Use of phytic acid and hyper-salting to eliminate Escherichia coli O157:H7 from napa cabbage for kimchi production in a commercial plant
Introduction
Kimchi is a traditional Korean side dish produced by processing trimmed, salted, washed, and drained vegetables (e.g., Chinese cabbage, cucumber, and radish) and mixing them with various seasonings (e.g., chopped garlic and ginger, glutinous rice paste, salted and fermented seafood, and sliced radish). The pH of the dish ranges from 4.0 to 4.5 (Kim, 2013, Lee et al., 1998). Kimchi came to the attention of consumers due to a variety of proposed anti-cancer, anti-diabetes, and anti-obesity effects (Choi et al., 2013, Islam and Choi, 2009, Kim et al., 2011, Kim et al., 2014, Kwak et al., 2014). A type of napa cabbage (Brassica rapa sub sp. pekinensis), called baechu in Korea, dà báicài in China, and hakusai in Japan, is the main ingredient of kimchi (CAC, 2001). Salted napa cabbage is also produced as side dish or sold as a ready-to-cook food ingredient in the Asia-pacific region (jjanji or kimchi (Korea), suan cai (China), and tsukemono (Japan)) and marketed worldwide in the form of different products (e.g., can, jar, and pack) (Solomon, 2014, Terebelski and Ralph, 2003).
Although this type of food is generally recognized as safe due to its high salinity (> 2.0%) and low pH (< 4.5), recent microbiological safety issues have thrown this assumption into doubt. Several large outbreaks of pathogenic Escherichia coli in Korea and Japan have been attributed to kimchi and pickled cabbage. In Korea in 2012, 1642 people were infected by pathogenic E. coli after eating kimchi products (Cho et al., 2014). Similarly, Japan suffered outbreaks in 2001 and 2011, in which 20 and 100 patients (seven of whom died), who had eaten kimchi and pickled napa cabbage, respectively, were infected (Iijima et al., 2013, Ozeki et al., 2003). Microbial testing revealed that salted napa cabbage is a major source of coliform bacteria and E. coli (detection rate: 79.0% and 5.0%, respectively) (Rhee et al., 2014). Therefore, steps must be taken to reduce the number of E. coli O157:H7, which is one of the most pathogenic forms of E. coli, and coliform bacteria present in salted napa cabbage used for kimchi production.
Different manufacturers of kimchi use different salinity and salting conditions to prepare salted napa cabbage. For example, the salt type (fine or bay salt), the sodium chloride concentration in the salting solution (8–30%, w/w), salting time (8–18 h), temperature (10–40 °C), and production method (e.g., dipping with pressure, decompression, or salting with tepid water) can all differ (Kim et al., 2009, Lee et al., 2002, Ryu et al., 2014). The appropriate level of salinity for salted napa cabbage used to prepare kimchi is generally between 1.5% and 2.0% (w/w) (Kim et al., 2010, Lee et al., 2009). In the past, some manufacturers used tepid water (30–40 °C) to accelerate the salting process (Ryu et al., 2014); however, warm temperatures can provide a favorable environment for mesophilic bacteria, including spoilage, fecal, or pathogenic bacteria that may pose a microbiological risk. Salting at cold temperatures, however, means that the time taken to achieve the proper level of salinity may be too long (more than 10 h) to be applicable in a commercial processing plant. Thus, the development of a safe and efficient salting method that inhibits bacterial growth at low temperatures (e.g., 10 °C) would be of benefit to the kimchi industry.
Few studies have attempted to devise a safe and effective disinfection procedure for salted and/or pickled napa cabbage. PA (2,3,4,5,6-pentaphosphonooxycyclohexyl dihydrogen phosphate) is a natural plant extract obtained from rice that is used as a food additive in Korea; it is also has a ‘Generally Recognized as Safe Notice’ in the USA (Keefe, 2012, MFDS, 2013). Several studies have examined its anti-cancer, anti-diabetes, and anti-oxidant properties of PA (Henderson et al., 2012, Lee et al., 2006, Norazalina et al., 2010, Stodolak et al., 2007). Although it shows potential as a natural antimicrobial agent, its bactericidal efficacy has not been studied in detail. Recently, we found that PA acts synergistically with sodium chloride to kill E. coli O157:H7 (unpublished data).
Here, was examined whether adding PA to the hyper-salting step during napa cabbage production in a typical kimchi manufacturing plant would have an anti-bacterial effect (Fig. 1). The aim of the study was to examine the antimicrobial efficacy of the PA/sodium chloride (NaCl) combination against E. coli O157:H7 in napa cabbage and to test the method on an industrial scale.
Section snippets
Experimental overview
This study comprised two separate experimental set-ups: 1) laboratory-scale tests (treatment unit of napa cabbage: 20 g) to examine the experimental variables and to determine the appropriate conditions for large-scale validation; and 2) field testing on a large-scale (treatment unit of napa cabbage: 50 kg) to evaluate the actual efficacy of the method for reducing the number of natural flora present in napa cabbage produced under manufacturing conditions. The laboratory tests were based on
Bactericidal effects of hyper-salting with PA
The initial population of GFP-labeled E. coli O157:H7 in prepared napa cabbage samples ranged from 7.8–8.3-log CFU/g. Hyper-salting (20% NaCl) for 30 and 60 min in the absence of PA had no bactericidal effect against E. coli O157:H7 (data not shown). Fig. 2 shows the log reductions in the inoculated E. coli O157:H7 population after hyper-salting in the presence of PA (1–3%) alone (Fig. 2a and c) or PA (1–3%) + NaCl (20% w/w) (Fig. 2b and d) for 30 or 60 min.
Comparisons between groups showed that the
Discussion
The present study examined the antimicrobial efficacy of PA as part of a novel antimicrobial salting procedure developed for the processed vegetable industry in Asia. Although a number of studies have examined the use of synthetic chemical agents (e.g., chlorine, hydrogen peroxide, ethanol, and ozonated water) for washing/sanitizing fresh produce, these methods have some limitations; for example, low antimicrobial efficacy, the generation of by-products and residues, and/or a natural consumer
Acknowledgments
This study was supported by a grant (#13162MFDS045) from the Korea Ministry of Food and Drug Safety (2013–2014) and a Korea University Grant. The authors thank the Institute of Biomedical Science and Food Safety, Korea University Food Safety Hall, for providing the equipment and facilities.
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