Structure and oxidation resistance of procyanidins biodegradable packaging film

[Chinese Packaging Network News] For many years, people have added various antioxidants to food to preserve it and prevent oxidation. However, in recent years, the safety of food additives has become a major concern. Researchers around the world have focused on packaging solutions that can inhibit oxidation and spoilage by incorporating or coating antioxidants into packaging materials. Natural antioxidants have gained significant attention due to their higher safety profile compared to synthetic ones and their health benefits. Liu Zhaoming and his team used ginger extract on kraft paper to create an antioxidant wrap, while Wessling extended this concept to films. Adding vitamin E to low-density polyethylene (LDPE) can reduce the oxidation of linoleic acid emulsions, but LDPE is not biodegradable and poses environmental risks. Elise Portes et al. incorporated tetrahydrocurcumin into chitosan films to introduce degradability.

Based on these ideas, this study proposes the development of a new natural, biodegradable antioxidant packaging film using solution blending and casting. The film is made from proanthocyanidins (PC) as the antioxidant and cellulose acetate (CA) as the base material. CA is non-toxic, fully biodegradable by microorganisms, and cost-effective. Proanthocyanidins are a type of bioflavonoid with strong antioxidant properties—50 times more effective than vitamin E and 20 times more than vitamin C. Their ability to neutralize free radicals is significantly higher than curcuminoids, which are only about 2.75 times as effective as vitamin C. The PC molecules contain numerous phenolic hydroxyl groups, especially ortho-hydroxy groups in catechol or pyrogallol, which oxidize easily into quinone structures, consuming oxygen and stabilizing the molecule. This structure is shown in Figure 1.

Faria et al. evaluated the antioxidant capacity of five different procyanidins by measuring oxygen consumption and conjugated diene formation. The results showed that as the polymerization degree increased, the antioxidant effect decreased, with dimers having the strongest performance. Therefore, the resulting film is expected to effectively delay oxidative decay, extend shelf life, and be environmentally friendly and biodegradable.

1 Experimental Section

1.1 Materials

Cellulose Acetate (CA): Contains 54.5% to 56.0% acetic acid, acidity ≤ 1.66 mmol/l00g, moisture content ≤ 5.0%, viscosity ≤ 300–500 mPa·s. The procyanidins used were grape seed extracts, rich in dimer structures. The powder was red-brown, astringent, soluble in water and most organic solvents, with a PC content of 99.52% and 4.8% water. Glacial acetic acid (analytical grade). Lard (commercial fresh plate oil made via warm-fire wet refining).

1.2 Sample Preparation

CA was weighed and soaked in glacial acetic acid for 2 hours. After dissolving, it was stirred to form a 18% mass fraction solution. PC was dissolved in glacial acetic acid and mixed with the CA solution for 2 hours. The mixture was left at room temperature for 1 hour to degas. A glass plate was used to cast the film, scraped evenly to form a 40μm thick transparent layer. After drying for 1 hour, the film was rinsed in distilled water to accelerate solidification. It was then dried at room temperature for storage.

1.3 Test Analysis

1.3.1 FT-IR Analysis

The infrared spectrum of PC was measured using the KBr tablet method, while the CA/PC film was analyzed using ATR. The wave number range was 700–2000 cm⁻¹.

1.3.2 XRD Analysis

The crystallinity of the CA/PC film was analyzed using a Thermo ARL XTRA X-ray diffractometer. Cu Ka radiation (λ = 0.154 nm), 40 kV×40 mA, scanning speed 5°/min, step size 0.02°, and 2θ range 5–50°.

1.3.3 AFM Observation

The surface morphology of the CA/PC film was observed using a PSIA XE-100E atomic force microscope in non-contact mode.

1.3.4 Antioxidant Performance Evaluation

To evaluate the antioxidant performance, fresh lard was packaged in the CA/PC film and stored. The peroxide value (POV) was measured throughout the process. Oil oxidation is a key reaction in food chemistry, and POV is an indicator of oxidation products. A lower POV indicates better antioxidant performance. The Schaal oven method (GB/T 5009.37-2003) was used for testing, and the POV was calculated based on standard formulas.

If the POV of the sample was lower than that of the control, the film showed antioxidant activity. The inhibition rate was calculated accordingly.

2 Results and Discussion

2.1 Structural Analysis

2.1.1 FT-IR Analysis

The infrared spectrum of PC showed characteristic peaks in the 1000–1650 cm⁻¹ and 700–850 cm⁻¹ regions, with a strong peak at 1520–1540 cm⁻¹ corresponding to aromatic ring stretching vibrations. Hydroxyl group changes were visible in the 730–780 cm⁻¹ region.

The IR spectrum of the CA/PC film showed characteristic peaks of CA, and the addition of PC caused a slight increase in absorption intensity, indicating no structural damage to CA. The presence of PC peaks confirmed its integration into the film, supporting its antioxidant potential.

2.1.2 XRD Analysis

The XRD pattern of the CA/PC film showed that PC had a broad diffraction peak at 23°, while CA had two distinct peaks at 8.5° and 22°. The CA/PC film retained these peaks, suggesting no disruption of CA's crystal structure. The diffraction peak at 21° shifted slightly, possibly due to interactions between CA and PC.

2.1.3 AFM Analysis

The AFM images showed that adding 1% PC improved the distribution of hard particles, while 3% PC led to uneven dispersion, reducing compatibility. The film surface remained smooth, with fluctuations within 50 nm, indicating good uniformity.

2.2 Antioxidant Properties

2.2.1 Effect of PC Content on Oxidation Resistance

CA/PC films with PC contents of 0%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, and 3.0% were tested. The results showed that 2.0% PC provided the best antioxidant effect, with a 37.65% inhibition rate. Higher PC content reduced compatibility and effectiveness, likely due to aggregation.

Using the Arrhenius equation, the relationship between temperature and reaction rate was discussed. At 60°C, the shelf life of the oil wrapped in 2.0% PC film was over 16 days, compared to 12 days for pure CA film. This suggests that the CA/PC film significantly extends shelf life.

3 Conclusion

This study successfully developed a novel natural, biodegradable antioxidant packaging film using PC and CA. PC did not alter the CA structure but enhanced its antioxidant potential through interactions. The film showed improved crystallinity and surface uniformity. The 2.0% PC film extended the shelf life of oils by more than two months, demonstrating excellent performance and environmental benefits.