Optimization of the Properties of pH-Sensitive Biodegradable Intelligent Films Based on Gelatin/Chitin Nanofibers Containing Malva and Amaranth Extracts

IntroductionFood packaging is one of the essential components of the food supply chain, playing a vital role in maintaining product quality, ensuring safety, and extending the shelf life of perishable commodities. Beyond its traditional role as a passive barrier against physical, chemical, and microbial deterioration, novel packaging is expected to perform active and intelligent functions that can enhance product stability, indicate spoilage, and reduce food waste. In this context, the development of intelligent biodegradable films has received growing attention over the past decade due to their environmental compatibility, sustainability, and ability to provide real-time information on food freshness through measurable color changes triggered by spoilage-related pH variations. Among various natural colorants, anthocyanin-rich plant extracts are considered ideal candidates to be aoolied as a pH indicator in smart packaging. These compounds exhibit distinct color transitions in response to pH changes while also contributing antioxidant and antimicrobial properties that can further enhance food preservation.Materials and MethodsIn the present study, a pH-sensitive, biodegradable intelligent film was developed using a gelatin-based matrix reinforced with chitin nanofibers (NCh). The film was incorporated with Malva sylvestris (malva) and Amaranthus cruentus (amaranth) extracts as natural colorimetric indicators and bioactive components. Both extracts are rich sources of anthocyanins and phenolic compounds, which can impart multifunctional properties to the packaging material. The experimental design was optimized using the Response Surface Methodology (RSM) based on a Central Composite Design (CCD) with two independent factors: the concentration of malva extract (0.05–0.35% w/v) and amaranth extract (0.3–0.8% w/v).The prepared films were evaluated for mechanical characteristics (tensile strength and elongation at break), water vapor permeability (WVP), moisture content, solubility, color attributes, thickness, and optical properties. Antioxidant activity was determined using the DPPH radical scavenging method, while antimicrobial activity of malva extract was tested against Staphylococcus aureus and Escherichia coli using the disk diffusion and minimum inhibitory concentration (MIC) assays. Structural and morphological characterizations were carried out using Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), and X-ray Diffraction (XRD), while Fourier-transform infrared spectroscopy (FTIR) was used to examine possible molecular interactions between film components. The color response of the films to pH variation was evaluated over a wide range (pH 1–14), and their practical application was assessed by monitoring spoilage in packaged common carp (Cyprinus carpio) fillets stored at refrigerator temperature for 72 hours.Results and Discussion The results indicated that the simultaneous increase in malva and amaranth extract concentrations significantly enhanced the tensile strength of the films from 1.687 to 4.654 MPa, while elongation at break decreased from 24.405% to 15.102%, reflecting increased structural rigidity and reduced flexibility. Water vapor permeability increased from 0.00162 to 0.00418 g·m⁻¹·s⁻¹·Pa⁻¹, whereas moisture content and solubility decreased from 26.501% to 19.001% and from 23.654% to 17.415%, respectively, suggesting improved hydrophobic interactions within the polymeric network. The film thickness increased from 0.141 to 0.231 mm, total color difference (ΔE) increased from 20.99 to 43.47, and whiteness index (WI) decreased from 82.71 to 51.11, demonstrating that the incorporation of extracts led to more intense coloration. The antioxidant activity, measured as DPPH radical inhibition, increased remarkably from 39.889% to 71.021% (p < 0.001), confirming the strong radical scavenging potential of the incorporated extracts. Malva extract showed notable antimicrobial effects, with inhibition zones of 8 mm (MIC = 13.86 ppm) against S. aureus and 9 mm (MIC = 24.67 ppm) against E. coli, highlighting its effectiveness as a natural antimicrobial agent.SEM and AFM analyses revealed improved surface uniformity and better compatibility between gelatin and chitin nanofibers in the presence of the extracts, while XRD results indicated that the semi-crystalline nature of the films remained largely unchanged. FTIR spectra confirmed hydrogen bonding interactions between the hydroxyl and amide groups of the extracts and the polymeric matrix, validating molecular-level compatibility. The pH-sensitivity evaluation demonstrated a distinct color shift across the pH range of 1 to 14, from red in strongly acidic conditions to yellow under alkaline environments. This visible and reversible color transition indicated the suitability of the films as pH-sensitive indicator. In practical application tests, the films successfully detected fish spoilage after 72 hours of storage at refrigerator temperature by exhibiting an obvious and easily perceivable color change corresponding to the increase in pH caused by microbial activity.ConclusionOverall, the developed gelatin/chitin nanofiber-based films incorporated with Malva sylvestris and Amaranthus cruentus extracts exhibited desirable physicochemical, mechanical, antioxidant, and antimicrobial properties, along with excellent pH sensitivity. These multifunctional attributes make the films a promising candidate for use as intelligent and active food packaging materials capable of real-time freshness monitoring. Furthermore, the biodegradable nature of the materials provides an environmentally sustainable alternative to conventional synthetic packaging. Future studies are recommended to investigate the color stability, performance, and long-term durability of these films under refrigerated and industrial storage conditions to support their potential commercialization in smart packaging systems.