Inactivation of Listeria monocytogenes on agar and processed meat surfaces by atmospheric pressure plasma jets
Highlights
► Pathogen inactivation of atmospheric pressure plasma (APP) jets was investigated. ► N2 + O2 gas mixture was the best for inactivation. ► APP jets were effective for inactivation of Listeria monocytogenes on meat surfaces. ► Shelf-life extension was also expected by APP jets.
Introduction
In recent years, as consumer interest in food safety has increased, many studies have been performed to identify means of securing foods against contamination with foodborne pathogens such as Escherichia coli, Salmonella typhimurium, Staphylococcus aureus, and Listeria monocytogenes (Kim et al., 2008). According to data from the Korea Food and Drug Administration, the number of foodborne diseases in 2008 increased 3.8-fold compared to the number in 2003. In the US, it was reported that foodborne pathogens result in 325,000 hospitalizations and 5000 deaths every year (WHO, 2007).
Thermal treatment has been used for many decades for the inactivation of microorganisms. However, its use is limited due to negative effects on the sensorial, nutritional, and functional characteristics of heat-sensitive foods (Awuah et al., 2007). Therefore, in order to develop appropriate sterilization methods without adverse changes to food qualities, researchers have developed non-thermal treatments such as irradiation, high pressure processing, application of natural antimicrobials, and active packagings (Aymerich et al., 2008, Devlieghere et al., 2004).
Among recently developed non-thermal treatments, the use of low temperature atmospheric pressure plasmas (APPs) has garnered much attention. Gas plasmas are ionized gases in a quasineutral condition. They consist of ions, electrons, and neutral particles, including atoms, molecules, radicals, and UV photons (Gweon et al., 2009, Wan et al., 2009). Many of the chemical species and UV light have been shown to be lethal toward microorganisms (Moisan et al., 2002).
APP has been used for surface modification, environmental, and biomedical applications (Bogaerts et al., 2002, Gweon et al., 2010). Although direct comparisons are not possible because of different APP systems employed for different studies, APP treatment is generally considered to be a candidate method for ensuring food safety during processing (Lee et al., 2006). Thus, Niemira and Sites (2008) applied APP to apples and observed efficient microbial inactivation with no changes in surface color or texture. Basaran et al. (2008) investigated the inactivation effects of an SF6 and air APP on different kinds of nut surfaces and found it very effective. Moon et al. (2009) reported that there was no electrical or thermal damage to pork or human skin when samples of those tissues were sterilized by He APP.
Other scientists have investigated the factors that possibly determine the inactivating effects of APP. Song et al. (2009) reported that L. monocytogenes inoculated on sliced cheese and ham was effectively reduced or eliminated by large area-type APP, but input power, exposure time, and the type of food affected the efficiency of inactivation. Yun et al. (2010) obtained similar results using inoculated food containers such as plastic trays, paper cups, and aluminum foil. Further, Kim et al. (2011) and Ragni et al. (2010) found that no quality changes in bacon and eggshell occurred in response to APP treatments.
However, the application of APP to food safety improvement is still very limited. In addition, based on previous studies, there are recommendations to develop a specific APP system for food application (Song et al., 2009, Yun et al., 2010). It is also necessary to evaluate the wide range of nutritional and quality attributes of APP treated foods for obtaining the evidences and general acceptance as a food decontamination process. Therefore, the objective of this study was to evaluate the efficiency of APP jets in the inactivation of L. monocytogenes on a model agar system and real food system, including slices of chicken breast and ham.
Section snippets
Sample preparation
Slices of chicken breast (Harim Co., Ltd., Iksan, Korea) and ham (CJ Co., Ltd., Jincheon, Korea) were purchased from a local market in Daejeon, Korea. Prior to the inoculation test, sliced chicken breast and ham were vacuum-packaged and sterilized by irradiation (40 kGy) using a cobalt-60 gamma irradiator at the Advanced Radiation Technology Institute, Jeongup, Korea. The agar plate used was composed of tryptic soy agar (Difco, Becton Dickinson Sparks, MD, USA).
Microorganism and inoculation
L. monocytogenes KCTC 3596
Results and discussion
Fig. 2 shows the effect of APP jets generated with different gas mixtures on the inactivation of L. monocytogenes cells seeded on agar plates. The numbers of L. monocytogenes were reduced by 0.87, 4.19, 4.26, and 7.59 log units after 2 min exposure with APP jets of He, He + O2, N2, or N2 + O2, respectively. The D-values, calculated from those reductions, of APP jets against L. monocytogenes were 135.19, 21.83, 21.70, and 7.72 s using He, He + O2, N2, or N2 + O2, respectively. Song et al. (2009)
Acknowledgment
This work was supported by a grant from the Next-Generation BioGreen 21 Program (No. PJ0081330), Rural Development Administration, Republic of Korea.
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