Multilayer zein/gelatin films with tunable water barrier property and prolonged antioxidant activity
Graphical abstract
Introduction
The demand of fresh-cut fruits and vegetables with the characteristics of freshness, convenience and high nutrition is increasing in the market, but because fresh-cut fruits and vegetables are easy to brown and rot during the storage process, making it difficult to preserve for a long time. Edible films based on biodegradable materials are considered as promising alternatives for conventional petroleum-derived polymers, which can prolong the shelf life of food by changing the food packaging environment and act as carrier matrix for active compounds, such as antioxidants or antimicrobial compounds (Mishra et al., 2018; Otoni et al., 2017). In preserving fresh-cut fruits and vegetables, edible films are required to possess suitable water vapor permeability (WVP) (Lin & Zhao, 2007). In other words, ideal edible films should form a moisture barrier outer layer and a moisture retention inner layer to achieve one-way moisture resistance, which will lead to a beneficial regulation over the relative humidity in packaging food system (Wang et al., 2018). To be specific, the moisture barrier layer is needed to prevent the infiltration of moisture from environment and to prolong the lifespan of the inner hydrophilic film layer, which will otherwise be reduced due to swelling and erosion caused by the moisture infiltration; and the moisture retention layer needs to adjust the humidity by absorbing moisture to avoid condensation occurring in the inner layer of the packaging film when fresh-cut fruits and vegetables are placed inside the package. The other critical requirement for fresh-cut fruits and vegetables packaging is that edible films containing or coated with antioxidant compounds should provide continuous protection against oxidation during storage through a prolonged antioxidant release profile (Liang & Ludescher, 2011; Min & Krochta, 2007; Mishra et al., 2018). However, most of the edible films show the same WVP values on both sides, and the other key problem yet to be addressed lies in how to protect the antioxidants of the edible films and prolong its release time.
One of the possible strategies to gain different WVP properties on both sides and overcome the drawback of burst-release of antioxidant is by developing the gradient multilayer structure by using materials with different water-induced structural responses. Zein, a hydrophobic corn protein with abundant nonpolar amino acids, owns excellent film-forming behavior and has been studied as a potential biomaterial for edible packaging (Liang & Ludescher, 2011; Sanchez-Garcia, Hilliou, & Lagaron, 2010). Zein-based films formed by solvent casting are hydrophobic, grease-proof, and resistant to microbial contamination (Shi, Kokini, & Huang, 2009; Shukla & Cheryan, 2001; Zhang et al., 2015). However, films formed solely by zein protein are opaque in appearance and exhibit brittle mechanical properties even with the aid of plasticizers. Gelatin, a biodegradable protein from animal, shows excellent film forming capacity and swelling ability (Adilah, Jamilah, & Hanani, 2018; Nilsuwan, Benjakul, & Prodpran, 2017). Besides, gelatin-based films generally have good optical properties and excellent oxygen barrier property (Nilsuwan et al., 2017; Pereda, Ponce, Marcovich, Ruseckaite, & Martucci, 2011). However, the application of pure gelatin-based films is compromised by its hydrophilic nature that imparts poor moisture barrier property to the derived films. Creating multilayer film by combining advantages of gelatin and zein film matrixes into one sheet is a promising solution to bypass their respective shortcomings. Although fabricating edible films from proteins of hydrophobic and hydrophilic natures has been rarely studied, we have successfully optimized a layer-by-layer solvent casting system that managed to achieve a tri-layered film from prolamine protein and gelatin in a previous research (Wang et al., 2018). Since prolamine protein and gelatin response differently to water penetration (prolamine moieties aggregated while gelatin matrix swells readily), by tuning the composition of film matrix and loading strategies of antioxidants, edible films with tunable water barrier property and prolonged antioxidant activity are expected.
The aim of this research was to develop gradient multilayer films by making full usage of the advantages of zein and gelatin to achieve one-way water barrier property and explore its potential in prolonging the release of antioxidant. In the multilayer film, the outer layer prepared by hydrophobic zein performed as moisture barrier; the middle layer prepared by blended zein/gelatin with different ratios functioned in regulating the mechanical and water barrier properties, and the inner layer prepared by hydrophilic gelatin acted as the moisture retention layer. Tea polyphenol, as the model antioxidant compound, at different concentrations, were then loaded to the inner and middle layer of the optimized multilayer film to prepare an antioxidant film with prolonged release property. The morphological, optical, mechanical properties of multilayer films were investigated to grasp the influences of lamination and film composition. And the water barrier properties were measured from both sides to identify the one-way water barrier behavior. The one-way release profile as well as release kinetic of the antioxidant loaded multilayer film was identified. Finally, the preservation effects of the active film were tested on freshly cut fruits.
Section snippets
Materials
Zein, gelatin (Type A, Bloom 300) and glycerol was purchased from Wako Chemical Co. (Japan), Sigma Chemical Co. (USA) and Biosharp Co. (China), respectively. Tea polyphenol (TP) (98% purity) were purchased from Yuanye Bio-Technology Co. Ltd (China). All other chemicals used were of analytical grade.
Preparation of multilayer film stock solution
Zein stock solution (15% w/v) (Z) was obtained by dissolving zein into 100% acetic acid (AA) and gelatin stock solution (10% w/v) (G) was prepared by dissolving gelatin into deionized water at 50 °C.
Color, light transmission and transparency properties of films
Attention is paid to the optical properties of films since film appearance affects the customer's selection of products. As shown in Table 2, G film had the lowest b* and ΔE values as well as the highest L and WI values. The b*, ΔE and WI values of multilayer films fell in between those of Z and G films, while a* values decreased and no obvious difference in L values existed between Z and multilayer films. The a*, b* and ΔE values of multilayer films at different ratios of zein/gelatin (2:1,
Conclusion
In this work, gradient multilayer films with one-way water barrier and prolonged antioxidant release properties were fabricated via layer-by-layer solvent-casting method. Multilayer films exhibited better UV-barrier effect and higher transparency than neat gelatin film and zein film, respectively. The zein/gelatin ratio in the middle layer affected the mechanical properties of films, indicating that the mechanical response of multilayer films can be regulated by the zein/gelatin ratio in middle
Acknowledgements
This work was financially supported by the National Key Research and Development Program of China (No. 2018YFD0400701), the National Natural Science Foundation of China (No. 31701556), the Scientific and Technological Breakthrough project for People's Livelihood of Guangzhou (No. 201803020020) and the Special Innovation Project of Guangdong Education Department (No. 2017KTSCX021).
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