Growth of Salmonella Spp. in Cantaloupe, Watermelon, and Honeydew Melons

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Growth of Salmonella spp. in Cantaloupe, Watermelon, and Honeydew Melons

DAVID A. GOLDEN*, E. JEFFERY RHODEHAMEL, and DONALD A. KAUTTER

Division of HACCP Programs, Food and Drug Administration, Washington, DC 20204 Downloaded from http://meridian.allenpress.com/jfp/article-pdf/56/3/194/1665082/0362-028x-56_3_194.pdf by guest on 26 September 2021 (Received for publication August 14, 1992)

ABSTRACT than 400 laboratory confirmed Salmonella poona infections and occurred in 23 states and Canada (5). Most illnesses in The ability of Salmonella spp. to grow on the interior tissues these two outbreaks were associated with the consumption of of cantaloupe, watermelon, and honeydew melons was investigated. cantaloupe from salad bars or in fruit salads. Some of the fruit Pieces of rind-free melons (pH 5.90-6.67) and tryptic soy broth salads also contained honeydew melon and watermelon. (TSB, pH 5.90) were inoculated with a mixed culture (approxi mately 100 CFU/g or ml) containing equal proportions of five The Food and Drug Administration (FDA) conducted species of Salmonella (S. anatum, S. Chester, S. havana, S. poona,fiel d surveys on imported melons during 1990 and 1991 in an and 5. senftenberg). Inoculated melon pieces and TSB were incu attempt to identify and eliminate the practices responsible for bated for 24 h at 5 or 23°C. Viable populations of salmonellae were melon rind contamination. Results of the surveys revealed determined by surface plating test portions on Hektoen enteric agar. that in 1990 and 1991, 0.76 and 1.06% of the melon rinds, Results indicated that Salmonella growth was rapid and prolific on respectively, harbored a variety of Salmonella spp. on their the melons and in TSB at 23°C incubation. Final populations on surfaces. Because these melons grow on the ground and are watermelons were approximately 1.0 log10 greater than populations exposed to microorganisms, they have the potential to be on cantaloupe and honeydew and in TSB. Although viable Salmo come contaminated with pathogens. The association of ill nella populations on melons and in TSB did not increase during the 24-h incubation at 5°C, little or no decrease in viable populations nesses with salad bars and fruit salads suggests that salmonel was observed. lae may have been introduced into the fruit from the rind by the physical act of cutting the melon, or contact by cut pieces Salmonellosis has been steadily increasing as a public of melon with contaminated rinds. In addition, the contami health problem in the United States since reporting began in nated fruit may have remained at room temperature for 1943 (10). Salmonella is the most frequently reported cause several hours after preparation (5). of foodborne outbreaks of gastroenteritis in the United States This study was undertaken to examine the ability of (4). Poultry and other meat products, eggs, and dairy products Salmonella spp. to grow on the interior tissues of cantaloupe, are the most commonly implicated foods in Salmonella watermelon, and honeydew melons at storage temperatures of outbreaks. Fruits and vegetables are implicated less often. In 5 and 23°C. fact, human salmonellosis attributed to the consumption of fruits and vegetables occurred in only 2.2% of all known MATERIALS AND METHODS cases reported to the Centers for Disease Control between 1983 and 1987 (8). Inoculum preparation Gayler et al. (7) reported that Salmonella miami and Five Salmonella spp. (S. anatum, S. Chester, S. havana, S. Salmonella bareilly were responsible for two salmonellosis poona, and S. senftenberg) were used in this investigation. Cultures outbreaks associated with precut wrapped watermelon. They were obtained from Dean Wagner, FDA Midwest Laboratories for showed that interior watermelon tissue could become con Microbiological Investigation, Minneapolis, MN. Test cultures were taminated if Salmonella was present either on the rind of the of the same serotypes as those isolated from melons during FDA watermelon or on the knife used for slicing. Salmonella field surveys. Cultures were grown at 35°C for 24 h in tryptic soy oranienburg was responsible for another watermelon-associ broth (TSB; Difco Laboratories, Detroit, MI), and viable popula ated outbreak and involved 18 cases (3). The possibility that tions of each species were determined by diluting each culture in watermelons were held at room temperature for long periods Butterfield's phosphate buffer (BPB) and surface plating 0.1 ml on tryptic soy agar (Difco). Appropriately diluted cultures were com of time was correlated with all three outbreaks. bined in a sterile test tube to obtain a mixed Salmonella culture, Two recent multistate outbreaks of salmonellosis were which contained approximately equal proportions of each of the associated epidemiologically with cantaloupes. The first in five species. The mixed culture was further diluted in buffer to volved Salmonella Chester and affected at least 245 individu provide an initial population of approximately 100 salmonellae per als (two deaths) in 30 states (9). The second involved more g or ml. SALMONELLA IN MELONS 195 Sample preparation ml, appropriately diluted in BPB, onto Hektoen enteric agar (Difco). Whole cantaloupes, watermelons, and honeydew melons were Viable Salmonella populations in TSB were determined similarly, purchased from retail markets in the Washington, DC area. The except that no BPB was added and test portions were not blended exterior surfaces of melons were washed thoroughly with soapy in a stomacher before plating. Hektoen enteric agar plates were water, rinsed with tap water, and towel dried. Melon surfaces were incubated at 35°C for 24 h before colonies were enumerated. Two then rinsed with 70% ethanol, blotted with paper towels, and replicate analyses were performed. allowed to air-dry. Rinds were carefully removed using stainless steel knives and utensils that had been sanitized with 70% ethanol. RESULTS AND DISCUSSION The inner seed portion of cantaloupes and honeydew melons was removed; seeds were not removed from watermelons. Melons were cut into cube-shaped pieces (approximately 25 g), which were The pH of uninoculated cantaloupe, watermelon, and placed into plastic stomacher bags and inoculated with 0.5 ml of the honeydew was 6.67, 5.90, and 5.95, respectively. Analysis of mixed Salmonella culture described above. Sterile TSB (9.0 ml) controls initially and after 24 h revealed that no salmonellae was inoculated with 0.2 ml of the mixed Salmonella culture. TSB or other endogenous microflora were recovered on HE A from was used as a control medium for culturing Salmonella to provide the interior tissues of melons. Results of this study show that a basis by which growth on melons could be compared. Before Salmonella is capable of rapid and prolific growth on canta Downloaded from http://meridian.allenpress.com/jfp/article-pdf/56/3/194/1665082/0362-028x-56_3_194.pdf by guest on 26 September 2021 inoculation, TSB was adjusted to pH 5.9 with 1 N HOI. A pH meter loupe, watermelon, honeydew, and in TSB (Fig. 1) when equipped with a flat-surface probe was used to determine the pH of melon pieces before inoculation with salmonellae. The pH was not incubated at 23°C. Although growth rates at 23°C were monitored during the 24-h incubation period. Inoculated melon similar on all melons and in TSB, final Salmonella popula pieces (in unsealed bags) and TSB were incubated aerobically at 5 tions incubated on watermelons were approximately 1.0 log10 or 23°C for 24 h. Populations of endogenous Salmonella and/or greater than those incubated on cantaloupe and honeydew background microflora were determined in controls that were and in TSB. Although viable Salmonella populations on examined initially and after 24 h of incubation. melons and in TSB did not increase during 24-h incubation at 5°C, little or no decrease in viable populations was ob Enumeration of salmonellae served. Duplicate test portions (25 g) of melons and TSB were taken at intervals throughout the 24-h incubation. Viable Salmonella Other investigators (6,7) have also reported that interior populations on melon pieces were determined by adding 18 ml of watermelon tissues support the growth of Salmonella spp. BPB to each stomacher bag, blending 1 min in a Stomacher 400 Gayler et al. (7) inoculated the cut surface of a watermelon (Dynatech Laboratories, Alexandria, VA), and surface plating 0.1 with S. miami, stored the melon at room temperature for one

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8 10 12 14 16 18 20 22 24 18 20 22 24 Incubation time (hours) Figure 1. Growth of Salmonella spp. in cantaloupe, watermelon, honeydew melon, and tryptic soy broth, incubated at 5 and 23°C. 196 GOLDEN, RHODEHAMEL AND KAUTTER day, and obtained innumerable Salmonella colonies on bis and sanitized utensils and surfaces when preparing cut mel muth sulfite agar. However, the authors failed to report the ons, maintain cut melons at or below 7°C (45°F), and limit initial inoculum size or the final population attained on the display or service of cut melons to 4 h if they are not watermelon. Escartin et al. (6) studied the growth of S. typhi refrigerated. in a sterile suspension of watermelon in distilled water (up to It is impossible to ensure that all domestic and imported 20% wt/vol; pH 5.03) at 22°C and reported a maximum melons are free of pathogens. The melon's natural biological population of 7.53 log10 CFU/ml. The final Salmonella popu barrier provides some protection against the invasion of lation on watermelon after 24 h in our study was 8.63 log10 foodborne pathogens until this barrier is compromised by CFU/g. This difference may reflect the dilution of nutrients in slicing and cutting. Application of the model food codes that the watermelon/distilled water suspension by Escartin et al. specify cleaning melons with potable water, and using clean, (6) in contrast to our use of watermelon cubes. Escartin et al. sanitized utensils and surfaces will help to minimize the (6) observed enhanced growth of Salmonella when the con potential for contaminating interior melon tissues. These centration of watermelon in distilled water was increased. practices along with proper temperature and time manage Other bacteria, including pathogens, also proliferate on ment of cut melons should significantly reduce the risk of watermelon tissues. Abbey et al. (1) reported that the pre contracting salmonellosis from consumption of fresh melonDownloaded from http://meridian.allenpress.com/jfp/article-pdf/56/3/194/1665082/0362-028x-56_3_194.pdf by guest on 26 September 2021 s dominant microflora naturally occurring on cut watermelon (2). included Pseudomonas spp., Escherichia coli, Enterobacter spp., and micrococci. They also inoculated Listeria monocy ACKNOWLEDGMENTS togenes into sterile watermelon juice and reported that the organism reached the exponential growth phase within 18 h We thank Dean Wagner, FDA Midwest Laboratories for Microbio logical Investigation, Minneapolis, MN, for supplying the Salmonella at 25°C but was not recovered after 21 d of incubation at 5°C cultures used in this study. (1). FDA field surveys of imported cantaloupe and water REFERENCES melon have shown a relatively low incidence of Salmonella on the exterior surfaces of intact melons. Because the meth 1. Abbey, S. D., E. K. Heaton, D. A. Golden, and L. R. 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