Plasma-Tuned Chitosan Films Turn Color to Reveal When Fish Has Spoiled

A package of fish sits in the refrigerator, and the film lining the lid quietly shifts from golden yellow to a deep orange-brown. That color change, triggered by nothing more than the volatile amines drifting off deteriorating flesh, is the central achievement of a new study published in Food Chemistry: X. Researchers in China have built a biodegradable, pH-responsive indicator film from chitosan loaded with curcumin and resveratrol nanoemulsions, then refined its performance with cold plasma treatment. The result is a packaging material that both preserves seafood and tells consumers, at a glance, whether tilapia fillets are still safe to eat.

The problem the team set out to solve is a familiar one in the seafood industry. Aquatic products spoil rapidly during storage and transit, driven by microbial growth and endogenous biochemical metabolism that degrade quality and create safety risks. Traditional monitoring approaches rely on destructive sampling and laboratory analysis, which cannot provide real-time information to retailers or shoppers. Meanwhile, conventional plastic packaging raises environmental concerns. Chitosan, a biopolymer derived from shellfish waste that is fully biodegradable and films well, has long been considered a promising replacement, and previous studies have shown chitosan coatings can extend the refrigerated shelf life of fish by roughly two days. But chitosan alone lacks the ability to signal spoilage, and its inherent antibacterial and antioxidant activity is too weak for prolonged preservation of highly perishable products.

The researchers’ solution pairs two natural polyphenols with complementary roles. Curcumin, the yellow pigment from turmeric, is a classic pH-sensitive colorant: its molecule contains a beta-diketone group that undergoes keto-enol tautomerism, adopting a ketone form that appears bright yellow under acidic or neutral conditions and converting to an enol form in alkaline environments. Resveratrol, rich in hydroxyl groups, contributes dual antioxidant and antibacterial activity by scavenging free radicals and suppressing pathogenic bacterial growth. Together they promise synergistic freshness indication and preservation. Both compounds, however, suffer from poor water solubility, low stability, and a tendency to aggregate, which would ruin their dispersion in a film matrix and blunt their color responsiveness if simply mixed in.

To overcome those limitations, the team turned to nanoemulsion technology. Zein, a corn protein, was dissolved with curcumin in aqueous ethanol and self-assembled into nanoparticles by dropwise addition into water; resveratrol was then injected into the dispersion, and sodium alginate was blended in to stabilize the particles. The mixture was combined with soybean oil and processed through a shear homogenizer followed by three cycles of high-pressure homogenization at 100 megapascals. The resulting nanoemulsions were remarkably uniform: an average droplet diameter of 200.2 nanometers, a polydispersity index of just 0.185, and a zeta potential of minus 44.1 millivolts, the negative charge from the alginate coating generating electrostatic repulsion that keeps the droplets from clumping. Encapsulation efficiencies reached 78.43 percent for curcumin and 71.2 percent for resveratrol, confirming that non-covalent interactions within the zein-alginate matrix effectively held both polyphenols. The nanoemulsion platform also shielded the compounds from ultraviolet degradation and promoted their uniform dispersion throughout the chitosan film.

The films themselves were cast from a chitosan solution blended with the nanoemulsions, then subjected to dielectric barrier discharge cold plasma in ambient air at three voltages: 10, 30, and 50 kilovolts. Cold plasma, an ionized gas operating near room temperature, modifies material surfaces through etching and the incorporation of polar functional groups without altering the bulk structure. Spectroscopic and diffraction analyses revealed what the treatment accomplished. X-ray diffraction showed that blending the nanoemulsions into chitosan weakened the polymer’s characteristic semicrystalline peak through hydrogen bond competition and steric hindrance, while plasma treatment broadened the peaks further and reduced crystallinity by inducing polymer chain rearrangement. Fourier transform infrared spectroscopy confirmed intermolecular hydrogen bonding between the nanoemulsion components and the chitosan matrix, with shifts in the hydroxyl, amide, and methylene bands that strengthened after plasma exposure, evidence of a tighter molecular network.

The 30-kilovolt treatment emerged as the sweet spot. Films modified at this voltage achieved the best barrier performance, with water vapor permeability falling from 7.21 to 4.87 times ten to the minus six grams per meter per day per pascal, and oxygen permeability dropping from 2.50 to 1.37 grams per meter per day per atmosphere. Water contact angle peaked at 78.7 degrees, moisture content, water solubility, and swelling all declined, and mechanical strength improved through plasma-induced surface cross-linking. The mechanism is a balance of effects: at moderate voltages, reactive oxygen and nitrogen species consume hydrophilic surface groups while mild etching compacts the matrix, but at 50 kilovolts the flood of high-energy particles causes over-etching, micropore formation, and polymer backbone scission that loosens the surface and degrades performance. Antioxidant activity, by contrast, rose with voltage, as plasma-induced etching exposed more reactive sites and accelerated the release of phenolic compounds from the matrix.

Release behavior followed a characteristic biphasic pattern in fatty food simulants: a rapid initial stage over the first 24 hours as ethanol dissolved the hydrophobic polyphenols and swelled the polymer network, followed by a sustained slow stage from 24 to 72 hours. Plasma voltage tuned this release in a dose-dependent manner, with low-voltage treatment accelerating diffusion through surface etching and medium-to-high voltages restraining it through enhanced cross-linking and stronger hydrogen bonding. More striking was the colorimetric performance. When immersed in buffers spanning pH 2 to 12, the films displayed bright golden yellow under acidic conditions and shifted to deep orange-brown at pH 10, a transition clearly visible to the naked eye. Exposure to ammonia and acetic acid vapors reproduced the same pattern over time, and the response was sharpest in the 30-kilovolt films, whose optimized surface porosity and active group distribution facilitated the permeation of gaseous molecules. Crucially, the color change was reversible when pH was reversed, confirming that the mechanism rests on reversible protonation and deprotonation of curcumin rather than irreversible chemical degradation.

The practical test came with fresh tilapia fillets stored at 4 degrees Celsius for four days, with indicator films fixed to the inside of the package lid without touching the fish. Over the storage period, total volatile basic nitrogen climbed from an initial 5.61 milligrams per 100 grams, well within China’s first-grade freshness limit of 15, to 23.34 milligrams per 100 grams on day four, exceeding the 20-milligram spoilage threshold. Fillet pH first fell as glycolysis produced lactic acid, then rose as microbial protein decomposition released ammonia and amines. Hardness dropped from 1503.96 grams to 774.15 grams and springiness from 0.83 to 0.64 millimeters as enzymes and bacteria dismantled muscle proteins, while rising thiobarbituric acid reactive substances documented continuous lipid oxidation. Throughout, the films’ color difference and redness values remained essentially unchanged during the first two days, matching the fresh fillets’ quality, then increased significantly as volatile nitrogenous compounds accumulated, shifting the films from yellow toward reddish yellow. The 30-kilovolt films showed the most pronounced response, and the color parameters tracked the spoilage indicators with a strong linear correlation.

The study’s authors argue that the work offers a green and feasible route to bio-based intelligent packaging that could substitute for traditional plastics in the aquatic food industry. By co-encapsulating curcumin and resveratrol in zein nanoemulsions, the design integrates dual bioactive functions into a single pH-sensitive film, while cold plasma provides a solvent-free physical means of optimizing surface structure, release behavior, and color sensitivity. The color transition occurs precisely at the pH threshold corresponding to fish spoilage, driven by the deprotonation of curcumin’s conjugated beta-diketone skeleton, with hydrogen bonds between resveratrol and curcumin stabilizing the dissociated phenoxide anions and sharpening the color development. For consumers, the appeal is immediacy: no instruments, no sampling, no laboratory, just a visible signal on the inside of the lid that changes as the fish inside changes. If such films can be scaled economically, the humble package label may become one of the most informative objects in the refrigerator.

Subject of Research: pH-responsive chitosan indicator films with curcumin-resveratrol nanoemulsions for monitoring fish freshness

Article Title: Intelligent indicator films based on cold plasma-modified chitosan loaded with curcumin-resveratrol nanoemulsions: pH-responsive mechanism, colorimetric performance and application for tilapia freshness monitoring

Article References: Qiu, M., Yang, T., Xia, G., Wang, J., Liu, Z., Liao, E., Xu, W., Zhang, L., & Wu, W. (2026). Intelligent indicator films based on cold plasma-modified chitosan loaded with curcumin-resveratrol nanoemulsions: pH-responsive mechanism, colorimetric performance and application for tilapia freshness monitoring. Food Chemistry: X, 39, Article 104512. https://doi.org/10.1016/j.fochx.2026.104512

Image Credits: AI Generated

DOI: 10.1016/j.fochx.2026.104512

Keywords: chitosan, cold plasma, curcumin, resveratrol, nanoemulsion, pH-responsive film, colorimetric indicator, tilapia, food freshness monitoring, intelligent packaging, food preservation, biodegradable packaging