A coating for use as an antimicrobial and antioxidative packaging material incorporating nisin and α-tocopherol
Introduction
Most foods deteriorate in quality during transport, processing, and storage through contamination, which occurs by growth of microorganisms, enzymatic or nonenzymatic chemical reactions, and from physical changes (Crosby, 1981; Kilcast & Subramaniam, 2000). Among all these modes of deterioration in quality, microbial spoilage and oxidative reactions have the greatest impact on limiting the shelf life of perishable foods. Packaging can maintain the quality and extend the shelf life of foods (Crosby, 1981; Kilcast & Subramaniam, 2000). To prevent and retard any deterioration in quality in packaged foods, active packaging, including the concept of the release of packaging components to foodstuffs, has showed the greatest potential to improve storage stability (Appendini & Hotchkiss, 2002; Miltz, Passy, & Manneheim, 1995; Vermeiren, Devlieghere, van Beest, de Kruijf, & Debevere, 1999). Antimicrobial packaging can inhibit the growth of pathogenic or spoilage organisms on food surfaces, and thus, can contribute to extending the shelf life of packaged foods. Antioxidative packaging can retard oxidative changes in packaged foods containing fatty components.
Many preservatives, such as sorbic acid, various plant extracts, silver-substituted zeolite, lysozymes, and chlorine dioxide, have been successfully incorporated in packaging materials to confer antimicrobial activity in food packaging (Appendini & Hotchkiss, 2002). Nisin, a natural polypeptide produced by Lactococcus lactis, has been shown to be able to be fabricated into various antimicrobial packaging films (An, Kim, Lee, Paik, & Lee, 2000; Daeschel, McGuire, & Al-Makhlafi, 1992; Lakmraju, Joseph, & Daeschel, 1996; Siragusa, Cutter, & Willett, 1999). Nisin-incorporated films have been reported to possess an antimicrobial activity for Gram-positive bacteria, such as Brochothrix thermosphacta, Lactobacillus helveticus, Listeria monocytogenes, M. flavus,and Pediococcus pentosaceus (An et al., 2000; Daeschel et al., 1992; Siragusa et al., 1999), and thus, have been shown to extend the shelf life of perishable foods by suppressing the growth of spoilage bacteria.
Antioxidants can also be incorporated into or coated onto food packaging materials to control the oxidation of fatty components and pigments, and thus can contribute to the preserved quality of foods (Vermeiren et al., 1999). Incorporation of synthetic antioxidant compounds, such as butylated hydroxytoluene and butylated hydroxyanisole, in high-density polyethylene has been shown to protect cereals from oxidation (Miltz et al., 1988; Wessling, Nielsen, & Andres, 2000). However, because of a growing concern regarding food safety, there is interest in using α-tocopherol in the fabrication of the active packaging materials, because it is a natural antioxidant. It has been reported that α-tocopherol is stable under polymer processing conditions, and a significant concentration of α-tocopherol usually remains in the final plastic films, and this may interact with foodstuffs packaged (Ho, Young, & Yam, 1998; Wessling, Nielsen, Leufven, & Jagerstad, 1999). Low-density polyethylene (LDPE) films incorporating α-tocopherol have been shown to have the potential to enhance the stability of linoleic acid emulsions (Wessling, Nielsen, & Andres, 2000).
Perishable foods that are sensitive to both microbial spoilage and oxidative deterioration may have their preservation properties enhanced by using packaging that has antimicrobial and antioxidative properties, which may be provided by the incorporation of both antimicrobial and antioxidant additives in the polymer matrix. Therefore, in this study, we have fabricated antimicrobial and/or antioxidant packaging materials that incorporate nisin and/or α-tocopherol, and have tested their effectiveness on a model emulsion and on milk cream.
Section snippets
Antimicrobial/antioxidative agents and coating binder
The nisin and α-tocopherol used were purchased from the Sigma Chemical Co. (St. Louis, MO, USA). The binder medium for incorporating the antimicrobial/antioxidant agent used was a vinyl acetate-ethylene copolymer (Elvace 40724; solid content = 54.5%; pH = 4.4; viscosity = 2000 cps), which was obtained from the Rohm and Hass Co. (Philadelphia, PA, USA).
Preparation of antimicrobial and/or antioxidative agent coated paper
The nisin and/or α-tocopherol were coated onto 0.231-mm thick paperboard (Daehan Pulp Co., Chungwon, Korea) according to the method of Kim, An, Park,
Evaluation of antimicrobial and antioxidative activity of the coated paper
The Gram-positive bacterium M. flavus ATCC 10240 was inoculated in the emulsion shown in Fig. 1 to test for antimicrobial activity of the paperboard coated by nisin and/or α-tocopherol in the binder medium. The M. flavus bacterium was selected because of its high susceptibility to antimicrobial packaged films (An et al., 2000; Ha, Kim, & Lee, 2001). The microbial strain had been cultured for 10 h at 30 °C in a nutrient broth medium (Difico Laboratories, Detroit, MI, USA) to reach a cell
Testing the effectiveness of the antimicrobial and antioxidant-coated paperboard on the microbial and chemical stability of milk cream
To carry out these tests, a pasteurized milk cream, ‘Fresh Milk’ (Seoul Milk Cooperative, Seoul, Korea), was purchased from a local supermarket. According to the manufacturer’s data, the cream had a crude fat content of 37–38%. A volume of 50 ml of the milk cream was poured into the same glass cell as used for the migration tests (see Fig. 1), which was then stored at 10 °C. The total aerobic bacterial count and the TBARS value were measured for the milk cream using the same method described
Migration of nisin and α-tocopherol from the coatings
Fig. 2 shows the progress of the migration of nisin and α-tocopherol from the coating to the emulsion at 10 °C. The migration of nisin from the coated paper was complete in eight days, and the maximum equilibrium concentration of nisin released into the solution was in the range of 222–241 μg/ml. This concentration corresponds to 8.6–9.3% of the total nisin content incorporated in the coating layer. Compared to migration of nisin, the migration of α-tocopherol reached a lower equilibrium level
Conclusions
Antimicrobial and/or antioxidant coated-paper was fabricated with a coating of nisin and α-tocopherol contained in a binder of vinyl acetate-ethylene copolymer, and the migration and potential activities in suppressing microbial growth and oxidative deterioration were tested for use in food packaging. At 10 °C, α-tocopherol migrated into an o/w-type emulsion at a faster rate, and reached an equilibrium level of about 6%, based on the initial incorporated concentration, compared to a maximum
Acknowledgements
This work was supported by the Korea Science and Engineering Foundation (Project #1999-2-220-009-4). Chan Ho Lee received a scholarship from the BK21 Program of the Korean Ministry of Education.
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