Physicochemical, mechanical and structural properties of composite edible films based on whey protein isolate/psyllium seed gum
Introduction
There is a growing interest to replace synthetic packaging materials with natural and non-toxic ones owing to consumer's increased awareness of food safety and environmental protection [[1], [2], [3], [4]]. As a packaging material, edible films can be manufactured from natural biopolymers such as proteins, polysaccharides, lipids, and/or their mixtures. These natural materials are abundant, low cost, renewable and degradable, which allows its application not only as a packaging material but also as fertilizers and soil conditioners [[5], [6], [7], [8]]. However, the films fabricated from single biopolymers have the characteristics of poor barrier and mechanical properties [1,9]. The disadvantages of a single biopolymer film can be alleviated by physical (heat, ultrasound or radiation) and chemical (cross linking) treatments, and/or synergism of composite biopolymers [8,10,11].
Whey proteins are by-product from cheese-making and recovered from whey through ultrafiltration and diafiltration because of its multiple functional properties for human health [12]. Whey proteins include whey protein isolate (WPI), whey protein concentrate (WPC) and whey protein hydrolysates. WPI contains >90% protein and the major components are α-lactalbumin and β-Lactoglobulin. Studies on the utilization of whey proteins mainly focus on their chemical, biochemical, and bioactive properties for the development of food, biotechnology, medicine, and biodegradable materials [13]. WPI can be used as an excellent film-forming material. WPI film has excellent oxygen, aroma, and lipid barrier properties at low relative humidity (RH). However, WPI film has poor water barrier and mechanical properties [9,14].
Psyllium (Plantago ovata Forsk) is an annual plant from the Plantago genus that is grown primarily in India and Iran [15]. Psyllium seed contains proteins, lipids, sterols, triterpenes, and aucubin glycoside. There is about 30% mucilage in the seed [[16], [17], [18]]. Psyllium seed mucilage or gum (PSG) is composed of a highly branched non-starch polysaccharide, which is composed of 85% of a soluble polysaccharide fraction dominated by D-xylose. The backbone of polysaccharide is a xylan with 1 → 3 and 1 → 4 linkages with no apparent regularity in their distribution. The monosaccharides primary chains of mucilage are substituted on C-2 or C-3 by L-arabinose, D-xylose, and α-D-galactouronyl- (1 → 2)- L-rhamnose. Its side chains, composed by single arabinofuranose and xylopyranose residues or short side chains consisting of these monosaccharides, are connected to the main chain via O-3 and/or O-2 linkage [15,17]. PSG is a neutral arabinoxylan and contains 22.6% arabinose, 74.6% xylose and some traces of other sugars. Furthermore, PSG has about 35% non-reducing terminal residues, and the polysaccharide is highly branched [19]. PSG have been applied in biomedical systems (cholesterol-lowering and bowel regulating), landscape industry (binding agent), environmental areas (adsorption to remove solid waste or toxic ion, and water purification), and additives in food, pharmaceutical, cosmetic industries (thickener, suspension aids, stabilizer, binding agents, emulsifiers) [16,20]. PSG also has received considerable attention in film production due to its abundance, renewability and gel-forming properties [15]. However, some drawbacks such as poor mechanical and barrier properties of PSG-based films have restricted its practical applications [10,15].
In recent years, a strategy to make films of different biopolymers through blending into a multifunctional film was put forward. This approach integrates the advantages of film formation from different components and compensate for the lack of desirable properties that single film forming biopolymers might possess [1,[21], [22], [23]]. Components of edible films can be divided into 3 categories: hydrocolloids (e.g. proteins and polysaccharides), lipids (e.g. waxes, acylglycerols, and fatty acids) and composites [5]. It is reported that hydrocolloids can be utilized to strengthen the properties of protein-based films [1,11,24]. WPI and PSG have a great potential for application in food packaging, but both single films have limitations. Blending of WPI and PSG might reinforce the properties of single films. Therefore, the aim of present study was to fabricate edible films by WPI and PSG at different ratios. Physicochemical, mechanical, and structural properties of edible films were also investigated to clarify the interaction between WPI and PSG.
Section snippets
Materials
Whey protein isolate and psyllium seed gum (PSG, 99.0% mucilage polysaccharide) were purchased from Shanghai Quanwang Biotechnology Co. Ltd. (Shanghai, China). The components of WPI were protein 87.00 ± 1.45%, total fat 2.60 ± 0.12%, moisture 4.50 ± 1.20%, and ash 2.58 ± 0.24%. Glycerol and all other chemical agents were of analytical grade.
Preparation of films
Five groups of composite films with different solid ratios of WPI/PSG (viz., 1:0, 3:1, 1:1, 1:3, 0:1) were prepared. The concentration of WPI film forming
Film thickness
Different WPI/PSG ratios affected the physical properties of the composite films. As shown in Table 1, the thickness range of the composite film was 0.057–0.128 mm. There were significant differences (p < 0.05) in film thickness of different WPI/PSG ratios (except thickness values between the films at WPI/PSG ratio of 1:0 and 3:1). The WPI only film (WPI/PSG = 1:0) was the thickest, while the pure PSG film (WPI/PSG = 0:1) was the thinnest. The composition of film forming dispersion solution
Conclusion
The WPI/PSG composite films were successfully developed; and the physical properties, barrier properties, mechanical properties and microstructure of films were investigated. The results demonstrated that WPI/PSG composite films improved the properties of single component films and have the potential for application in food packaging. The addition of PSG improved TS of the composite film, and the EB of composite films were both higher than the individual material films. These findings indicated
CRediT authorship contribution statement
Xin Zhanga: Investigation, Software, Data curation, Writing - original draft. Ya Zhaoa: Conceptualization, Methodology, Investigation, Data curation, Validation. Yuanyuan Lia: Investigation, Data curation. Lanlan Zhua: Formal analysis, Visualization. Zhongxiang Fang: Writing - review & editing. Qilong Shia: Supervision, Conceptualization, Methodology, Writing - review & editing, Funding acquisition, Project administration.
Declaration of competing interest
The authors declared no potential conflicts of interest.
Acknowledgments
This work was funded by the National Natural Science Foundation of China (No. 31171708). It was also supported by the Innovative Research Teams of Postgraduate Eduation of Shandong University of Technology (No. 219028). Furthermore, the authors would like to thank Dr. William Leonard of the University of Melbourne for the excellent language polish.
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These authors contributed equally to this work and are co-first authors.