Effects of plasticizer type and concentration on rheological, physico-mechanical and structural properties of chitosan/zein film
Introduction
Packaging can interrupt the link between the external environment and food so as to ensure food quality and extend its shelf life [1]. However, the excessive use of petroleum-based packaging material causes environmental pollution and damages the ecology due to its poor biodegradability [2]. Currently, there is an urgent need to develop biomaterials to replace it. As a new and environment-friendly material, edible film has good barrier, antibacterial and antioxidant properties [3,4], which can effectively solve the problem of environmental pollution caused by petroleum-based synthetic packaging films [5]. Edible film has been a hotspot in the field of food packaging.
Among these biomaterials, there is a lot of demand for chitosan due to its safety, natural source, outstanding biocompatibility, fine film-forming performance and affordable price [6]. Chitosan is obtained by deacetylation of chitin, and it is a linear polysaccharide constituted of β-(1,4)-linked N- acetyl-D-glucosamine units [7]. But, because of the existence of hydrophilic groups, the chitosan films exhibit a tendency to bloat due to water absorption, as well as poor water vapor and gas barrier properties in previous research [8]. Zein, an abundant and inexpensive byproduct of ethanol manufacture, has a high proportion of non-polar amino acid residues [9]. Thus, zein has fine solubility in alcohol and high hydrophobicity [10]. However, the mechanical properties of pure zein films is unsatisfying. We found that the edible film with better properties could be obtained by mixing chitosan and zein. According to our previous studies, the optimal formulation of chitosan/zein blend film had been determined [11].
Compared with the single-component film, chitosan/zein blend film has better mechanical properties [12], but its weak flexibility and strong brittleness still greatly limits its wide application in the food industry. Plasticizer could be added to improve the flexibility and workability of films, because it could weak the intermolecular forces among the polymer chains [13], form new hydrogen bonds with polymers [14], increase the free volume of polymer molecules [15] and improve the mobility of polymer chains [16]. Glycerol, PEG-400 and sorbitol are often used as plasticizers owing to their good compatibility and water solubility [17], which makes bio-films overcome brittleness and be equipped with the desirable stretchability and flexibility. The specific improvement effect was determined by the interaction between plasticizer and polymer. The same plasticizer might have different effects on different polymers. The tensile strength of sage seed gum films decreased as the plasticizer increased [18], while tensile strength of cassia gum films increased as the plasticizer increased [19]. Besides, the properties of film were correlated with the rheological properties of solution [20]. Rheological behaviors of film-forming solution could affect the thickness, uniformity and mechanical properties of film. The study of the rheology could illuminate the polymerization reaction so as to produce edible films with excellent performance. Ghanbarzadeh et al. [21] found that sorbitol and glycerol significantly reduced the viscoelastic modulus of zein resin film, and had an excellent plasticizing effect.
For all we know, there is no report about the effects of plasticizer type and concentration on properties of chitosan/zein blend film. And little was known about the influence of plasticizer on rheological properties of the film-forming solution. The rheological behaviors were sensitive to the changes of molecular structure, thus rheological analysis of the film-forming solutions could provide a basic understanding of whether the change of film-forming solution was conducive to the preparation of film [21]. In this study, different types and concentrations (0%, 15%, 30% and 45%, w/w) of plasticizers were added into zein/chitosan film solutions. The goals of the study were to evaluate their impacts on rheological properties and to determine the physico-mechanical and structural properties of plasticized blend films. The modified film had potential application as packaging materials with superior properties.
Section snippets
Materials
Chitosan (average molecular weight was 8 × 105 Da, deacetylated degree was 90%) was provided by Jinan Huaxing Biotechnology Co. Ltd. (Shandong, China). Zein (92% protein mass fraction) was obtained from the Hebei Huayang Biochemistry Co., Ltd. (Hebei, China). Ethanol was purchased from Shandong Yusheng Biotechnology Co. Ltd. (Shandong, China). Acetic acid was supplied by Shanghai Kelin Biochemical Technology Co. Ltd. (Shanghai, China). Glycerol,PEG-400 and sorbitol (analytical reagent) were got
Steady shear behavior
Steady shear behavior of chitosan/zein film-forming solution was performed to elucidate the processability of the blends. The thickness, spread-ability, and uniformity of the blend film would be seriously affected by the flow characteristics of film-forming solution [28]. Fig. 1 presented the effect of shear rate on the apparent viscosity of film-forming solution. In all cases, the apparent viscosity of the blend solution decreased with the increase of shear rate. Blend solution exhibited a
Conclusion
This work demonstrated that the addition of plasticizer could be effective in modifying the properties of chitosan/zein blend film. And the results also showed that the rheological analysis of film-forming solutions was an effective way to understand the differences in the final properties of films. The apparent viscosity of film solutions decreased and the fluidity of film solutions increased with the increase of the plasticizer concentration. Dynamic oscillatory rheology of C/Z with
CRediT authorship contribution statement
Yang Sun: Methodology, Investigation, Formal analysis, Writing - original draft. Zhanli Liu: Writing - review & editing, Funding acquisition. Liming Zhang: Investigation. Xiaomin Wang: Investigation. Ling Li: Writing - review & editing.
Acknowledgement
This work was financially supported by the National Natural Science Foundation of China (No. 31301570).
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