Physicochemical, mechanical and structural properties of composite edible films based on whey protein isolate/psyllium seed gum

Highlights

• Composite films exhibited better barrier properties than single films.

• Mechanical properties of WPI/PSG composite films were strengthened.

• WPI/PSG films showed a typical semi-crystalline amorphous structure.

Abstract

A composite film composed of whey protein isolate (WPI) and psyllium seed gum (PSG) was investigated. Its physicochemical, mechanical and structural properties were determined at different ratios of WPI/PSG (1:0, 3:1, 1:1, 1:3, 0:1). WPI/PSG composite films had higher water contact angle and water vapor permeability, as well as lower oxygen permeability and light transmittance as compared with single WPI or PSG films. With the increase in PSG concentration, higher film brightness and whiteness index, and smaller total color difference were observed. The tensile strength and elastic modulus of the composite film at WPI/PSG ratio of 1:1 was the highest. Elongation at break of composite films was higher than that of the single films. WPI/PSG composite films were more effective than single films in reducing the surface cracks and degree of cracks. XRD revealed a typical semi-crystalline amorphous structure of the composite films. With the increase of the PSG content, higher diffraction peak strength and crystallinity of the films were observed. The results indicated that the properties of the WPI/PSG composite film were superior to that of PSG or WPI film alone. The composite film at WPI/PSG ratio of 1:1 resulted in the highest comprehensive physicochemical and mechanical performance.

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.