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From Science & Research

From Science & Research Use of Biopolymers in Antimicrobial Food Packaging The demand for safe, minimally processed, ‘fresh’ food products presents major challenges to the food-packaging industry to develop packaging concepts for maintaining the safety and quality of packaged foods. Recent outbreaks of foodborne pathogens such as Escherichia coli O157:H7, Salmonella spp. and Listeria monocytogenes continue to push for innovative ways to inhibit microbial growth in foods while maintaining quality, freshness and safety. As an additional hurdle to nonthermal processes, antimicrobial packaging can play an important role at reducing the risk of pathogen contamination of minimally processed foods. Antimicrobial packaging systems incorporate antimicrobials into the packaging to prevent microbial growth on the surface of solid foods and to reduce the need for excessive antimicrobials in liquid foods. Currently, food application of an antimicrobial packaging system is limited due to the availability of suitable antimicrobials, new polymer materials, regulatory concerns and appropriate testing methods. Polylactic acid (PLA) is a biodegradable and compostable polymer well known as suitable for different kind of packaging of foods such as milk, water, bakery, cheese, and produce. The special characteristics of PLA, such as GRAS status (i.e. Generally Recognized As Safe (FDA)), biodegradability and being a bio-resource put PLA in a unique position for food applications. Pectin is a water soluble, hygroscopic polymer. Pectin has been used as a thickening, coating and encapsulating material. It can be used as a vehicle to carry and deliver a variety of bioactive substances. Relatively few studies have been reported on the use of pectin or PLA, alone or in combination, as a base packaging material for antimicrobial food packaging. However, neither PLA nor pectin possess antimicrobial properties; therefore, a natural antimicrobial called nisin was combined with the polymers. Nisin is nontoxic, heat stable and does not contribute to off-flavors. Additionally it is commercially used in a variety of foods including dairy, eggs, vegetables, meat, fish, beverages and cereal-based products. In this study, an extruded composite food packaging film containing pectin and polylactic acid polymers was developed. Nisin was loaded into pectin/PLA and PLA films by a diffusion coating method post extrusion. Experiments were conducted to evaluate the potential use of these films in antimicrobial food packaging to inhibit cells of pathogenic Listeria monocytogenes. Listeria monocytogenes was used as a model in this study because of recent implications in several fatal outbreaks of foodborne illness. The presence of L. monocytogenes in ready-to-eat foods is a special concern for at-risk populations. The USDA has set a zero tolerance level for L. monocytogenes in ready-toeat food products. Microbial growth medium (pH 6.9), orange juice (pH 3.8), and liquid egg white (pH 8.7) were selected for this study because they represented neutral, high acid and low acid foods, respectively. PLA from NatureWorks, pectin and nisin (Nisaplin®) from Danisco were used in this project. Adding pectin to PLA slightly reduced film strength but increased film flexibility. The addition of nisin to films had no effect on the film thickness or other mechanical properties. To simulate a test for films used to wrap a solid food, each film sample was placed on a surface-inoculated microbial growth agar plate, on which 106 CFU (Colony Forming Units 1 ) per ml of L. monocytogenes were seeded. The agar plates were incubated at 37 ºC for 24 h. Zones of inhibition were formed after incubation. The larger the zone of inhibition indicated higher antimicrobial activity of the film. Figure 1 indicated that there was a zone of inhibition formed around a film sample containing pectin/PLA/nisin. In contrast, there was no zone of inhibition observed around the film with PLA/nisin, indicating the PLA film lost nisin during the coating process. Therefore, pectin played an important roll at embedding nisin into the film. When used in a liquid medium, nisin was gradually released from pectin/PLA films causing an inhibition 30 bioplastics MAGAZINE [02/09] Vol. 4

From Science & Research Article contributed by Tony Jin and LinShu Liu Eastern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, Wyndmoor, Pennsylvania, USA of bacteria in the liquid medium. Figure 2 suggested the inhibitory effect of pectin/PLA film on L. monocyto-genes in liquid egg white. The cell population of L. mono-cytogenes in liquid egg white with pectin/PLA+nisin film was reduced from 6.8 log units to 2 log units while the film sample without nisin remained at 6.5 log units after 48 hours. When cells of L. monocytogenes were inoculated into orange juice, the cell populations decreased over 48 hours of incubation at 24ºC (Figure 3). Pectin/PLA+nisin film significantly reduced the bacterial populations by 3 to 4 log units from 8 to 48 hours. In this study, pectin/PLA films incorporating nisin showed promise for the inhibition of pathogenic L. monocytogenes in orange juice or liquid egg. The use of pectin and PLA in combination with nisin has a great potential in antimicrobial food packaging to reduce post process growth of food pathogens. The central idea behind the project was to develop a biodegradable PLA and/or pectin-based packaging system that would improve food safety; the incorporation of antimicrobials in the packaging system would target only at food borne pathogens and would not affect the biodegradability of the packaging system under composting conditions. The use of nisin containing films as packaging materials with activity against L. monocytogenes for solid foods, such as meat products, will be published in the Journal of Food Protection. In addition, the antimicrobial packaging systems with PLA or pectin in combination with other antimicrobials against other food borne pathogens will be further explored at the Eastern Regional Research Center, Agricultural Research Service, USDA. The influence of antimicrobials on the biodegradability of the packaging systems will be investigated. The commercial applications of the antimicrobial packaging systems will be evaluated in collaboration with industry partners. 1: After you plate out the milliliter of growth medium (milk, egg white or the like), the individual bacteria will start to multiply. Each one will form a colony, or a visible circle on the growth medium. You can count these, whereas you cannot count the individual bacteria with the naked eye (source: com). Fig. 1. Antilisterial activity of films determined by agar diffusion method. 1. Pectin/PLA film with nisin; 2. Pectin/ PLA film without nisin; 3. PLA film with nisin. Log Colony Forming Unit per ml Log Colony Forming Unit Log per Colony ml Forming Unit per ml 8 7 6 5 4 3 2 1 0 Pectin/PLA on L. 7 monocytogenes in liquid egg white. 6 5 8 4 7 3 6 2 5 1 4 0 Pectin/PLA 0 8 24 48 Incubation time (hour) Fig. 2. Inhibitory effect of pectin/PLA+nisin film 3 2 1 0 Pectin/PLA 0 8 24 48 Incubation time (hour) Pectin/PLA on L. 7 monocytogenes in orange juice. Pectin/PLA+Nisin Pectin/PLA+Nisin 0 8 24 48 Incubation time (hour) Fig. 3. Inhibitory effect of pectin/PLA+nisin film Pectin/PLA+Nisin Pectin/PLA+Nisin ng Unit per ml 6 5 4 bioplastics MAGAZINE [02/09] Vol. 4 31

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