Cameron R. Hackney

Use of a modified Gompertz equation to model nonlinear survival curves for Listeria monocytogenes Scott A

The heat resistance of Listeria monocytogenes was determined in 0.1 M KH2PO4 buffer at three temperatures (50, 55, and 60°C), three pH levels (5, 6, and 7), and three NaCl concentrations (0, 2, and 4%). Survival curves were fit using nonlinear regression with a modified Gompertz equation. The Gompertz equation is capable of fitting survival curves which are linear, those which display an initial lag region followed by a linear region, and those which are sigmoidal. Parameter estimates were used to describe the lag region, death rate, and the tailing region of a survival curve. These estimates were also used to predict single and interactive effects of temperature, pH, and percentage of NaCl on the log surviving fraction (LSF) of bacteria. Interactions among these variables significantly (P < .05) affected the LSF. Generally, increased pH or NaCl concentration lead to an increased (P < .05) LSF, where as increased time or temperature lead to a decreased (P < .05) LSF. All multiple factor interactions significantly (P < .05) affected the LSF. These interactions differed depending on the heating medium and the region of the survival curve. The correlation of observed LSF and predicted LSF (R2 = .89) indicated that the Gompertz equation was in close agreement with the observations. This study demonstrated that the Gompertz equation and nonlinear regression can be used as an effective means to predict survival curve shape and response to heat of L. monocytogenes in many different environmental conditions.

Efficacy of ultraviolet light for reducing Escherichia coli O157:H7 in unpasteurized apple cider.

This study examined the efficacy of UV light for reducing Escherichia coli O157:H7 in unpasteurized cider. Cider containing a mixture of acid-resistant E. coli O157:H7 (6.3 log CFU/ml) was treated using a thin-film UV disinfection unit at 254 nm. Dosages ranged from 9,402 to 61,005 microW-s/cm2. Treatment significantly reduced E. coli O157:H7 (P < or = 0.0001). Mean reduction for all treated samples was 3.81 log CFU/ml. Reduction was also affected by the level of background microflora in cider. Results indicate that UV light is effective for reducing this pathogen in cider. However, with the dosages used in this experiment, additional reduction measures are necessary to achieve the required 5-log reduction.

EXAMINATION OF ATTACHMENT AND SURVIVAL OF TOXOPLASMA GONDII OOCYSTS ON RASPBERRIES AND BLUEBERRIES

The consumption of Toxoplasma gondii oocysts on fresh produce may be a means of its transmission to humans. Cats shed T. gondii oocysts, which contaminate produce directly or contaminate water sources for agricultural irrigation and pesticide and fertilizer applications. Cyclospora cayetanensis is a related coccidial parasite, and outbreaks of diarrhea caused by C. cayetanensis have been associated with the ingestion of contaminated raspberries. The oocysts of these coccidians are similar in size and shape, indicating that they may attach to and be retained on produce in a similar manner. In the present study the attachment and survival of T. gondii oocysts on 2 structurally different types of berries were examined. Raspberries and blueberries were inoculated individually with 1.0 × 101 to 2.0 × 104 oocysts of sporulated T. gondii. Berries inoculated with 2.0 × 104 oocysts were stored at 4 C for up to 8 wk. Oocyst viability and recovery were analyzed by feeding processed material to mice. Mice fed T. gondii–inoculated berries stored at 4 C for 8 wk developed acute infections. In other experiments mice fed raspberries inoculated with ≥1.0 × 101 oocysts became infected, whereas only mice fed blueberries inoculated with ≥1.0 × 103 oocysts became infected. This study demonstrates that T. gondii oocysts can adhere to berries and can be recovered by bioassays in mice and that raspberries retain more inoculated oocysts than do blueberries. The results suggest that T. gondii may serve as a model for C. cayetanensis in food safety studies.

The Effect of Sublethal Heat Shock and Growth Atmosphere on the Heat Resistance of Listeria monocytogenes Scott A

Log phase cells of Listeria monocytogenes Scott A were heat shocked in Trypticase Soy + 0.6% yeast extract (TSYE) broth at 48°C for 10 min, followed by heating at 55°C for up to 50 min. Heat resistance was determined using nonselective (TSYE) and selective (McBride Listeria ) enumeration media which were incubated under aerobic and anaerobic environments. D55°C-values for heat shocked cells were 2.1-fold higher than nonheat shocked cells (18.7 min vs. 8.89 min) when cells were enumerated on TSYE agar aerobically and 2.2-fold higher (26.4 min vs. 12.0 min) for cells enumerated anaerobically on TSYE agar. When cells were enumerated aerobically on McBride Listeria (ML) agar, D55°C-values for heat shocked cells were 1.4-fold higher than nonheat shocked cells (9.55 min vs. 6.69 min). No growth was observed on ML agar anaerobically. The physiological condition of the microorganism, the enumeration medium, and the growth environment greatly affected the heat resistance of logphase cells of Listeria monocytogenes Scott A.

Use of a Modified Gompertz Equation to Predict the Effects of Temperature, pH, and NaCl on the Inactivation of Listeria monocytogenes Scott A Heated in Infant Formula

The heat resistance of Listeria monocytogenes was determined in infant formula for all possible combinations of temperature (50, 55, and 60°C), pH level (5, 6, and 7), and NaCl concentration (0, 2, and 4%). Survival curves were fit using nonlinear regression with a Gompertz equation. The Gompertz equation was flexible enough to fit the three most commonly observed survival curves: linear curves, those with an initial lag region followed by a linear region, and sigmoidal shaped. Parameter estimates obtained by the method of nonlinear least squares were used to describe the effect(s) of different heating treatments on the lag region, death rate, and tailing region of survival curves. These estimates were further used to predict single and interactive effects of temperature, pH, and percentage of NaCl on the log of the surviving fraction (LSF) of bacteria. Interactions among these variables significantly (P ≤ .05) affected the LSF. Generally, increased pH or NaCl concentration lead to an increased LSF, whereas increased time or temperature lead to a decreased LSF. All multiple-factor interactions significantly (P ≤ .05) affected the LSF. The correlation of observed LSF versus predicted LSF (R2 = .92) indicated that the estimated Gompertz equation was in close agreement with the observation. This study demonstrated that the Gompertz equation and nonlinear regression can be used as an effective means to predict survival curve shape and response to heat of L. monocytogenes under many different environmental conditions.

Low temperature growth and thermal inactivation of Listeria monocytogenes in precooked crawfish tail meat

Growth of Listeria monocytogenes in precooked crawfish tail meat at 0, 6, and 12°C was determined. Thermal death times were also determined. Growth curves for L. monocytogenes revealed that little multiplication was observable for the entire storage time of 20 d at 0°C. At 6 and 12°C, exponential growth began immediately with no observed lag phase. Generation times of 72.2, 17.0, and 6.9 h were calculated at 0, 6, and 12°C, respectively. Observed D values at 55, 60, and 65°C were 10.23, 1.98, and 0.19 min, respectively. The z value for L. monocytogenes in precooked crawfish tail meat was calculated to be 5.5°C. Results from this study indicate that a refrigeration temperature of 6°C (42.8°F) will support growth of L. monocytogenes and short-term temperature abuse at 12°C will induce very rapid growth of the organism on crawfish tail meat. Thermal treatment values from this study can be used to establish postpicking heat treatments that would eliminate L. monocytogenes from packaged crawfish tail meat prior to retail sale.

Effect of Organic Acids and Hydrogen Peroxide on Cryptosporidium parvum Viability in Fruit Juices

Cryptosporidium parvum has historically been associated with waterborne outbreaks of diarrheal illness. Foodborne cryptosporidiosis has been associated with unpasteurized apple cider. Infectious oocysts are shed in the feces of common ruminants like cattle and deer in and near orchards. In this study, the ability of organic acids and hydrogen peroxide (H2O2) added to fruit juice to inhibit the survival of C. parvum was analyzed. Oocyst viability was analyzed by a cell culture infectivity assay with the use of a human ileocecal cell line (HCT-8) whose infectivity pattern is similar to that for human oral infectivity. Cell monolayers were infected with 10(6) treated oocysts or a series of 10-fold dilutions. Parasitic life stages were visualized through immunohistochemistry with 100 microscope fields per monolayer being counted. In vitro excystation assays were also used to evaluate these treatments. Organic acids and H2O2 were added to apple cider, orange juice, and grape juices on a weight/volume basis. Malic, citric, and tartaric acids at concentrations of 1 to 5% inhibited C. parvums infectivity of HCT-8 cells by up to 88%. Concentrations ranging from 0.025 to 3% H2O2 were evaluated. The addition of 0.025% H2O2 to each juice resulted in a >5-log reduction of C. parvum infectivity as determined with a most-probable-number-based cell culture infectivity assay. As observed with differential interference contrast and scanning electron microscopy, reduced infectivity may be mediated through effects on the oocyst wall that are caused by the action of H2O2 or related oxygen radicals. The addition of low concentrations of H2O2 can represent a valuable alternative to pasteurization.

Predictive model with improved statistical analysis of interactive factors affecting the growth of Staphylococcus aureus 196E

The growth of pathogenic bacteria in foods is affected by several factors which may interact to enhance or inhibit microbial growth. A model to predict the growth of Staphylococcus aureus 196E in microbiological media was developed using a modified Gompertz function and response-surface methodology. The predictive equation required the estimation of 23 parameters which describe singular and interactive effects of the growth factors studied. S. aureus 196E was inoculated into brain heart infusion broth formulated with either 0.5, 4.5, or 8.5% NaCl, adjusted to pH 5.0, 6.0, or 7.0, and incubated aerobically at 12, 20, or 28°C. Several interactive relationships between time, temperature, pH, and NaCl concentration were significant. The model adequately predicted the growth of S. aureus 196E. Predicted responses to multiple-factor interactions were displayed with three-dimensional and contour plots. A second model developed from a smaller subset of the growth data demonstrated that models could be produced with much less data collection. This methodology can provide important information to food scientists about the growth kinetics of microorganisms and prediction ranges or confidence intervals for growth parameters. Consequently, the effects of food formulations and storage conditions on the growth kinetics of foodborne pathogens or spoilage microorganisms could be predicted.

Levels of Vibrio vulnificus and Organoleptic Quality of Raw Shellstock Oysters (Crassostrea virginica) Maintained at Different Storage Temperatures

Temperature abuse during raw oyster harvesting and storage may allow for the multiplication of natural spoilage flora as well as microbial pathogens, thus posing a potential health threat to susceptible consumers and compromising product quality. The objective of this study was to provide a scientific basis for determining whether different refrigeration and abuse temperatures for raw oysters would result in a spoiled product before it became unsafe. Raw shellstock oysters (Crassostrea virginica) purchased from a commercial Virginia processor were subjected to different temperature abuse conditions (7, 13, and 21 degrees C) over a 10-day storage period. Salinity, pH, halophilic plate count (HPC), total culturable Vibrio counts, and culturable Vibrio vulnificus counts were determined at each abuse condition. V. vulnificus isolates were confirmed by a specific enzyme-linked immunosorbent assay. Olfactory analysis was performed to determine consumer acceptability of the oysters at each abuse stage. The pH of the oysters decreased over time in each storage condition. The HPC increased 2 to 4 logs for all storage conditions, while olfactory acceptance decreased over time. V. vulnificus levels increased over time, reaching 10(5) to 10(6) CFU/g by day 6. The length of storage had a greater effect on the bacterial counts and olfactory acceptance of the oysters (P < 0.05) over time than did the storage temperature (P < 0.05).

Nonproteolytic Clostridium botulinum toxigenesis in cooked turkey stored under modified atmospheres.

The ability of nonproteolytic Clostridium botulinum type B spores to grow and produce toxin in cooked, uncured turkey packaged under modified atmospheres was investigated at refrigeration and mild to moderate abuse temperatures. Cook-in-bag turkey breast was carved into small chunks, surface-inoculated with a mixture of nonproteolytic C. botulinum type B spores, packaged in O2-impermeable bags under two modified atmospheres (100% N2 and 30% CO2:70% N2), and stored at 4, 10, and 15 degrees C. Samples were analyzed for botulinal toxin and indigenous microorganisms, as well as subjected to sensory evaluation, on days 0, 7, 14, 28, 42, and 60. Given sufficient incubation time, nonproteolytic C. botulinum type B grew and produced toxin in all temperature and modified atmosphere treatment combinations. At moderate temperature abuse (15 degrees C), toxin was detected by day 7, independent of packaging atmosphere. At mild temperature abuse (10 degrees C), toxin was detected by day 14, also independent of packaging atmosphere. At refrigeration temperature (4 degrees C), toxin was detected by day 14 in product packaged under 100% N2 and by day 28 in product packaged under 30% CO2:70% N2. Reduced storage temperature significantly delayed toxin production and extended the period of sensory acceptability of cooked turkey, but even strict refrigeration did not prevent growth and toxigenesis by nonproteolytic C. botulinum. At all three storage temperatures, toxin detection preceded or coincided with development of sensory characteristics of spoilage, demonstrating the potential for consumption of toxic product when spoilage-signaling sensory cues are absent.