 1.51 newton-millimeters, and springiness, gumminess, and chewiness all climbed significantly as well. Rheological testing showed that all samples behaved as pseudoplastic, shear-thinning fluids, with apparent viscosity decreasing as shear rate increased from 0.1 to 100 reciprocal seconds. The flow curves fitted the power-law model with coefficients of determination between 0.989 and 0.996, and the consistency index rose from 37.29 pascal-seconds to the power of n in the control to roughly 50 at the two highest substitution levels. The researchers attribute this firmer, more cohesive structure to two mechanisms: the reduced free oil that would otherwise act as a lubricant, and the long molecular chains of dietary fiber, which intertwine into a three-dimensional network that binds oil and restricts its movement through the matrix.

The antioxidant evidence was equally compelling. Radical scavenging assays showed that spreads containing 7.5 and 10 percent embryonic axis displayed significantly higher activity against both DPPH and ABTS free radicals than the control, and the 10 percent formulation outperformed the 2.5 and 5 percent versions in the DPPH assay. Spearman correlation analysis confirmed that fiber content was strongly and positively associated with radical scavenging, with correlation coefficients of 0.903 for DPPH and 0.888 for ABTS. The enriched triterpenoid fraction likely contributed as well, since compounds such as maslinic acid, ginsenosides, and soyasaponins have demonstrated free radical neutralization and shelf-life extension in other food systems. Although roasting can partially degrade certain heat-labile saponins, the authors note that degradation is compound-specific and unlikely to have eliminated the overall antioxidant capacity of the enriched spreads.

Storage performance under accelerated conditions provided the decisive test. Samples packed in PET jars were held at 37 degrees Celsius for four weeks and monitored for oil separation and peroxide value. At the outset, all axis-fortified spreads showed markedly lower oil separation than the control, and a clear dose-dependent relationship emerged, with higher substitution levels corresponding to less oil loss over time. The researchers suggest that surface-active triterpenoid saponins may lower interfacial tension and form stable viscoelastic films that inhibit the coalescence and migration of oil droplets, while the denser fiber network physically entraps liquid oil. Peroxide values told a similar story: the control started at 5.25 millimoles per kilogram and climbed to 7.78 by week four, whereas the fortified spreads began between 3.28 and 4.77 and reached only 6.7 to 7.0 after the same period, demonstrating that the axis genuinely retarded lipid oxidation rather than merely masking its symptoms.

Crucially, none of these functional benefits came at the cost of appearance or palatability. Colorimetric analysis in the CIELab system showed no significant changes in brightness, redness, or yellowness across formulations, and the overall color difference relative to the control remained low, between 0.92 and 1.54, well within the range considered visually acceptable. A semi-trained sensory panel of twelve regular peanut butter consumers rated all spreads on a nine-point scale covering appearance, flavor, consistency, taste, spreadability, and overall acceptability. The 2.5 percent substitution earned the highest overall score of 8.0, compared with 7.4 for the control, and also received the best flavor rating at 7.8. Scores declined modestly at higher inclusion levels, likely because triterpenoid saponins impart a characteristic bitterness, but even the 10 percent formulation matched the control at 7.4, indicating that the axis never pushed the product beyond consumer tolerance.

The implications extend well beyond a single spread. The study demonstrates that a bitter, discarded fraction of the peanut, available essentially for free as a processing by-product, can simultaneously reduce fat, boost fiber, enhance antioxidant activity, suppress oil separation, and slow rancidity, all while preserving the clean-label, single-ingredient character that modern shoppers increasingly demand. The authors caution that their metabolomics results are semi-quantitative and based on limited biological replicates, so targeted absolute quantification will be needed to confirm the triterpenoid findings, and sensory testing was performed only at the initial time point rather than after storage. Future work may explore synergies between the embryonic axis and other natural antioxidants and extend the approach to other oil-rich food systems. For now, the humble peanut heart, once destined for the waste stream, has earned a place at the center of a smarter, more sustainable peanut butter.

Subject of Research: Use of peanut embryonic axis as a natural functional ingredient to improve the texture and oxidative stability of peanut butter

Article Title: Composition characterization of peanut embryonic axis and its application in peanut butter: Effects on texture and oxidative stability

Article References: Gao, Y., Fu, B., Wu, J., Yu, L., Bi, J., Song, Y., Wang, L., Jiang, C., & Wang, M. (2026). Composition characterization of peanut embryonic axis and its application in peanut butter: Effects on texture and oxidative stability. Food Chemistry: X, 39, Article 104532. https://doi.org/10.1016/j.fochx.2026.104532

Image Credits: AI Generated

DOI: 10.1016/j.fochx.2026.104532

Keywords: peanut butter, peanut embryonic axis, lipid oxidation, triterpenoids, dietary fiber, natural antioxidants, oxidative stability, oil separation, peroxide value, food by-product valorization, rheology, clean-label foods