Merle D. Pierson

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.

Chemical, nutritional and physiological aspects of dry bean carbohydrates—A review

Abstract The current knowledge of dry bean carbohydrates related to their composition, nutritional value and physiological attributes in humans is reviewed. Dry bean carbohydrates represent up to 60% of the total seed weight and starch is the major constituent. Molecular and physicochemical properties of legume starches are also discussed. Data to indicate the possible involvement of the raffinose family of oligosaccharides in flatulence production are given.

Antibacterial effects of hydrogen peroxide and methods for its detection and quantitation

Hydrogen peroxide is responsible for certain bactericidal effects observed in biological systems, such as growth inhibition of one bacterial species by another and killing of invading microorganisms by activated phagocytic cells. H2O2 might be generated in bacteriological media by their exposure to light and/or oxygen and become an important mediator of toxic effects. H2O2 cytotoxicity is apparently due to its capacity-generally mediated by transition metal ions-to generate more reactive and cytotoxic oxygen species such as the hydroxyl radical, which is a powerful oxidant, and which can initiate oxidation of biomolecules. The conversion of H2O2 into more cytotoxic compounds may be potentiated by reducing agents and by peroxidases. Cells may protect themselves against H2O2 toxicity either by the action of catalases or, in the case of DNA damage, by repairing the damage after it has taken place. Assays for the detection and quantitation of H2O2 in cell cultures include those based on (i) catalase-dependent oxidation of formate to CO2, (ii) generation of fluorescent products due to a H2O2- mediated oxidative reaction, (iii) the loss of fluorescence upon the oxidation of scopoletin, (iv) change in absorbance upon oxidation of phenol red, or (v) formation of complexes with peroxidases. Some possible antimicrobial uses of H2O2 in the food industry are presented.

Efficacy of UV light for the reduction of Listeria monocytogenes in goat's milk.

Certain types of goats cheeses are produced using unpasteurized milk, which increases the food safety concerns for these types of products. Popularity and consumption of goats milk products have increased, and the niche market includes gourmet goats cheeses. The U.S. Code of Federal Regulations and the Pasteurized Milk Ordinance both address the possibility for processing alternatives to heat treatment, and the use of UV light treatment may be a viable alternative that still ensures the safety of the product. Fresh goats milk was inoculated with Listeria monocytogenes (L-2289) at 10(7) CFU/ml and exposed to UV light using the CiderSure 3500 apparatus (FPE Inc., Macedon, NY). Inoculated milk was exposed to a UV dose range between 0 and 20 mJ/cm2 to determine the optimal UV dose. A greater than 5-log reduction was achieved (P < 0.0001) when the milk received a cumulative UV dose of 15.8 +/- 1.6 mJ/cm2. The results of this study indicate that UV irradiation could be used for the reduction of L. monocytogenes in goats milk.

Nitrite, nitrite alternatives, and the control of Clostridium botulinum in cured meats.

Historically, nitrite has been a component of meat-curing additives for several centuries. In recent years the safety of nitrite as an additive in cured meats has been questioned mainly because of the possible formation of carcinogenic nitrosamines. Nitrite has many important functions in meat curing including its role in color development, flavor, antioxidant properties, and antimicrobial activity. The inhibition of Clostridium botulinum growth and toxin production is an especially important antimicrobial property of nitrite. This review discusses the effects of processing, curing ingredients (especially nitrite), and storage of cured meats in relation to the control of C. botulinum. If nitrite is eliminated from cured meats or the level of usage decreased, then alternatives for the antibotulinal function of nitrite need to be considered. Several potential alternatives including sorbates, parabens, and biological acidulants are discussed.

Influence of Environmental Stress on the Kinetics and Strength of Attachment of Listeria monocytogenes Scott A to Buna-N Rubber and Stainless Steel

Attachment and detachment of Listeria monocytogenes Scott A to Buna-N rubber and stainless steel under varying conditions of temperature and pH were investigated using model systems. Numbers of attached cells increased with increasing attachment temperature (10 to 45 degrees C) and time (up to 120 min) for both test surfaces. Compared to Buna-N rubber, the rate of attachment to stainless steel was markedly more rapid for all temperature and pH conditions studied and could not be calculated. Rate of attachment to Buna-N rubber was found to be significantly lower when cells were attached at 10 degrees C. Growth temperature did not significantly affect rates of adhesion to Buna-N rubber. Altering the medium pH during attachment between 4 and 9 demonstrated that rates of adhesion were slower under alkaline conditions. Growth pH was also found to significantly affect rates of attachment to Buna-N rubber. Detachment of cells adhered to Buna-N rubber was significantly affected by growth temperature but not growth pH. Significant differences in detachment were also found between Buna-N rubber and stainless steel, inferring stronger attachment to Buna-N rubber. Cell surface hydrophobicity was found to be affected by both growth temperature and growth pH. However, changes in hydrophobicity could not be correlated to differences in rates of attachment. Addition of 0.01% trypsin to the attachment medium during cell exposure to either test surface resulted in a 99.9% reduction in the adhered cell population when compared to controls. This would suggest that proteins play a role in the initial attachment process of L. monocytogenes.

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.

Effect of environmental stress on the ability of Listeria monocytogenes Scott A to attach to food contact surfaces.

Attachment of Listeria monocytogenes Scott A to Buna-N rubber and stainless steel under different temperature and pH conditions at the time of cell growth or at the time of attachment was investigated. All experiments were conducted using sterile phosphate buffer to avoid cell growth during exposure to the test surfaces. Numbers of attached cells increased with increasing attachment temperature (10 to 45 degrees C) and exposure time for both test surfaces. Maximum levels of attached cells were obtained when cell growth occurred at 30 degrees C. Downward, but not upward, shifts in the cell suspension holding temperature prior to attachment to Buna-N rubber resulted in reduced adhered cell populations. Maximum levels of adhered cells to Buna-N rubber were not affected by adjustments of the attachment medium pH between 4 and 9. However, after short contact times (i.e., less than 30 min), levels of attached cells were lower when attachment occurred under alkaline conditions. Growth pH was also found to affect the levels of adhered cell populations to Buna-N rubber. L. monocytogenes Scott A attached to stainless steel at higher levels for all temperature and pH parameters evaluated in this study.

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.