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

The problem the researchers set out to solve is a familiar one. Seafood is densely packed with nutrients, and even when it is chilled or frozen, endogenous enzymes and microorganisms gradually break down proteins and other compounds, releasing volatile amines such as ammonia and biogenic amines. These gases are the key contributors to total volatile basic nitrogen, or TVB-N, the gold-standard chemical indicator of seafood freshness. Conventional laboratory methods for measuring TVB-N, including chemical-microbiological assays, chromatography, mass spectrometry, and spectroscopy, are accurate but slow, destructive to the sample, and dependent on expensive instrumentation and skilled technicians. They cannot tell a shopper whether the shrimp in the display case is fresh, and they cannot track freshness continuously inside a sealed package.

Fluorescent sensing tags have emerged as a promising alternative because they can be placed inside packaging and photographed with a smartphone. Early versions, however, relied on a single fluorescence signal that switched on or off, which made them vulnerable to environmental interference and difficult to judge by eye. The field responded with ratiometric probes, which carry two well-resolved emission bands so that one signal serves as an internal reference for the other. But many dual-emission probes built from dyes or quantum dots suffer from aggregation-caused quenching, in which the fluorescent signal collapses when the molecules cluster together on a solid surface. Aggregation-induced emission, or AIE, flips that behavior, glowing brighter when aggregated, and it has become a favored design principle for solid-state sensors.

The team, led by Wenyang Zhang with co-authors You Tian, Yanwu Chen, and Min Wei, built their tag around a relatively unexplored class of AIE materials: bimetallic nanoclusters. Using a microwave-assisted synthesis run at 90 degrees Celsius for just 150 seconds, they prepared silver-copper nanoclusters capped with D-penicillamine, abbreviated DPA-AgCuNCs. Transmission electron microscopy showed dense aggregates of particles averaging about 3 nanometers in diameter, and energy-dispersive X-ray mapping confirmed a homogeneous distribution of silver, copper, sulfur, carbon, and nitrogen. Spectroscopic characterization revealed a strong orange emission at 585 nanometers when excited at 360 nanometers, a large Stokes shift of 225 nanometers, and a long photoluminescence lifetime of about 4 microseconds, all hallmarks of a ligand-to-metal charge transfer from the sulfur atom of the capping ligand to the metal core.

Exposure to ammonia vapor quenched the nanocluster fluorescence in a linear fashion across concentrations from 10 to 120 parts per million, with a detection limit of 2.95 parts per million. Mechanistic experiments showed that the quenching is a synergistic combination of static and dynamic pathways. Ultraviolet-visible spectroscopy revealed two new absorption bands after ammonia exposure, evidence of a non-fluorescent ground-state complex, while fluorescence lifetime measurements showed the average lifetime plummeting from 4.023 microseconds to 3.27 nanoseconds, a signature of dynamic quenching. X-ray photoelectron spectroscopy showed the nitrogen content of the material rising after exposure and shifts in the copper oxidation-state features, and electron microscopy captured the physical disaggregation of the nanocluster aggregates. The researchers attribute the dominant pathway to static quenching through formation of an ammonia-coordinated copper-silver complex.

To convert this single-signal quenching into a visually readable ratiometric response, the team combined the orange-emitting nanoclusters with fluorescein isothiocyanate, or FITC, a dye that behaves in the opposite way. In the presence of the alkaline environment created by ammonia, the lactone ring of FITC opens to its carboxylate form and its green fluorescence at 510 nanometers brightens. Drop-casting the two probes together onto circular cellulose fiber paper produced a tag that glows orange-yellow under 365 nanometer ultraviolet light but shifts progressively toward green as amine concentrations rise. Scanning electron microscopy confirmed that both components, sheet-like FITC aggregates and petal-shaped nanocluster particles, coexisted on the paper fibers. After optimizing the volume ratio of nanoclusters to FITC at 4 to 0.5, the tag displayed a clean, graduated orange-to-yellow-to-green progression.

The practical test came with fresh sand shrimp stored at 4 degrees Celsius, with a tag affixed inside the lid of a sealed Petri dish where it never touched the sample. Independent measurements of TVB-N and total viable counts classified the shrimp as fresh at days zero to one, less fresh at days two to three, and spoiled at day four. The tag’s color changes tracked these grades precisely: bright orange when fresh, pale yellow when declining, and distinctly green when spoiled. Based on this behavior, the team built a fluorescence color card allowing anyone to grade freshness at a glance. The tag also proved rugged, retaining 94.47 percent of its initial fluorescence after 30 days at 4 degrees Celsius, more than 91 percent after a month across relative humidities from roughly 33 to 90 percent, and 95.47 percent after five days of continuous ultraviolet exposure.

Visual color cards, however, remain hostage to human subjectivity, since individual differences in color perception can produce inconsistent judgments. To eliminate that variability, the researchers trained five convolutional neural network architectures on a dataset of 1,842 labeled fluorescence images spanning the three freshness categories, augmented with flipping, rotation, scaling, cropping, translation, and Gaussian noise. ResNet-34 decisively outperformed the alternatives, achieving 98.39 percent accuracy, 98.48 percent precision, 98.41 percent recall, a 98.41 percent F1-score, and an area under the receiver operating characteristic curve of 99.02 percent on held-out data. VGGNet-16 and DenseNet managed only about 75 to 76 percent accuracy, GoogLeNet 53 percent, and AlexNet a dismal 36 percent. Stratified five-fold cross-validation confirmed ResNet-34’s robustness, with accuracy of 96.81 plus or minus 0.92 percent across folds, and on an independent test set of 122 unseen images the model achieved perfect precision for fresh and spoiled samples and 95.24 percent for the intermediate category.

The optimized network was then packaged into a proof-of-concept smartphone application called FreshSense, in which a user photographs the illuminated tag and the app displays an automatic freshness prediction, with inference currently handled on a remote server. The system generalized well beyond shrimp: applied to salmon, cod, and grass carp stored at 4 degrees Celsius, it achieved classification accuracies of 93.52, 90.74, and 92.59 percent, respectively, showing that the sensing chemistry is not tied to a single species. The authors note that future work will target model compression and on-device deployment so the tool can function fully offline, along with systematic evaluation of computational costs. Selectivity testing against thirteen volatile substances produced during spoilage, including methanol, hydrogen sulfide, formaldehyde, and acetic acid, showed strong responses only to ammonia, trimethylamine, putrescine, and a mixed-gas cocktail, precisely the compounds that matter for spoilage, suggesting the platform could become a practical, intelligent fixture in cold chains, retail displays, and eventually home refrigerators.

Subject of Research: A CNN-integrated ratiometric fluorescent nanocluster tag for rapid seafood freshness monitoring at refrigerated temperatures

Article Title: CNN-integrated ratiometric fluorescent tag for rapid monitoring of seafood freshness at refrigerated temperatures

Article References: Zhang, W., Tian, Y., Chen, Y., & Wei, M. (2026). CNN-integrated ratiometric fluorescent tag for rapid monitoring of seafood freshness at refrigerated temperatures. Current Research in Food Science, 13, Article 101560. https://doi.org/10.1016/j.crfs.2026.101560

Image Credits: AI Generated

DOI: 10.1016/j.crfs.2026.101560

Keywords: seafood freshness, ratiometric fluorescent tag, aggregation-induced emission, silver-copper nanoclusters, volatile amines, ammonia detection, convolutional neural network, ResNet-34, fluorescein isothiocyanate, TVB-N, smartphone sensing, refrigerated storage