Development of biocomposite films incorporated with different amounts of shellac, emulsifier, and surfactant
Graphical abstract
Introduction
Safety and quality properties of food products which incorporate chemical changes (enzymatic browning and oxidation), microbial stability, sensorial (texture) and physical characteristics, determine the product quality and shelf life (Phan The, Debeaufort, Luu, & Voilley, 2008). The application of edible films and coatings to reduce deteriorative processes, as well as increasing shelf-life and appearance of food, has attracted significant research and industry interest (Bosquez-Molina, Guerrero-Legarreta, & Vernon-Carter, 2003). Edible films made from starch are suitable for food preservation because their resultant films are odorless, tasteless, and transparent with good oxygen barrier properties. Although starch based films have some limitations due to their affinity to water adsorption and retrogradation phenomena which affect their mechanical and barrier properties of films (Cano et al., 2014, Jiménez et al., 2013). One approach to overcome these shortcomings is development of composites with other polymers or reinforcement substances (Ortega-Toro, Jiménez, Talens, & Chiralt, 2014).
Guar gum (GG) derived from a legume plant Cyamopsis tetragolonoba is an appropriate biopolymer for formation of biodegradable films owing to its high molecular weight and wide availability (Saurabh, Gupta, Variyar, & Sharma, 2016). GG is a galactomannan with a backbone of 1, 4-linked β-d-mannose residues and galactose as a side group linked by (1–6) α-d-galactopyranose at every second mannose which establishes short side-branches (Fernandes, Gonçalves, & Doublier, 1993).
Our previous studies demonstrated that GG in combination with pea starch (PS) improved physical, barrier and mechanical properties of films (Saberi et al., 2016a, Saberi et al., 2017). The water vapor permeability (WVP) value of pea starch-guar gum (PSGG) biocomposite films exhibited better moisture barrier characteristics than pure PS films (Saberi et al., 2016b), but these were higher than those of low density polyethylene (LDPE) (Phan The et al., 2008). GG improved the functional characteristics of PS edible film as both biopolymers are hydrophilic; however, the incorporation of hydrophobic substances for decreasing the water sensitivity of biocomposite films is required.
Lipid components including natural waxes, fatty acids, essential oils, surfactants and resins are commonly applied to reduce water vapor transmission rate in the hydrocolloid matrix (Villalobos, Hernández-Muñoz, & Chiralt, 2006). Shellac is currently used as a moisture barrier in the food industry to extend the shelf-life of products (Phan The et al., 2008), and in the pharmaceutical industry for the moisture protection of drugs, controlled drug delivery system and as an enteric coating for drugs and probiotics (Pearnchob et al., 2004, Soradech et al., 2013, Stummer et al., 2010). Shellac is a purified resinous secretion of lac insects, Kerria lacca, a parasitic insect found on trees in Southeast Asia (Phan The et al., 2008). However issues related to the application of shellac include its insolubility in an aqueous system, lack of mechanical strength and lower stability, which lead to the reduction in its use (Limmatvapirat et al., 2007, Luangtana-anan et al., 2007). The production of composite films by combination of two natural polymers and the incorporation of some plasticizers, is a novel approach to counter these issues (Soradech et al., 2013).
In addition, the incorporation of fatty acids has been used to decrease water transmission through edible films. Fatty acids are polar lipids and their chain length and unsaturation degree has a significant effect on film properties (Fernández, de Apodaca, Cebrián, Villarán, & Maté, 2007). Other hydrophobic compounds with the potential to improve film characteristics are surfactants. These compounds, such as Tween-20, are amphiphilic substances, which are necessary for preparation emulsion films with suitable properties (Tongnuanchan, Benjakul, & Prodpran, 2014).
The widespread availability and low cost of PS and GG make the use of these compounds ideal for film formation and in combination with shellac as a hydrophobic substance make the combination of these compounds ideal in the development of packaging films. However it is critical to understand the interaction and relationships between these compounds to optimize film properties. In this study, the influence of different amounts of shellac, surfactant and emulsifier based on dry film matter on mechanical (tensile strength and percent of elongation at break) and barrier properties (WVP) of PSGG based films was investigated using response surface methodology (RSM). In addition, different emulsifiers were added to PSGG-Sh composite film based on optimized amount to determine the appropriate emulsifier to make films with improved mechanical, water vapor barrier and optical properties.
Section snippets
Materials
Canadian non-GMO yellow pea starch with 13.2% moisture, 0.2% protein, 0.5% fat, 0.3% ash, and 36.25 ± 0.32% amylose was used in all experiments (supplied by Yantai Shuangta Food Co., Jinling Town, China). Guar gum (E-412) was purchased from The Melbourne Food Ingredient Depot, Brunswick East, Melbourne, Australia. Food grade shellac was purchased from Castle Chemicals (castlechem.com.au), NSW, Australia. Stearic acid (SA), lauric acid (LA), oleic acid (OA), butyric acid (BA), palmitic acid (PA)
Preliminary studies
Initial studies showed that using a shellac concentration lower than 20% was not adequate to form a compact film structure. Conversely, the films with shellac concentrations greater than 60% were very soft to peel off; therefore concentrations between 20% and 60% shellac based on dry film matter, were selected for further examination. Similarly different amounts of Tween-20 as the surfactant were also exposed to preliminary assessment to examine their effects on the flexibility of PSGG films.
Conclusion
Three-levels Box–Behnken response surface design was applied to evaluate main and interaction influence of different amounts of shellac, SA, and Tween 20 on thickness, moisture barrier, and mechanical characteristics of PSGG films. A film formulation composed of 40% shellac, 1% SA and 0.3 ml Tween 20 provided the film with minimum thickness and WVP, as well as improved mechanical properties. After selecting the best concentration of film components, different emulsifiers, BA, LA, PA, SA and OA
Conflict of interest
The authors declare no conflict of interest.
Acknowledgement
This study was funded by the University of Newcastle, NSW Australia. The authors greatly acknowledge University of Newcastle EMX-ray center for providing access to SEM.
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