O. Joseph Scullen

Radio frequency energy effects on microorganisms in foods

Abstract Liquids containing microorganisms were exposed to radio frequency (RF) energy to study non-thermal inactivation. RF energy was applied to the liquids while heat was simultaneously removed to control temperature. Turbulent flow was maintained to minimize localized heating. An 18 MHz RF processor applied an approximately 0.5 kV/cm electric field strength to the liquids. It was capable of pasteurizing the liquids provided that cooling was minimized. There were no non-thermal effects of RF energy detected on Escherichia coli K-12, Listeria innocua , or yeast in apple cider, beer, deionized water, liquid whole egg, and tomato juice; nor were there any synergistic effects of RF energy with heat. The low temperature effects of RF energy at 18 MHz and 0.5 kV/cm were due to heat.

Effects of antimicrobial coatings and cryogenic freezing on survival and growth of Listeria innocua on frozen ready-to-eat shrimp during thawing.

Foodborne pathogens such as Listeria monocytogenes could pose a health risk on frozen ready-to-eat (RTE) shrimp as the pathogen could grow following thawing. In this study, antimicrobial-coating treatments alone, or in combination with cryogenic freezing, were evaluated for their ability to inhibit the growth of Listeria innocua, a surrogate for L. monocytogenes, on RTE shrimp. Cooked RTE shrimp were inoculated with L. innocua at 3 population levels and treated with coating solutions consisting of chitosan, allyl isothiocyanate (AIT), or lauric arginate ester (LAE). The treated shrimp were then stored at -18 °C for 6 d before being thawed at 4, 10, or 22 °C for either 24 or 48 h. Results revealed that antimicrobial coatings achieved approximately 5.5 to 1 log CFU/g reduction of L. innocua on RTE shrimp after the treatments, depending on the inoculated population levels. The coating-treated shrimp samples had significantly (P < 0.05) less L. innocua than controls at each thawing temperature and time. Cryogenic freezing in combination with coating treatments did not achieve synergistic effects against L. innocua. Antimicrobial coatings can help to improve product safety by reducing Listeria on RTE shrimp.

Revised model for aerobic growth of Shigella flexneri to extend the validity of predictions at temperatures between 10 and 19°C

Although Shigella is a major foodborne pathogen, its growth in foods has received little attention. Growth of S. flexneri 5348 inoculated into commercially available sterile foods (canned broths, meat, fish, UHT milk, baby foods) was studied at 10 to 37 degrees C. S. flexneri was enumerated by surface-plating on Tryptic Soy Agar and growth curves were fitted by means of the Gompertz equation. Observed growth kinetics values and values calculated using a previously developed response surface model compared favorably for growth at 19 to 37 degrees C, but not at < 19 degrees C. To refine the model, additional data were collected for growth at 10 to 19 degrees C. A total of 844 tests in BHI broth, representing 197 variable combinations of temperature (10-37 degrees C), pH (5.0-7.5), NaCl (0.5-5.0%) and NaNO2 (0-1000 ppm) was used for the revised model. The revised model, developed in BHI, gave significantly better agreement of calculated growth kinetics values with those observed in foods at 10 to 19 degrees C.

Growth of Shigella flexneri in foods : comparison of observed and predicted growth kinetics parameters

Shigella causes foodborne gastrointestinal illness; however, little information is available on its ability to grow in foods. Commercially available sterile foods (UHT milk, beef broth, chicken broth, vegetable broth, meats, vegetables) were inoculated with S. flexneri 5348 and incubated at 12, 15, 19, 28 or 37 degrees C. Growth curves were fitted from plate count data by the Gompertz equation and exponential growth rates, generation times, lag times and maximum population densities were derived. The observed kinetics values, expressed as T1000 (time, h, required for a 3 log increase in bacterial population), were compared with values calculated using published growth models. Observed and calculated values compared favorably for growth at 19-37 degrees C. S. flexneri grew well in milk at 15-37 degrees C but growth at 12 degrees C was variable. The bacteria readily grew in most foods, even at 12 degrees C; but died off in carrots at 19 and 28 degrees C. Factors other than those used in the growth model may influence bacterial growth in specific foods.

Inactivation of Francisella tularensis Utah-112 on Food and Food Contact Surfaces by Ultraviolet Light

Francisella tularensis is the causative agent of tularemia, a plague-like illness that affects animals and humans, and has caused large illness pandemics in the last century. It has also been used as a biological warfare agent, and tularemia can be contracted through consumption of contaminated food and water. In this study the use of a U.S. Food and Drug Administration approved technology, 254 nm ultraviolet light (UV-C), to inactivate F. tularensis Utah-112 (a rodent pathogen) on food and food contact surfaces was investigated. The D10 value, the UV-C dose needed to inactivate one log of microorganism, was approximately 0.71 mJ/cm2 on agar plates using a low UV-C intensity of 100μW/cm2/s. When a commercial UV-C conveyor was used (5 mW/cm2/s) 0.5 J/cm2 inactivated >7 log CFU of F. tularensis Utah 121 on agar plates. At 0.5 J/cm2 UV-C inactivated >4 log CFU of Utah-112 in beef, chicken, catfish, frankfurter, and bratwurst exudates inoculated onto stainless steel coupons, and >7 log CFU was eliminated at 1 J/cm2 UV-C. Similar results were obtained when the exudates were inoculated onto high density polypropylene. Approximately 0.5 log CFU was inactivated on chicken breast, beef steak, and catfish fillets, and approximately 1.9 log CFU on frankfurters and bratwurst at a UV-C dose of 1 J/cm2. These results indicate routine use of UV-C during food processing would provide workers and consumers some protection against F. tularensis.

Inactivation of Shiga toxin-producing Escherichia coli in lean ground beef by gamma irradiation.

In this study the radiation resistance of 40 Shiga Toxin-Producing Escherichia coli (STEC) isolates which contained various combinations of the shiga toxin 1 (stx1), shiga toxin 2 (stx2), intimin (eae), and hemolysin (ehx) genes were determined. The STEC were suspended in lean ground beef and irradiated at 4 °C. D10 values, the radiation dose needed to reduce 1 log (90%) of a microorganism, ranged from 0.16 to 0.48 kGy, with a mean of 0.31 kGy for the 40 isolates. Isolates associated with illness outbreaks had a mean D10 of 0.27 kGy, while non-outbreak isolates had a mean D10 of 0.36 kGy (p < 0.05). The presence or absence of stx1, stx2, or both stx1 and 2 had no affect on D10 (p > 0.05). The presence (0.30 kGy) or absence (0.35 kGy) of ehx had no affect on D10 (p > 0.05). However, the mean D10 of isolates lacking eae (0.37 kGy) were significantly higher than those containing eae (0.27 kGy) (p < 0.05). There was no difference in D10 for isolates lacking eae regardless of whether or not they were associated with a foodborne illness outbreak (p > 0.05). It may be possible to use some of the STEC isolates which lacked eae, ehx, or both (D10 > 0.30) as avirulent surrogates in food irradiation research. The data presented in this study provides risk assessors data for metagenomic analysis as well as food and radiation processors with valuable information to control of STEC in meat.