Fabrication and characterization of multilayered kafirin/gelatin film with one-way water barrier property
Graphical abstract
Introduction
Bio-based edible films for food packaging have gained constant research fever in the last decade for its eco-friendly, safe and promising shelf-life extension features (Etxabide, Uranga, Guerrero, & de la Caba, 2017; Sanches-Silva et al., 2014). Among various film forming materials, several proteins are known for their satisfying film forming properties and excellent natural barrier capacity against oxygen and carbon dioxide diffusion. For instance, with the aid of suitable plasticizers, collagen, gelatin, soy protein isolates, zein, kafirin etc., readily form continuous solid matrixes upon drying from their solutions (Alves, Gonçalves, & Rocha, 2017; Liu et al., 2017; Shi, Yu, Lee, & Huang, 2013; Zhang et al., 2015). Protein based biodegradable films extend the shelf life of food products when used as packaging material by providing barrier properties towards moisture, oxygen and microbacterial, as well as by serving as the carrier matrix for functional additives, such as antioxidants or antibacterial agents (Abdelhedi et al., 2018; Adilah, Jamilah, & Hanani, 2018; J.; Liang & Ludescher, 2011).
When evaluating the effectiveness of protein-based films as food packaging materials, apart from their visual and mechanical properties, the other critical criterion lies in their moisture barrier property (Gao et al., 2017; Li, Yin, Yang, Tang, & Wei, 2012; Jun; Liang et al., 2015). The transfer of moisture to food from the environment and the absorption of moisture in food, which determined the relative humidity in packaging food system, exert critical effects on food quality. Ideal protein based films for food-packaging usage should exhibit different moisture barrier property on both sides. That is, the external layer should have low water vapor permeability so as to prevent the invasion of moisture from environment, and the internal layer should possess moisture retention capacity in order to maintain the initial relative humidity for food preservation. Yet, to the best of our knowledge, research attempts of fabricating protein-based film with one-way water barrier property have rarely been reported.
Gelatin, attainable by the controlled hydrolysis of collagen (Etxabide et al., 2017), exhibits great film forming properties with satisfactory mechanical property and good oxygen barrier property. Yet, its application are compromised by its hydrophilic nature which imparts poor moisture barrier property to the derived films (Ahmad, Benjakul, Prodpran, & Agustini, 2012). As a consequence, several strategies have been proposed to improve the moisture resistance of gelatin films including chemical, physical or enzymatic cross-linking method (Martucci, Accareddu, & Ruseckaite, 2012), compositing it with other moisture resistant polymers (Moreno, Gil, Atares, & Chiralt, 2017; Rhim, Mohanty, Singh, & Ng, 2006), or fabricating multilayered films (Martucci & Ruseckaite, 2009, Martucci and Ruseckaite, 2010a, Martucci and Ruseckaite, 2010b). Kafirin protein, an alcohol soluble prolamin protein extracted from sorgum grain, readily forms film with great water barrier property upon solvent casting from either acetic or alcoholic solution. Classified as prolamin due to its high proportion of non-polar hydrophobic amino acids, kafirin is considered to be even more hydrophobic than zein, and is capable of self-assemble upon change in solvent polarity (Xiao et al., 2015). The hydrophobicity guarantees its promising performance for barrier coatings, formation of kafirin coating or blending with hydrophilic protein based films is thus promising in improving the barrier capability of film. However, films formed solely by prolamin protein are opaque in appearance and exhibit brittle mechanical properties even with the aid of plasticizers (Li, Li, Lee, & Huang, 2013; Panchapakesan, Sozer, Dogan, Huang, & Kokini, 2012). Creating multilayered film by combining advantages of gelatin and kafirin film matrixes into one sheet is a promising solution to bypass their respective shortcomings. With the kafirin film serving as the external layer, the gelatin film functioning as the internal layer and a hybrid kafirin/gelatin film performing as the intermediate transition layer, the kafirin-kafirin/gelatin-gelatin multilayered film is expected to perform the one-way water barrier property.
The present study aims to fabricate a multilayered film structure with one-way water barrier property, that is, the film possesses better water barrier property from one direction than from the reverse direction. A co-dissolving solution was used to prepare the hybrid transitional layer of alcohol-soluble kafirin and water-soluble gelatin. Multilayered film was fabricated with the stacking order of kafirin layer, blend kafirin/gelatin layer and the gelatin layer (abbreviated as Kaf-Kaf/Gel-Gel). The surface and cross-section morphology of multilayered film was investigated in the first place, then influences of lamination on visual inspection and mechanical properties of films were evaluated. Intermolecular interactions, surface properties of multilayered film were also analyzed to gain insights to the structure of multilayered film. The water barrier properties of multilayer films were then evaluated from both the external to internal direction and the internal to external direction. The structural responses after contact with water were further tested to identify the swelling behavior of the multilayered film. A schematic illustration of the multilayered kafirin/gelatin films was proposed to relate its water-induced structural responses with its one-way water barrier behavior.
Section snippets
Materials
Kafirin protein with a purity of 92% was extracted from whole sorghum grain as described in our previous report (Xiao et al., 2015). Gelatin from porcine skin (Type A, 300g Bloom) was purchased from Sigma Chemical Co. (St. Louis, MO, USA). Glycerol and glacial acetic acid were purchased from Bioshapp Co. Ltd. (Shanghai, China) and Sigma-Aldrich, Inc. (Steinheim, Germany), respectively. Water purified by Milli-Q system was used for sample preparation.
Preparation of stock solutions
Kafirin stock solution was prepared by
Color, light transmission and transparency of films
Color and transparency are essential parameters in visual inspection of food package, since they are key factors that affect the choice of consumers. Visually, Kaf is yellowish and semi-opaque, Gel, Kaf/Gel and Kaf-Kaf/Gel-Gel are clear and transparent (Fig. S1). As presented in Table 2, neat kafirin film displayed the lowest lightness (L = 52.5) and highest yellowness (b* = 18.5) values, which resulted in the highest E* value and lowest WI value. On the contrary, gelatin film displayed the
Conclusion
This work demonstrated a novel design of multilayered film with one-way water barrier property based on water-soluble and water insoluble proteins. As revealed by the cross-section microstructure observation, Kaf/Gel layer showed better compatibility with Kaf layer than that of the Gel layer. Multilayered film showed UV light protection property, and the lamination of Gel layer contributed to the improved transparency of multilayered film. The FTIR analysis suggested that interactions existed
Acknowledgement
This work was financially supported by the Scientific and Technological Breakthrough project for People's Livelihood of Guangzhou, National Natural Science Foundation of China (No. 31701556) and the Higher Education Research Promotion Project of Guangdong Province. The authors wish to thank also the colleagues from the Center of Polymer from Renewable Resources in South China University of Technology for precious collaboration and help.
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