Capsaicin is the predominant pungent alkaloid in chili peppers. It is lipophilic and has poor aqueous solubility, which can restrict its access during oral delivery, while its amide bond and phenolic hydroxyl group make it susceptible to degradation under light, heat and alkaline conditions. The compound activates transient receptor potential vanilloid 1, or TRPV1, and has been investigated for analgesic, anti-inflammatory and other biological activities. High concentrations can also irritate mucosal surfaces and produce burning sensations. The study therefore focused on an encapsulation approach in which capsaicin was dissolved in soybean oil, emulsified as droplets and surrounded at the oil-water boundary by solid food-derived particles. The authors assessed whether this arrangement could improve chemical stability, simulated gastrointestinal release and bioaccessibility without evaluating the formulation in a living organism.

Pickering emulsions differ from conventional emulsions stabilized by small-molecule surfactants because solid nano- or microscale particles adsorb at the oil-water interface and form a physical barrier around droplets. This mechanism can limit coalescence, Ostwald ripening and phase separation. Whey protein was selected as one component because it is a food-derived protein complex with emulsifying activity, self-assembly behavior and the capacity to change conformation. Its limitations include sensitivity to thermal denaturation, degradation in gastric conditions and batch-to-batch variation. Carboxylated cellulose nanofibers, produced by chemical oxidation of cellulose nanocrystals, were added as a more rigid, negatively charged and fibrous component. At pH 3.0, electrostatic attraction between carboxyl groups on the fibers and charged amino-acid residues in whey protein, together with hydrogen bonding, supported formation of composite nanoparticles with altered protein structure.

The composition strongly affected particle size. Pure whey protein had an average hydrodynamic diameter of 193 ± 4.68 nanometers, while pure carboxylated cellulose nanofibers measured 221.7 ± 7.34 nanometers. Composite particles were smaller, with their average size decreasing from 165.18 ± 6.2 nanometers at 20% fiber content to 97.26 ± 7.96 nanometers at 80%. The 1:4 whey protein-to-fiber formulation therefore produced the smallest measured particles among the composites. Fourier-transform infrared spectroscopy showed a redshift and broadening of whey protein amide bands as fiber content increased, alongside a reduction in the fiber carboxyl signal, consistent with hydrogen bonding and partial extension of compact protein domains. Tryptophan fluorescence was progressively quenched without a significant shift in emission wavelength, indicating a change in the protein’s tertiary structure rather than a complete alteration of the fluorescent tryptophan environment. X-ray diffraction indicated that the composite retained the amorphous character of whey protein while increasing the ordering associated with cellulose nanofibers, and electron microscopy showed spherical or near-spherical structures connected within a three-dimensional fibrous network.

Interfacial wettability provided one possible explanation for the differences in stabilization. Pure whey protein and pure carboxylated cellulose nanofibers had water contact angles of 21.1 and 24.2 degrees, respectively. The 1:4 composite reached 68.1 ± 2.3 degrees, a value the authors associated with more favorable wetting at an oil-water interface and stronger interfacial anchoring. This interpretation was not tested independently of the other molecular and structural changes produced by altering the fiber-to-protein ratio. The optimized composite also showed the strongest combined performance in two chemical radical-scavenging assays, with DPPH and ABTS scavenging rates of 67.5 ± 5.22% and 65.1 ± 3%, respectively. Pure cellulose nanofibers had the highest ABTS rate at 66.2 ± 2.45%. These measurements describe the capacity of the materials to intercept radicals in laboratory assays and do not demonstrate antioxidant benefits in people.

When used at 1% of the aqueous phase to stabilize emulsions containing 10% soybean oil by weight, the composite particles produced narrow, nanoscale droplet distributions. The 1:4 formulation had the smallest droplets at both the start of storage and after 21 days at ambient conditions, showed no obvious phase separation and maintained an average droplet-size change below 20%. Emulsions stabilized by pure cellulose nanofibers showed less visible coalescence but creamed upward, which the authors associated with the high hydrophilicity of the particles and stabilization through a continuous-phase network rather than robust interfacial anchoring. The emulsions displayed shear-thinning behavior over 0.1–100 inverse seconds, with apparent viscosity decreasing as shear rate increased. Viscosity generally rose with cellulose content, consistent with a network combining rigid nanofibers and flexible protein chains. After centrifugation at 6,000 revolutions per minute for 20 minutes, the 1:4 emulsion had the highest measured water retention, 51.8 ± 3.1%.

Fluorescence microscopy supplied direct visual evidence for the emulsion’s architecture. Nile Red stained the oil phase red, Calcofluor White labeled the cellulose nanofibers blue, and merged images showed composite particles surrounding spherical oil droplets in an oil-in-water structure. Oxidation tests detected increasing levels of lipid hydroperoxides and malondialdehyde in every emulsion during storage, demonstrating that oxidation occurred. Their concentrations were lower as the proportion of cellulose nanofibers increased, and the 1:4 composite had the strongest inhibition among the formulations. The authors attributed this observation to the fibers’ radical-scavenging capacity and to a denser interfacial barrier that could restrict oxygen diffusion. The capsaicin-loaded 1:4 formulation also maintained comparatively stable size, polydispersity and zeta potential after 30 minutes of heating at 50–90 degrees Celsius, ultraviolet irradiation for as long as 12 hours, and exposure to sodium chloride concentrations as high as 300 millimolar.

Release behavior was evaluated with a two-stage simulated gastrointestinal model rather than in animals or humans. Samples were first mixed with an equal volume of simulated gastric fluid at pH 2.0, containing 0.7% hydrochloric acid, 3.2 grams per liter pepsin and 8.5 grams per liter sodium chloride, and incubated for two hours at 37 degrees Celsius with shaking at 100 revolutions per minute. They were then combined with simulated intestinal fluid at pH 7.0, containing 8 grams per liter bile salts, 120 millimolar sodium chloride, 10 millimolar calcium chloride and 1 gram per liter trypsin, and incubated for another two hours under the same conditions. Droplets increased in size and flocculated during gastric digestion as low pH screened electrostatic repulsion and pepsin hydrolyzed interfacial protein. During the intestinal phase, bile salts and pancreatic lipase acted on the oil cores and produced smaller droplets. Across the digestion stages, the 1:4 formulation showed the most stable size distribution, zeta potential and polydispersity index, with limited gastric flocculation and uniform redispersion in the intestinal phase.

Capsaicin release was slow and incomplete under gastric conditions, and cumulative release from the optimized formulation remained below 25% at the end of the gastric phase. Release then accelerated in simulated intestinal fluid and reached approximately 60% by the end of the experiment. After complete in vitro digestion, capsaicin dissolved only in soybean oil had a bioaccessibility of 19.74 ± 0.28%, calculated from the amount recovered in the micellar phase. The optimized whey protein-cellulose nanofiber emulsion reached 46.23 ± 0.41%. Pure cellulose nanofiber and pure whey protein emulsions produced values of 42.17 ± 0.64% and 45.17 ± 0.39%, respectively, so the optimized formulation’s advantage over either individual stabilizer was modest. The authors proposed that bile salts and fatty acids released during lipid digestion promoted mixed-micelle formation, increasing capssaicin solubilization. They further proposed that the cellulose network partly delayed lipolysis by limiting bile-salt and lipase access to the oil-water interface, rather than by directly inhibiting digestive enzymes.

The researchers also tested whether encapsulation retained capsaicin’s intrinsic antibacterial activity. In disk-diffusion assays using Escherichia coli and Staphylococcus aureus, the tested emulsions produced inhibition zones around loaded discs. Measured inhibition-zone diameters increased as the proportion of cellulose nanofibers rose, and the 1:4 formulation produced the largest zones among the formulations. This assay shows inhibition of bacterial growth around the discs under the experimental conditions; it did not quantify reductions in viable bacterial counts, establish a clinical antibacterial effect or demonstrate superiority to antibiotics. Overall, the laboratory findings support whey protein and carboxylated cellulose nanofibers as a composite stabilizer for an experimental oral capsaicin delivery emulsion, with the strongest results obtained at a 1:4 mass ratio. Large-scale production feasibility and the system’s long-term behavior in living organisms remain untested, so the work does not establish an established oral treatment or a food preservative for commercial use.

Subject of Research: In vitro development of a whey protein-carboxylated cellulose nanofiber Pickering emulsion for oral capsaicin delivery

Article Title: Pickering emulsion stabilized by whey protein-carboxylated cellulose nanofiber composite for oral capsaicin delivery: Interfacial structure, physicochemical stability and antibacterial efficacy

Article References: Zhao, W., Zhang, Z., Guo, T., Zhang, Y., Cao, J., Sun, P., Xie, J., & Wang, Z. (2026). Pickering emulsion stabilized by whey protein-carboxylated cellulose nanofiber composite for oral capsaicin delivery: Interfacial structure, physicochemical stability and antibacterial efficacy. Food Chemistry: X, Article 104640. https://doi.org/10.1016/j.fochx.2026.104640

Image Credits: AI Generated

DOI: 10.1016/j.fochx.2026.104640

Keywords: capsaicin, Pickering emulsion, whey protein, carboxylated cellulose nanofibers, oral delivery, in vitro digestion, bioaccessibility, nanoparticle stabilizer, emulsion stability, antibacterial assay, Pickering, emulsion