Microbial risk assessment of staphylococcal food poisoning in Korean kimbab

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Abstract

The objective of this study was to conduct a preliminary microbial risk assessment for Staphylococcus aureus in kimbab, and then propose appropriate guidelines for its preparation and storage. Kimbab is a ready-to-eat product, popular in Korea, made of various foods surrounded by rice and seaweed. Because it is usually prepared by hand and stored at room temperature, it can be contaminated with S. aureus. Data on the prevalence, concentration and growth of S. aureus in kimbab as well as typical ambient temperatures in Seoul were collected; probability distributions were then selected to describe these data. A Monte Carlo simulation model was created using @risk. When current Korean retail data on S. aureus prevalence and concentration were used as inputs, the simulation predicted that the S. aureus concentration would be between 4.0 and 4.9 log CFU/g after 1 h of storage at ambient temperature during the warmest part of the year (March through November). Conversely, by assuming a starting concentration of 1 cell/g, the simulation predicted that the S. aureus concentration would reach 4.4 log CFU/g after 5 h of ambient storage in May and 3.4 log CFU/g in October. Our results suggest that, given the current prevalence and concentration of S. aureus found in kimbab at retail outlets, the product should be consumed within 1 h of purchase. Our results also indicate that if Korean consumers wish to safely store kimbab for 5 h at ambient temperature, S. aureus concentration should not exceed 1 CFU/g at the time of preparation.

Introduction

Kimbab is the most popular ready-to-eat food in Korea. It is similar to sushi, but does not contain raw fish. It is made of various foods (e.g. ham, eggs, cooked vegetables) surrounded by rice and seaweed. Because kimbab is usually prepared by hand after all the ingredients have been cooked and cut, it can be contaminated with Staphylococcus aureus. Recently, Park et al. (2005) tested 32 samples of kimbab from several restaurants and found that 20 were contaminated with S. aureus. Kimbab can be eaten promptly after preparation and purchase, but it is typically consumed later. Many Koreans, especially students, enjoy partaking of this product as a picnic lunch, which means it is consumed several hours after preparation. It is considered a potentially hazardous food (Institute of Food Technologists, 2001) and requires time/temperature control. However, people usually store it at room temperature to avoid retrogradation of the rice starch. This makes the rice hard and is considered organoleptically unacceptable by Korean consumers.

The Korean Food Code stipulates that pathogens should not be detected in kimbab (Korea Food and Drug Administration, 2006). Unfortunately, pathogenic microorganisms often are found in this product and the Korea Food and Drug Administration reports that kimbab has been implicated in many foodborne disease outbreaks. Woo (2005) has determined that, for the year 2004, “lunch box” (an assortment of Korean foods commonly served in a plastic box) and kimbab together are the primary cause of foodborne illness outbreaks in Korea, followed by fishery and then meat products. The Korea Food and Drug Administration reports that S. aureus was the third leading causative bacteria of foodborne illness outbreaks in Korea in 2004 (Korea Food and Drug Administration, 2005).

Some research on S. aureus in kimbab has already been conducted. Lee et al. (2004) used a predictive growth model to estimate the maximum edible time of kimbab to be 1.9 h at 30 °C and 17.7 h at 15 °C. Bahk et al., 2005, Bahk et al., 2006 developed a quantitative microbial risk assessment (QMRA) model for S. aureus in kimbab and suggested that it be stored at less than 10 °C. The latter study was limited, however, by using data from only 20 samples in its exposure assessment. More studies are needed to understand the risk factors associated with kimbab preparation and storage so that S. aureus foodborne disease outbreaks associated with this popular Korean food can be reduced.

QMRA is a useful method in the ongoing efforts to ensure food safety. It is traditionally defined as consisting of four components: hazard identification, dose–response assessment, exposure assessment and risk characterization, which together provide numerical expressions of risk (Codex Alimentarius Commission, 1999). Because the estimation of microbial risk inherently contains variability and uncertainty, simulation methods such as Monte Carlo are typically used to conduct QMRA (Schlundt, 2000, FAO/WHO, 2002).

The objective of this study was twofold: (1) to conduct a preliminary microbial risk assessment for S. aureus in kimbab using Monte Carlo simulation and (2) to conduct a scenario analysis of various storage times and initial contamination levels to propose appropriate guidelines for the preparation and storage of this food product.

Section snippets

Initial prevalence and concentration

Two factors, contamination levels of S. aureus in kimbab at time of purchase and the amount of growth before consumption, were considered and a simulation model was constructed to estimate the distribution of the contamination level of kimbab at time of consumption. A description of the parameters used in the simulation model can be found in Table 1.

To estimate the contamination level at time of purchase, data on the prevalence and concentration of S. aureus in kimbab at retail establishments

Consumption time after purchasing

The concentration of S. aureus in kimbab 1 to 6 h after purchase was estimated for the time period of March through November (Fig. 2, Fig. 3, Fig. 4). To demonstrate how the distribution of the contamination level of S. aureus changes over time, the concentration of S. aureus in contaminated kimbab after storage was plotted versus the initial concentration of S. aureus at retail for the month of May. As seen from Fig. 2, S. aureus concentration increases with time, but because storage

Discussion

QMRA can be a useful tool to estimate the risk of pathogen growth in foods and to make decisions to manage food safety. Unfortunately, a lack of data means that risk estimates are uncertain (Hoornstra and Notermans, 2001, Oscar, 2004). One source of uncertainty in the current risk assessment stems from the lack of a published dose–response model for S. aureus toxin. In situations like these, the use of a specific cell concentration to determine if a food is safe may be the best course of action

Conclusions

Given the current prevalence and concentration of S. aureus in kimbab at the retail level, one risk management option might be to suggest that the product be consumed within 1 h of purchase. If consumers wish to store kimbab for 5 h at ambient temperatures, another risk management option might be to stipulate that the S. aureus concentration should not exceed 1 cell per g at the time of purchase. It should be noted that since no data are currently available on kimbab serving sizes and number of

Acknowledgements

This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2005-214-F00022). The authors wish to thank Ms. Kristin Schaffner for her editorial assistance.

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