Susan S. Sumner

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.

Antibacterial Activity of the Lactoperoxidase System: A Review

The lactoperoxidase (LP) system is a naturally occurring system which was first discovered in raw milk. Different groups of bacteria show a varying degree of resistance to the LP system. Gram-negative catalase-positive organisms, such as pseudomonads, coliforms, salmonellae, and shigellae, are inhibited by the LP system. Depending on the medium pH, temperature, incubation time, cell density, and the particular donor, these microorganisms may be killed. It has been shown that the LP system can increase storage times of raw milk by delaying growth of psychrotrophs; perhaps this method could be used to extend the shelf life of other foods.

Stability of Fumonisins in Thermally Processed Corn Products

Little is known about the stability of fumonisins in corn-based foods during heating. This study investigated the effects of canning, baking, and roasting (dry heating) processes on the stability of fumonisins in artificially contaminated and naturally contaminated corn-based foods. All samples were analyzed for fumonisin levels by both a commercial enzyme-linked immunosorbent assay (ELISA) and a high-performance liquid chromatographic (HPLC) method. Canned whole-kernel corn showed a significant (P < or = 0.05) decrease in fumonisins by both ELISA (15%) and HPLC (11%) analyses. Canned cream-style corn and baked corn bread showed significant (P < or = 0.05) decreases in fumonisin levels at an average rate of 9% and 48%, respectively, as analyzed by ELISA. Corn-muffin mix artificially contaminated with 5 micrograms of fumonisin B1 (FB1) per g and naturally contaminated corn-muffin mix showed no significant (P < or = 0.05) losses of fumonisins upon baking. Roasting cornmeal samples artificially contaminated with 5 micrograms of FB1 per g and naturally contaminated cornmeal samples at 218 degrees C for 15 min resulted in almost complete loss of fumonisins.

Survival of Escherichia coli O157:H7 and Salmonella in Apple Cider and Orange Juice as Affected by Ozone and Treatment Temperature

Inactivation of Escherichia coli O157:H7 and Salmonella in apple cider and orange juice treated with ozone was evaluated. A five-strain mixture of E. coli O157:H7 or a five-serovar mixture of Salmonella was inoculated (7 log CFU/ml) into apple cider and orange juice. Ozone (0.9 g/h) was pumped into juices maintained at 4 degrees C, ambient temperature (approximately 20 degrees C), and 50 degrees C for up to 240 min, depending on organism, juice, and treatment temperature. Samples were withdrawn, diluted in 0.1% peptone water, and surface plated onto recovery media. Recovery of E. coli O157:H7 was compared on tryptic soy agar (TSA), sorbitol MacConkey agar, hemorrhagic coli agar, and modified eosin methylene blue agar; recovery of Salmonella was compared on TSA, bismuth sulfite agar, and xylose lysine tergitol 4 (XLT4) agar. After treatment at 50 degrees C, E. coli O157:H7 populations were undetectable (limit of 1.0 log CFU/ml; a minimum 6.0-log CFU/ml reduction) after 45 min in apple cider and 75 min in orange juice. At 50 degrees C, Salmonella was reduced by 4.8 log CFU/ml (apple cider) and was undetectable in orange juice after 15 min. E. coli O157:H7 at 4 degrees C was reduced by 4.8 log CFU/ml in apple cider and by 5.4 log CFU/ml in orange juice. Salmonella was reduced by 4.5 log CFU/ml (apple cider) and 4.2 log CFU/ml (orange juice) at 4 degrees C. Treatment at ambient temperature resulted in population reductions of less than 5.0 log CFU/ml. Recovery of E. coli O157:H7 and Salmonella on selective media was substantially lower than recovery on TSA, indicating development of sublethal injury. Ozone treatment of apple cider and orange juice at 4 degrees C or in combination with mild heating (50 degrees C) may provide an alternative to thermal pasteurization for reduction of E. coli O157:H7 and Salmonella in apple cider and orange juice.

Response of Salmonella and Escherichia coli O157:H7 to UV Energy

To determine the efficacy of a UV light treatment at 253.7 nm (UVC light) on microbial growth, plates containing tryptic soy agar plus 50 ppm of nalidixic acid (TSAN) were inoculated with known concentrations of five-strain cocktails of Salmonella and Escherichia coli O157:H7 and subjected to different UVC treatments. The concentration of the cocktail inoculum was determined with TSAN prior to inoculation. Serial dilutions were carried out, and inoculation levels of 10(0) to 10(8) CFU/ ml were tested for each pathogen. Multiple replications of doses of UV light ranging from 1.5 to 30 mW/cm2 were applied to different cocktail concentrations, and doses of > 8.4 mW/cm2 resulted in a 5-log reduction of Escherichia coli O157:H7, while a 5-log reduction of Salmonella was observed with doses of > 14.5 mW/cm2. Results for both organisms yielded sigmoidal inactivation curves. UVC light is effective in reducing microbial populations of pathogens on agar surfaces.

Microbial evaluation of vegetable ingredients in salad bars

Vegetable salad ingredients (lettuce, tomatoes, broccoli, and cauliflower) purchased from three grocery-store deli operations were analyzed for total plate count, coliforms, yeasts, and molds. The temperature of the vegetable ingredients was measured at the time of purchase and the pH was measured on all samples within one-half hour after purchase. In the second phase, fresh broccoli was processed into florets, inoculated with E. coli ATCC 23742, and subjected to three washing treatments. The temperature of the salad ingredients ranged from 5.1°C to 18.9°C. The pH ranges for the vegetables were broccoli, 5.46 to 6.39; cauliflower, 5.82 to 6.65; lettuce, 4.92 to 6.38; and tomatoes, 3.30 to 4.47. The total aerobic count for the vegetables ranged from 5.51 to 6.63 log CFU/g. Coliforms on the vegetables ranged from 4.89 to 6.30 log CFU/g. Yeasts and molds were found on all vegetables. The results of the study indicate that the temperature conditions and pH ranges for the broccoli, cauliflower, and lettuce could support microbial growth. The pH range of the tomatoes was below 4.6, but if contaminated and added to low acid foods, the tomatoes may also act as a vehicle for microbial contamination. When a chlorine wash solution was used, it slightly reduced the aerobic microbial load on previously inoculated broccoli and reduced the coliform population of the broccoli by approximately one log unit.

Inhibition of Salmonella typhimurium on Agar Medium and Poultry Skin by Ultraviolet Energy

Ultraviolet radiation (UV) was effective in destroying Salmonella typhimurium on agar plates and poultry skin. Agar plates inoculated with varying numbers of colony-forming units (CFU) of S. typhimurium (1.2 x 10(2) to 1.7 x 10(9) were subjected to different doses of UV light to determine optimal killing. Poultry skin was also inoculated with varying CFU of S. typhimurium per 2 cm2 of skin and subjected to UV light. UV light treatment of inoculated agar plates revealed almost complete elimination (99.9%) of S. typhimurium at 2,000 microW x s x cm(-2). Bacterial reduction was less effective on the surface of poultry skin when a 80.5% reduction in S. typhimurium was obtained at 2,000 microW x s x cm(-2).

Occurrence of Fumonisins in Corn-Based Food Products†

Corn-based food products obtained from commercial outlets in three different parts of the U.S., Maryland, Nebraska, and Arizona were analyzed for total fumonisins by a commercial competitive direct enzyme-linked immunosorbent assay (CD-ELISA) and for fumonisin B1 (FB1) by high-performance liquid chromatography (HPLC). The highest fumonisin concentrations were found in samples collected in Maryland, where all 18 samples were found positive for fumonisins (200 to 7,450 ng/g of food) by CD-ELISA and 15 of the 18 samples (83%) were found positive for FB1 (< 75 to 5,916 ng/g) by HPLC. Fumonisins were also detected by CD-ELISA in 14 of 15 samples collected in Arizona with concentrations ranging from 200 to 1,450 ng/g, but analyses by HPLC showed that only 8 of 15 samples (53%) were positive for FB1 (< 75 to 1,565 ng/g of food). Of the 23 samples collected in Nebraska, 20 (87%) were positive for fumonisins (200 to 2,500 ng/g) by CD-ELISA, but only 10 (44%) were positive for FB1 (< 75 to 927 ng/g) by HPLC. The highest fumonisin and FB1 concentrations were found in cornmeal samples, ranging up to 7,450 ng/g of cornmeal by CD-ELISA and 5,916 ng/g by HPLC. These findings indicate that there may be a risk of human exposure to fumonisins through the consumption of some corn-based foods.

Internalization of Salmonella enterica serovar Montevideo into greenhouse tomato plants through contaminated irrigation water or seed stock.

Tomatoes have been linked to outbreaks of salmonellosis, demonstrating the need to identify sources of contamination. Objectives of this study included determining the ability for Salmonella enterica serovar Montevideo to be internalized into tomatoes from contaminated irrigation water and seed stock, and establishing whether Salmonella Montevideo can survive in fertilizer solutions. Six treatment groups (five plants per group) were irrigated with 350 ml of 7 log CFU/ml of Salmonella Montevideo every 14 days for 70 days, each group receiving an increased number of contaminated water events progressively: group 1 received one contaminated watering at day 0, and group 6 received a total of six contaminated waterings. Group 7 was a control, and group 8 was grown from seeds soaked in 8 log CFU/ml of Salmonella Montevideo for 24 h. All plants were watered daily with uncontaminated water. Three replications were completed. Fruit from every plant, and roots, stems, and leaves of one plant per treatment were sampled. All tomatoes were negative for Salmonella Montevideo; five root samples tested positive. For fertilizer studies, a commercially available fertilizer, two custom mixed and 1.0% dilutions of each (total of six solutions), and sterile water were inoculated with 8 log CFU/ml of Salmonella Montevideo and stored at 25 degrees C. Solutions were sampled at 24, 48, and 72 h. There were no differences (P > or = 0.05) between survival of Salmonella Montevideo in diluted fertilizers and the control. Results indicate Salmonella Montevideo is unable to contaminate tomato fruit via irrigation water and seed stock but can survive in fertilizer solutions.

Detection Method for Histamine-Producing, Dairy-Related Bacteria using Diamine Oxidase and Leucocrystal Violet

A detection method for histamine-producing, dairy-related bacteria was developed that involves a two-step sequential enzyme system. First, isolated bacteria are incubated in MRS broth or trypticase soy broth fortified with histidine. The histamine formed during this incubation period is reacted with diamine oxidase, which catalyzes the oxidation of histamine to form imidazole acetaldehyde, ammonia, and hydrogen peroxide. The hydrogen peroxide is then detected by the formation of crystal violet from the leuco base in the presence of horseradish peroxidase. Liquid culture medium containing bacteria that produce greater than 1200 nmole histamine per ml will develop a positive purple color. Cultures containing bacteria that produce little or no histamine will not develop a purple color. Other amines often found in cheese, such as tyramine, cadaverine, or putrescine, will not interfere with the color formation.