Turmeric and Madder Pigments Turn Nanofiber Labels Into a Visual Pork Freshness Gauge

A team of researchers in China has built a color-changing nanofiber label that can tell shoppers and retailers, at a glance and without any laboratory equipment, whether the pork in a package is fresh, past its prime, or outright spoiled. The platform, described in the journal Food Chemistry: X, combines two natural plant-derived pigments, curcumin from turmeric and alizarin from the madder root, inside a water-resistant mat of electrospun polymer fibers. When the volatile amines that betray bacterial spoilage drift from aging meat, the fibers shift through a graded series of colors that map directly onto standardized freshness categories.

The motivation is straightforward. Pork is one of the world’s most consumed meats, and projections cited by the Organization for Economic Co-operation and Development suggest global consumption will reach 129 million metric tons by 2032, accounting for roughly a third of projected growth in total meat consumption. Yet pork’s high lipid and protein content, stored in the humid microclimate of a sealed package, makes it an ideal substrate for microbial spoilage. Consumers still judge freshness by color and odor, subjective cues that vary from person to person, while conventional laboratory tests for spoilage are costly, slow, and require trained personnel and elaborate sample preparation.

Smart labels offer a way around that bottleneck. As meat spoils, microbes and enzymes release volatile compounds that raise the pH inside the package. Freshness indicators exploit this chemistry by embedding pH-sensitive pigments that visibly change color as the atmosphere shifts. Synthetic pigments work well but raise health and environmental concerns, so researchers have turned to natural dyes. Curcumin, the polyphenol that gives turmeric its golden hue, is yellow under acidic conditions and turns orange-red when alkaline. Alizarin, an anthraquinone from madder roots, goes from light yellow in acid to bluish-purple in base. Used alone, however, each pigment responds over a narrow pH window with limited color contrast.

The research team, led by Bingcan Liu, Siling Zhang, and Zeyu Wu of Hefei University of Technology, solved both the pigment problem and a materials problem at once. They blended curcumin and alizarin at six different mass ratios, from pure curcumin to pure alizarin, and electrospun each mixture into nanofibers made of polyvinyl alcohol (PVA) blended with zein, a hydrophobic corn protein. Electrospinning draws a charged polymer jet from a Taylor cone under a 16-kilovolt electric field; as the jet whips and stretches toward the collector, it dries into solid fibers with enormous surface area and porosity, ideal for gas sensing. Crucially, the process involves no high temperatures, so the delicate pigments survive intact.

The polymer blend was the key to durability. Pure PVA films dissolve in humid conditions because of their abundant hydroxyl groups, which would be fatal for a label inside a moist meat package. By mixing in zein, rich in nonpolar amino acids, the researchers produced fibers averaging roughly 76 to 120 nanometers in diameter, far finer than the 347-nanometer fibers of pure PVA. Scanning electron microscopy showed smooth, continuous, well-fused fibers, evidence of good compatibility between the two polymers. Fourier transform infrared spectroscopy confirmed hydrogen bonding across the network, with characteristic zein amide bands appearing alongside PVA’s hydroxyl and carbon-oxygen stretches.

Water resistance improved dramatically. Pure PVA films showed 100 percent water solubility and a 100 percent swelling index, meaning they disintegrated completely in water. With zein and pigments added, water solubility fell to between 31 and 35 percent, moisture content dropped from 7.58 percent to as low as 4.5 percent, and the swelling index fell by about 35 percent. The hydrophobic protein shields PVA’s polar groups from water, while hydrogen bonds between the pigments and the matrix occupy binding sites that water molecules would otherwise attack. The films also held their color during ten days of storage at both 4 and 25 degrees Celsius, with total color differences remaining below 3, well under the threshold of 5 that untrained observers can perceive.

Sensitivity testing sealed squares of each film in a chamber with controlled vapors of ammonia, dimethylamine, and trimethylamine, the signature gases of meat decay, at concentrations from 1 to 40 parts per million. The mixed-pigment films outperformed single-pigment versions on both ends of the spectrum. Films containing both dyes detected ammonia and dimethylamine at just 1 part per million, five times more sensitive than the alizarin-only film, while mixed formulations responded to trimethylamine at 10 parts per million, better than curcumin alone. The three intermediate blends, with curcumin-to-alizarin ratios of 4:1, 3:2, and 2:3, showed the most pronounced and varied color changes, because the paired pigments broaden the pH response range and multiply the distinguishable hues.

The decisive test came with real pork. The team affixed a circular array of six indicator films inside the lids of glass containers holding about 60 grams each of chilled pork tenderloin, then refrigerated the samples at 4 degrees Celsius for six days. Daily measurements of total volatile basic nitrogen, the regulatory benchmark for meat spoilage, tracked the meat’s decline: below 10 milligrams per 100 grams on day one, between 10 and 15 on days two and three, and 15.54 on day four, crossing the Chinese national limit of 15. Lipid oxidation, measured by the thiobarbituric acid assay, rose from 0.16 to 0.52 milligrams per kilogram, and pH climbed from 5.96 to 6.58 as protein-degrading bacteria took over from glycogen fermenters.

The array’s color output mirrored this trajectory almost perfectly. The films drifted from light yellow through reddish-brown to purple as spoilage progressed, and a composite chromatic parameter calculated from all six films jumped sharply on day four, exactly when the meat crossed into the spoiled category. Pearson correlation coefficients between the color parameter and the three chemical spoilage markers were strikingly high: 0.98 for total volatile basic nitrogen, 0.98 for lipid oxidation, and 0.95 for pH. Single-pigment labels, by contrast, often lagged or produced changes too subtle to notice, which is precisely why the array design matters. By reading several films together, the system converts a fuzzy qualitative signal into a reliable three-tier grading of fresh, less fresh, and spoiled.

The researchers describe the platform as eco-friendly, low-cost, and scalable, and the ingredients support that claim: biodegradable PVA, an abundant food-grade corn protein, and non-toxic dyes already common in the food supply. Because the label works inside ordinary packaging and needs no instrument beyond the human eye, it could plausibly move from the laboratory to supermarket shelves and home refrigerators. The work also points toward broader applications, since the same dual-pigment, dual-polymer strategy could be tuned for fish, shrimp, beef, or any protein-rich food whose spoilage chemistry raises the local pH. For a food system that loses enormous value to spoilage and foodborne illness alike, a label that quietly turns from yellow to purple may prove one of the simplest and most consequential safety tools yet devised.

Subject of Research: A pH-sensitive curcumin-alizarin electrospun nanofiber array for colorimetric monitoring of pork freshness

Article Title: Synergistic PVA/zein electrospun nanofiber array with curcumin-alizarin: a water-resistant, sensitive colorimetric platform for graded pork freshness monitoring

Article References: Liu, B., Zhang, S., Geng, H., Hu, C., Zhou, A., & Wu, Z. (2026). Synergistic PVA/zein electrospun nanofiber array with curcumin-alizarin: a water-resistant, sensitive colorimetric platform for graded pork freshness monitoring. Food Chemistry: X, Article 104591. https://doi.org/10.1016/j.fochx.2026.104591

Image Credits: AI Generated

DOI: 10.1016/j.fochx.2026.104591

Keywords: electrospinning, nanofibers, curcumin, alizarin, smart packaging, pork freshness, food safety, pH indicator, volatile amines, zein, polyvinyl alcohol, colorimetric sensor