Physico-mechanical and antimicrobial properties of gelatin film from the skin of unicorn leatherjacket incorporated with essential oils
Graphical abstract
Highlights
► Bergamot and lemongrass oil as glycerol substitute affected the mechanical properties of gelatin film. ► Gelatin film incorporated with bergamot and lemongrass oils had antimicrobial activity. ► Properties and antimicrobial activity of gelatin film were governed by essential oils.
Introduction
Nowadays, eco-friendly materials from natural polymers have gained increasing attention for the development of packaging to reduce environmental pollution and ecological related problems caused by non-biodegradable plastic packaging (Petersen et al., 1999). Packaging is widely used for the protection of food quality, thereby ensuring hygiene and extending the shelf life of perishable items, especially those susceptible to oxidative and microbiological deterioration (Coma, 2008). Proteins, lipids and polysaccharides are employed for the preparation of bio-based packaging for food industry (Bao, Xu, & Wang, 2009). Gelatin is one of the versatile biomaterial obtained from collagen by the controlled hydrolysis of the fibrous insoluble collagen present in the bones and skin generated as waste during animal slaughtering and fish processing (Ahmad, Benjakul, Ovissipour, & Prodpran, 2011). It has been known for the excellent film forming ability (Hoque, Benjakul, & Prodpran, 2010). Gelatin-based film used for coating or packaging could maintain the quality of foods during storage, due to its barrier to oxygen, light and prevention of dehydration and lipid oxidation (Jongjareonrak, Benjakul, Visessanguan, & Tanaka, 2011). Several properties of gelatin film, such as mechanical properties, permeability, light absorption, transparency, antimicrobial activity and antioxidant ability, are influenced by the addition of active substances (Pires et al., 2009). Essential oils derived from natural sources have been added to the film as antimicrobial agent (Pereda, Ponce, Marcovich, Ruseckaite, & Martucci, 2011).
Essential oils categorised as GRAS (generally recognised as safe) by U.S. Food and Drug Administration (Persico et al., 2009) can be considered as the potential alternatives to synthetic additives (Valentao et al., 2002). Essential oils especially from bergamot and lemongrass appear as interesting natural compounds with great potential use in foodstuffs preservation. In vitro studies have revealed significant antimicrobial effects of these essential oils (Lv, Liang, Yuan, & Li, 2011; Naik, Fomda, Jaykumar, & Bhat, 2010). Thus the incorporation of bergamot essential oil and lemongrass essential oil into gelatin films offers the possibility not only of imparting antimicrobial activity, but also improving the physicochemical properties. In general, essential oils, the odorous volatile products of an aromatic plant's secondary metabolism, are well-known as antimicrobial agents that could be used to control food spoilage and food-borne pathogenic bacteria (Kalemba & Kunicka, 2003). Essential oil-based antimicrobial packaging has high potential for food applications (Emiroglu, Yemis, Coskun, & Candogan, 2010). The antimicrobial activity of essential oils is assigned to a number of small terpenoids and phenolic compounds (thymol, carvacrol, eugenol) (Oussalah, Caillet, Saucier, & Lacroix, 2007). Terpenes have the ability to disrupt and penetrate into lipid structure of the cell wall of bacteria, leading to denaturation of proteins and destruction of cell membrane, cytoplasmic leakage, cell lysis and eventually cell death (Emiroglu et al., 2010). The decrease in pH that occurs due to cell membrane disruption resulted in a loss of control of cellular processes such as DNA transcription, protein synthesis and enzyme activity (Oussalah et al., 2007). Essential oils also penetrate into mitochondrial membrane, leading to the greater permeability of organelle and the K+ ion leakage process (Oussalah et al., 2007).
Unicorn leatherjacket (Aluterus monoceros) is one of the marine tropical fish, which is harvested in large quantities throughout the world. This species has been used for fillet production in Thailand, in which a large quantity of skin has been produced as by-product (Ahmad & Benjakul, 2010). Due to its thick skin, it can be a potential source for gelatin production (Ahmad & Benjakul, 2011). As a consequence, the increased revenue for processors can be achieved. Furthermore, gelatin from unicorn leatherjacket skin can be used for smart film/packaging with antimicrobial activity. Selected natural antimicrobial agents, especially essential oils can be incorporated into films to provide their preventive role. Therefore, the objectives of this investigation were to prepare and characterise the gelatin film from unicorn leatherjacket skin and to assess the antimicrobial property of film added with bergamot and lemongrass essential oils.
Section snippets
Chemicals
Essential oils of bergamot (BO, Citrus bergamia) and lemongrass (LO, Cymbopogon citratus) were purchased from Botanicessence essential oils (Suanlung, Bangkok, Thailand). Bovine serum albumin (BSA) and high molecular weight protein markers were purchased from Sigma Chemical Co. (St. Louis, MO, USA). Glycerol, phosphoric acid, p-dimethylaminobenzaldehyde and tris(hydroxymethyl) aminomethane were obtained from Merck (Darmstadt, Germany). Sodium dodecyl sulphate (SDS), N,N,N′,N′-tetramethyl
Thickness
Thickness of gelatin films incorporated with bergamot (BO) and lemongrass oil (LO) at various concentrations is shown in Table 1. Thickness of films incorporated with BO (5–25%) increased in comparison with the control (P < 0.05). It ranged from 0.045 to 0.048 mm. However, no significant difference in thickness was observed between films incorporated with BO in the range of 10–25% (P > 0.05). The results suggested that peptide chains of gelatin could not form the compact film network in the
Conclusion
The properties of gelatin films were affected by the addition of BO and LO. The BO and LO incorporated films with porous structure showed the decreased tensile performance (i.e., tensile strength and elongation at break). Both BO and LO affected the film differently due to the different modes of action determined by compounds presented in essentials oils. Films incorporated with LO exhibited more antibacterial activity than BO added films against microorganisms studied. The films were generally
Acknowledgements
The authors would like to express their sincere thanks to the TRF senior research scholar program and Graduate School, Prince of Songkla University for the financial support.
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