Structure and oxidation resistance of procyanidins biodegradable packaging film

[Chinese Packaging Network News] For many years, people have added various antioxidants to food in order to preserve it and prevent oxidation. However, in recent years, the safety of food additives has become a growing concern. Researchers around the world have focused on packaging solutions, using antioxidants either added or coated onto packaging materials to inhibit oxidation and spoilage. Natural antioxidants are gaining more attention due to their higher safety compared to synthetic ones and their potential health benefits. For instance, Liu Zhaoming et al. used ginger extract on kraft paper to create an antioxidant wrap, while Wessling et al. extended this idea to films. Adding vitamin E to low-density polyethylene (LDPE) can help prevent oxidation of linoleic acid emulsions, but LDPE is not biodegradable, raising environmental concerns. Elise Portes et al. introduced degradability into antioxidant films by adding tetrahydrocurcumin to chitosan films.

Based on these ideas, this study proposes the development of a new natural, biodegradable antioxidant packaging film. The film was prepared through solution blending and casting using proanthocyanidin (PC) as the antioxidant and cellulose acetate (CA) as the base material. CA is non-toxic, fully biodegradable by microorganisms, and cost-effective. Proanthocyanidins, which belong to bioflavonoids, are a new type of natural antioxidant with significantly higher radical-scavenging ability than vitamin E and vitamin C. PC molecules contain numerous phenolic hydroxyl groups, especially ortho-hydroxy groups in catechol or pyrogallol, which can be oxidized into quinone structures, consuming oxygen and stabilizing the system. As shown in Figure 1, Faria et al. found that dimeric procyanidins exhibit the strongest antioxidant activity among five different structures.

This study aims to develop a sustainable and effective packaging solution that can extend the shelf life of packaged products while being environmentally friendly. The film is expected to suppress oxidative degradation, enhance product preservation, and ensure biodegradability.

1 Experimental Section

1.1 Materials

Cellulose Acetate (CA): Contains 54.5%–56.0% acetic acid, acidity (measured as H+) ≤ 1.66 mmol/100g, moisture content ≤ 5.0%, viscosity ≤ 300–500 mPa·s. The procyanidins used were grape seed extracts, containing a high concentration of procyanidin dimers. The extract was a red-brown powder with astringency, soluble in water and most organic solvents. The PC content was 99.52%, with 4.8% moisture and 0.3% ash. Glacial acetic acid (analytical grade) and lard (commercial fresh plate oil from warm-fire wet refining) were also used.

1.2 Sample Preparation

CA was weighed and immersed in glacial acetic acid for 2 hours. The partially dissolved CA solution was stirred thoroughly with an electric mixer to obtain a uniform 18% mass fraction solution. A certain amount of PC was dissolved in glacial acetic acid as a solvent. The PC solution was added to the CA solution and stirred for 2 hours. The mixture was left at room temperature for 1 hour to remove air bubbles.

A clean glass plate was placed on a water platform, and the cooled solution was cast onto one end. Using a homemade scraper, the solution was evenly spread across the glass plate to form a transparent film about 40 μm thick. The film was dried at room temperature for 1 hour, then rinsed with distilled water to accelerate solidification. After soaking for 30 minutes, the film was removed from the glass plate, dried at room temperature, and stored.

1.3 Test Analysis

1.3.1 Infrared Spectroscopy (FT-IR)

FT-IR analysis was performed using a Nicolet 5700 spectrometer. PC was analyzed using the KBr pellet method, while the CA/PC film was analyzed via ATR. The spectral range was 700–2000 cm⁻¹.

1.3.2 X-ray Diffraction (XRD)

XRD analysis was conducted using an ARL XTRA diffractometer. Cu Ka radiation (λ = 0.154 nm), 40 kV × 40 mA, scan speed 5°/min, step size 0.02°, and scanning range 2θ = 5–50° were used.

1.3.3 Atomic Force Microscopy (AFM)

Surface morphology of the CA/PC film was observed using a PSIA XE-100E AFM in non-contact mode.

1.3.4 Antioxidant Performance Testing

To evaluate the antioxidant performance of the CA/PC film, fresh lard was packaged and stored. The peroxide value (POV) was measured over time. POV is a key indicator of oil oxidation, with higher values indicating greater oxidative damage. The Schaal oven test method (GB/T 5009.37-2003) was used to determine oil oxidation stability. The formula for calculating POV is provided in the text.

If the POV of the sample oil is lower than that of the control, it indicates the CA/PC film has antioxidant properties. The inhibition percentage of oxidation is calculated accordingly.

2 Results and Discussion

2.1 Structural Analysis

2.1.1 FT-IR Analysis

The infrared spectrum of PC shows characteristic peaks in the 1000–1650 cm⁻¹ and 700–850 cm⁻¹ regions. The aromatic ring stretching vibration appears around 1520–1540 cm⁻¹. The hydroxyl group of the benzene ring is reflected in the low-frequency region (730–780 cm⁻¹).

The IR spectrum of the CA/PC film shows four characteristic peaks of CA: 1750 cm⁻¹ (C=O stretching), 1370 cm⁻¹ (CH₂ deformation), 1200 cm⁻¹ (C–O stretching), and 1050 cm⁻¹ (C–O skeleton). With increasing PC content, the peak positions remained unchanged, indicating no structural disruption. The absorption peak near 1600 cm⁻¹ increased with PC content, showing the presence of PC in the film.

2.1.2 XRD Analysis

The XRD pattern of the CA/PC film shows broad diffraction peaks for PC around 2θ = 23°, indicating an amorphous structure. The CA film has two distinct peaks at 8.5° and 22°, and similar peaks are present in the CA/PC films with 1%, 2%, and 3% PC. The peak at 21° is smaller, possibly due to interactions between CA and PC. The CA/PC film shows a peak at 8.0°, indicating the presence of CA's crystalline structure.

2.1.3 AFM Analysis

The AFM images show that the pure CA film has uneven surface features, consistent with XRD results. When 1% PC is added, the hard phase particles aggregate, forming larger, more uniform structures. At 3% PC, the particles disperse again, suggesting poor compatibility at higher concentrations. The surface fluctuation remains within 50 nm, indicating a smooth and uniform film.

2.2 Antioxidant Properties

2.2.1 Effect of PC Content on Oxidation Resistance

Films with 0%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, and 3.0% PC were tested. The inhibition rate of lipid peroxidation was highest at 2.0% PC, reaching 37.65%. Higher PC content led to reduced effectiveness, likely due to aggregation and uneven distribution. The shelf life of oil packaged with 2.0% PC was extended by more than two months compared to pure CA film.

Based on the Arrhenius equation, increasing the storage temperature by 10°C doubles the reaction rate. At 60°C, the shelf life of oil wrapped in 2.0% PC film was 16 days, while the control group reached the upper limit of POV at 12 days. This demonstrates the superior antioxidant performance of the CA/PC film.

3 Conclusion

A novel natural, biodegradable antioxidant packaging film was successfully developed using solution blending and casting. PC does not alter the structure of CA and introduces antioxidant properties through its hydroxyl groups. The interaction between CA and PC improves the crystallinity and surface quality of the film. The CA/PC film with 2.0% PC showed excellent antioxidant performance, extending the shelf life of packaged oils by more than two months. This study provides a promising alternative to traditional packaging materials, offering both functional and environmental benefits.