Myoung-Su Park

Improved multiplex PCR assay for simultaneous detection of Bacillus cereus emetic and enterotoxic strains

Toxin producing Bacillus cereus can cause enterotoxic and/or emetic food poisoning. In the present study, a multiplex PCR assay was developed to detect all toxin genes known to be involved in food poisoning of B. cereus in a single reaction. Specific primers for the detection of enterotoxic (entFM, hblC, nheA, and cytK) genes and emetic toxin production (2 primer pairs: ces, CER) were designed based on the GeneBank sequences. The developed multiplex PCR assay was evaluated in pure culture and artificially inoculated milk, using 43 B. cereus strains and non-target strains. In brief, sensitivity in pure culture was 10-fold or more higher than artificially inoculated milk in multiplex PCR detection limit assay. The presented PCR assay is a developed molecular tool for the rapid simultaneous detection of emetic and enterotoxin producing B. cereus strains.

Modeling the response of Listeria monocytogenes at various storage temperatures in pork with/without electrolyzed water treatment

The objective of this study was to develop a model of the growth of Listeria monocytogenes in pork untreated or treated with low concentration electrolyzed water (LcEW) and strong acid electrolyzed water (SAEW), as a function of temperature. The experimental data obtained under different temperatures (4, 10, 15, 20, 25, and 30°C) were fitted into the modified Gompertz model to generate the growth parameters including specific growth rate (SGR) and lag time (LT) with high coefficients of determination (R2 >0.97). The obtained SGR and LT were employed to develop square root models to evaluate the effects of storage temperature on the growth kinetics of L. monocytogenes in pork. The values of bias factor (0.924–1.009) and accuracy factor (1.105–1.186), which were regarded as acceptable, demonstrated that the obtained models could provide good and reliable predictions and be suitable for the purpose of microbiological risk assessment of L. monocytogenes in pork.

Analysis of microbiological contamination in mixed pressed ham and cooked sausage in Korea.

The objective of this study was to investigate the microbial contamination levels (aerobic bacteria plate count [APC], coliforms, Escherichia coli, Staphylococcus aureus, and Listeria monocytogenes) in mixed pressed ham and cooked sausage. A total of 180 samples were collected from factories with and without hazard analysis critical control point (HACCP) systems at four steps: after chopping (AC), after mixing (AM), cooling after the first heating process, and cooling after the second heating process. For ham, APCs and coliform and E. coli counts increased when ingredients were added to the meat at the AC step. Final product APC was 1.63 to 1.85 log CFU/g, and coliforms and E. coli were not detected. S. aureus and L. monocytogenes were found in nine (15.0%) and six (10.0%) samples, respectively, but only at the AC and AM steps and not in the final product. Sausage results were similar to those for ham. The final product APC was 1.52 to 3.85 log CFU/g, and coliforms and E. coli were not detected. S. aureus and L. monocytogenes were found in 29 (24.2%) and 25 (20.8%) samples at the AC and AM steps, respectively, but not in the final product. These results indicate that the temperature and time of the first and second heating are of extreme importance to ensure the microbiological safety of the final product regardless of whether a HACCP system is in place. Microorganism contamination must be monitored regularly and regulations regarding sanitization during processing should be improved. Education regarding employee personal hygiene, environmental hygiene, prevention of cross-contamination, ingredient control, and step-by-step process control is needed to reduce the risk of food poisoning.

A probability model for enterotoxin production of Bacillus cereus as a function of pH and temperature.

Bacillus cereus is frequently isolated from a variety of foods, including vegetables, dairy products, meats, and other raw and processed foods. The bacterium is capable of producing an enterotoxin and emetic toxin that can cause severe nausea, vomiting, and diarrhea. The objectives of this study were to assess and model the probability of enterotoxin production of B. cereus in a broth model as affected by the broth pH and storage temperature. A three-strain mixture of B. cereus was inoculated in tryptic soy broth adjusted to pH 5.0, 6.0, 7.2, 8.0, and 8.5, and the samples were stored at 15, 20, 25, 30, and 35°C for 24 h. A total of 25 combinations of pH and temperature, each with 10 samples, were tested. The presence of enterotoxin in broth was assayed using a commercial test kit. The probabilities of positive enterotoxin production in 25 treatments were fitted with a logistic regression to develop a probability model to describe the probability of toxin production as a function of pH and temperature. The resulting model showed that the probabilities of enterotoxin production of B. cereus in broth increased as the temperature increased and/or as the broth pH approached 7.0. The model described the experimental data satisfactorily and identified the boundary of pH and temperature for the production of enterotoxin. The model could provide information for assessing the food poisoning risk associated with enterotoxins of B. cereus and for the selection of product pH and storage temperature for foods to reduce the hazards associated with B. cereus.

Analysis of Transfer Rate on Listeria monocytogenes Contaminated Pork Meat During Processing

In this study, the transfer rate of wild type Listeria monocytogenes (LM) was investigated to estab- lish the standard of safety management during pork meat processing for meat to meat and meat to food contact sur- faces contamination at 5 and 10 o C. The transfer rate of LM from meat to meat during the processing increased from 0.02% after 30 min to 0.42% after 120 min at 5 o C, while for conveyor belt and stainless steel, it decreased from 0.015% and 0.013% after 30 min to 0.002% and 0.0003% after 120 min at 5 o C, respectively (p < 0.05). When temper- ature increased to 10 o C, the transfer rates of LM from meat to meat, conveyor belt and stainless steel were the highest at 60 min exposure, and all decreased after 120 min. In reverse, the transfer rate from food contact surface to pork meat was significantly higher than that from pork meat to food contact surface (p < 0.01). Also, the transfer rate to con- veyor belt was significantly higher than stainless steel (p < 0.05) and it was highest at 30 min exposure time in both 5 and 10 o C. This study indicates that the transfer and adherence rates of LM are influenced by the contact time and tem- perature. Consequently, these results were utilized to develop a predictive model with a high level of confidence which can lead to prevent cross-contamination during pork meat processing.

Combination of Slightly Acidic Electrolyzed Water, Ultrasound and Water Wash to Maximize the Sanitization Effect against Microbial Contamination on Lettuce

Microbial contamination of fresh produce is still a major concern to the food industry regardless of all recent improvements in food safety technologies. Among the numerous technologies used slightly acidic electrolyzed water (SlAEW) and ultrasound, are both known as environmental friendly technologies. Although, SlAEW still needs more improvement to be used in food sanitization and ultrasound alone does not effectively reduce microbial numbers in food samples. Hence, the aim of this study was to develop an improved hurdle for fresh produce sanitization using combined treatments of SlAEW (pH 5.4, available chlorine concentration 22 mg/L, oxidation reduction potential 544-592 mV) with ultrasound. Lettuce samples were treated with distilled water (DW), SlAEW, SlAEW followed by water wash (SlAEW + WW), SlAEW with ultrasound (SlAEW + US) and SlAEW with ultrasound followed by water wash (SlAEW + US + WW) at room temperature (24 ± 2 °C), respectively. Following treatments, number of yeasts and molds (YM), total bacteria (TBC) and artificially inoculated Escherichia coli O157:H7 were enumerated. The results showed that ultrasound and water wash alone did not significantly increase the sanitation efficacy of SlAEW. However, treatment of the sample with SlAEW and ultrasound simultaneously, followed by water wash (SlAEW + US + WW) resulted in a great enhancement in microbial reduction. This combined treatment resulted in the reduction of 3.7, 4.1 and 3.8 log CFU/g in YM, TBC and E. coli O157:H7 respectively. Therefore, there is a great chance that this simple hurdle technology can be used in fresh produce as well as other types of food industry to improve the safety of product. Furthermore, addition of mild heat or other treatments should be investigated. This can result in higher numbers of microbial inactivation with cheap, easy to operate and environmental friendly technology.