Behavior of Listeria monocytogenes inoculated on cantaloupe surfaces and efficacy of washing treatments to reduce transfer from rind to fresh-cut pieces

Attachment and survival of Listeria monocytogenes on external surfaces (rind) of inoculated cantaloupe, resistance of the surviving bacteria to chlorine or hydrogen peroxide treatments, transfer of the pathogen from unsanitized and sanitized rinds to fresh-cut tissues during cutting and growth, and survival of L. monocytogenes on fresh-cut pieces of cantaloupe were investigated. Surface treatment with 70% ethanol to reduce the native microflora on treated melon, followed by immersion in a four-strain cocktail of L monocytogenes (10(8) CFU/ml) for 10 min, deposited 4.2 log10 CFU/cm2 and 3.5 log10 CFU/cm2 of L monocytogenes on treated and untreated cantaloupe rinds, respectively. L. monocytogenes survived on the treated or untreated cantaloupe rinds for up to 15 days during storage at 4 and 20 degrees C, but populations declined by approximately 1 to 2 log10 CFU/cm2. Fresh-cut pieces prepared from inoculated whole cantaloupes stored at 4 degrees C for 24 h after inoculation were positive for L. monocytogenes. Washing inoculated whole cantaloupes in solutions containing 1,000 ppm of chlorine or 5% hydrogen peroxide for 2 min at 1 to 15 days of storage at 4 degrees C after inoculation resulted in a 2.0- to 3.5-log reduction in L. monocytogenes on the melon surface. Fresh-cut pieces prepared from the sanitized melons were negative for L. monocytogenes. After direct inoculation onto fresh-cut pieces, L. monocytogenes survived, but did not grow, during 15 days of storage at 4 degrees C. Growth was evident by 4 h of storage at 8 and 20 degrees C. It is concluded that sanitizing with chlorine or hydrogen peroxide has the potential to reduce or eliminate the transfer of L. monocytogenes on melon surfaces to fresh-cut pieces during cutting.     

Effect of hydrogen peroxide treatment on microbial quality and appearance of whole and fresh-cut melons contaminated with Salmonella spp

The efficacy of hydrogen peroxide treatment on the inactivation of Salmonella spp. inoculated on the external surface of cantaloupe and honeydew melon was investigated. Salmonella was inoculated onto whole cantaloupe and honeydew melon to a final concentration of 4.65 log(10) CFU/cm(2) and 3.13 log(10) CFU/g, respectively. Inoculated whole melons stored at 5 degrees C for up to 7 days were washed with water, 2.5% and 5% hydrogen peroxide at day 0 and 5. Hydrogen peroxide (2.5% and 5%) treatments of whole melon for 5 min caused a 3 log(10) CFU/cm(2) reduction of the indigenous surface microflora and a 3.0 log(10) CFU/cm(2) reduction in Salmonella spp. on all melon surfaces. The efficacy of the hydrogen peroxide treatments was less when the interval between inoculation and treatment of cantaloupe exceeded 24 h. Unlike cantaloupe fresh-cut pieces, Salmonella was not recovered from fresh-cut pieces prepared from treated whole honeydew melon. Growth of Salmonella occurred in cantaloupe fresh-cut pieces stored at 10 or 20 degrees C, and by 2 weeks, levels reached approximately 1 log CFU/g. A rapid decline in appearance and overall acceptability was observed in fresh-cut pieces prepared from untreated whole cantaloupe. While Salmonella was recovered from fresh-cut pieces from and whole treated cantaloupe, sanitizing the surface of contaminated whole melons with hydrogen peroxide before and after cutting and storage of the fresh-cut pieces at 5 degrees C can enhance the microbial safety and acceptability rating for about 2 weeks after processing.     

Effect of hot water and hydrogen peroxide treatments on survival of salmonella and microbial quality of whole and fresh-cut cantaloupe

Cantaloupe melon has been associated with outbreaks of salmonellosis. Contamination might be introduced into the flesh from the rind by cutting or by contact of cut pieces with contaminated rinds. Our objectives were to investigate the efficacy of hot water or hot 5% hydrogen peroxide treatments in reducing the population of native microflora and inoculated Salmonella on cantaloupe rind and transfer to fresh-cut tissue during cutting. Whole cantaloupes, inoculated with a cocktail of Salmonella serovars to give 4.6 log CFU/cm2 and stored at 5 or 20 degrees C for up to 5 days, were treated with hot water (70 or 97 degrees C) or 5% hydrogen peroxide (70 degrees C) for 1 min at 0, 1, 3, or 5 days postinoculation. Aerobic mesophilic bacteria and yeast and mold on treated whole melon and fresh-cut pieces were significantly (P < 0.05) reduced by all three treatments. Treatments with hot water (70 and 97 degrees C) caused a 2.0- and 3.4-log CFU/cm2 reduction of Salmonella on whole cantaloupe surfaces irrespective of days of postinoculation storage prior to treatment up to 5 days at 5 or 20 degrees C, respectively. Treatment with 5% hydrogen peroxide (70 degrees C) caused a 3.8-log CFU/cm2 reduction of Salmonella. Fresh-cut pieces prepared from untreated inoculated melons and those treated with 70 degrees C hot water were positive for Salmonella. However, fresh-cut pieces prepared from inoculated whole melon dipped in water (97 degrees C) or hydrogen peroxide (70 degrees C) for 60 s were negative for Salmonella, as determined by dilution plating onto agar medium, but were positive after enrichment at days 3 and 5 of storage at 5 degrees C. The ability to detect Salmonella in fresh-cut pieces was dependent on the initial level of inoculation. The results of this study indicate that the use of hot water (97 degrees C) or heated hydrogen peroxide to reduce the population of Salmonella on contaminated whole cantaloupes will enhance the microbial safety of the fresh-cut product.     

Use of hydrogen peroxide in combination with nisin, sodium lactate and citric acid for reducing transfer of bacterial pathogens from whole melon surfaces to fresh-cut pieces

Hydrogen peroxide (2.5%) alone or hydrogen peroxide (1%) in combination with nisin (25 microg/ml), sodium lactate (1%), and citric acid (0.5%) (HPLNC) were investigated as potential sanitizers for reducing Escherichia coli O157:H7 or Listeria monocytogenes populations on whole cantaloupe and honeydew melons. Whole cantaloupes inoculated with E. coli O157:H7 and L. monocytogenes at 5.27 and 4.07 log10 CFU/cm2, respectively, and whole honeydew melons inoculated with E. coli O157:H7 and L. monocytogenes at 3.45 and 3.05 log10 CFU/cm2, respectively, were stored at 5 degrees C for 7 days. Antimicrobial washing treatments were applied to inoculated whole melons on days 0 or 7 of storage and surviving bacterial populations and the numbers transferred to fresh-cut pieces were determined. At days 0 and 7 treatment with HPLNC significantly (p<0.05) reduced the numbers of both pathogens, by 3 to 4 log CFU/cm2 on both types of whole melon. Treatment with HPLNC was significantly (p<0.05) more effective than treatment with 2.5% hydrogen peroxide. While fresh-cut pieces prepared from stored whole melons were negative for the pathogens by both direct plating and by enrichment, fresh-cut pieces from cantaloupe melons treated with 2.5% hydrogen peroxide were positive for both pathogens and pieces from honeydew melons were positive for E. coli 0157:H7. The native microflora on fresh-cut melons were also substantially reduced by HPLNC treatment of whole melons. The results suggest that HPLNC could be used to decontaminate whole melon surfaces and so improve the microbial safety and quality of fresh-cut melons.     

Effect of time before storage and storage temperature on survival of Salmonella inoculated on fresh-cut melons

The effects of a waiting period at room temperature ( approximately 22 degrees C) before refrigerating fresh-cut watermelon, cantaloupe and honeydew pieces contaminated with Salmonella on survival of the inoculated pathogen were investigated. Whole cantaloupes, honeydew melons and watermelons were washed with water, and fresh-cut pieces from individual melons were prepared and inoculated with a five strain cocktail of Salmonella at 10(5)cfu/ml. Populations of aerobic mesophilic bacteria, yeast and mold and Pseudomonas spp. were higher for fresh-cut cantaloupe than for fresh-cut watermelon and honeydew immediately after preparation. Populations of Salmonella, aerobic mesophilic bacteria, yeast and mold and Pseudomonas ssp. in fresh-cut melons left at room temperature for up to 5h before refrigeration were significantly (P<0.05) higher than populations in fresh-cut melons stored at 5 degrees C immediately after preparation. Populations of Salmonella recovered in fresh-cut melon after inoculation with the cocktail of Salmonella strains averaged 2 log(10)cfu/g for all three types of melons. Populations in fresh-cut watermelon and honeydew pieces declined by 1 log when stored immediately at 5 degrees C for 12 days, while the populations in fresh-cut cantaloupe did not show significant (P>0.05) changes. Populations of Salmonella in fresh-cut melons stored immediately at 10 degrees C for 12 days increased significantly (P<0.05) from 2.0 to 3.0 log(10)cfu/g in watermelon, 1.9 to 3.0 log(10)cfu/g in honeydew and 2.0 to 3.6 log(10)cfu/g in cantaloupe pieces. Holding freshly prepared, contaminated fresh-cut melon pieces at 22 degrees C for 3h or more prior to refrigerated storage would increase the chances of Salmonella proliferation, especially if the fresh-cut melons were subsequently stored at an abusive temperature.     

ATP bioluminescence assay for estimation of microbial populations of fresh-cut melon

Estimation of microbial numbers in foods by conventional microbiological techniques takes days, so there is a need for faster methods that can give results in minutes. Research was undertaken to investigate the use of bioluminescent ATP determination and a firefly luciferase assay to estimate the initial population of aerobic mesophilic bacteria on fresh-cut melons immediately after preparation and during storage at 5 or 15 degrees C for up to 12 days. Populations of aerobic mesophilic bacteria on fresh-cut cantaloupe prepared immediately from unsanitized whole melons averaged 3.42 log CFU/g, corresponding to an ATP value of 5.40 log fg/g. Populations for fresh-cut honeydew prepared from unsanitized whole melon averaged 1.97 log CFU/g, corresponding an ATP value of 3.94 log fg/g. Fresh-cut pieces prepared from cantaloupe or honeydew melons sanitized with either chlorine (200 ppm free chlorine) or hydrogen peroxide (2.5%) had similar ATP values: 3.1 log fg/g (corresponding to bacterial counts 1.7 log CFU/g) for cantaloupes and 2.6 log fg/g (corresponding to bacterial counts of 0.48 CFU/g) for fresh-cut honeydew. Positive linear correlations for ATP concentrations and microbial populations were found for fresh-cut cantaloupe (R2 = 0.99) and honeydew R2 = 0.95) during storage at 5 degrees C for up to 12 days. ATP values in fresh-cut melons inoculated with either aerobic mesophilic bacteria or yeast and mold were significantly higher (P < 0.05) than control values and parallel total plate counts on plate count agar. Results of this study indicate that the bioluminescent ATP assay can be used to monitor total microbial populations on fresh-cut melon after preparation and during storage for quality control purposes to establish specific sell-by or consume-by dates.     

Survey of yeasts for antagonistic activity against Salmonella Poona in cantaloupe juice and wounds in rinds coinfected with phytopathogenic molds

Application of yeasts as biocontrol agents to prevent mold decay of fruits and vegetables has been described. We examined 10 yeasts for potential antagonistic activity against survival and growth of Salmonella Poona in cantaloupe juice and decay by Cladosporium cladosporioides and Geotrichum candidum in wounds on cantaloupe rind. Cantaloupe juice was inoculated using five schemes: Salmonella Poona only (1.10 log CFU/ml), high (3.93 to 5.21 log CFU/ml) or low populations (1.79 to 3.26 log CFU/ml) of yeasts only, and Salmonella Poona combined with high or low populations of yeasts. High initial populations of Debaryomyces hansenii, Pichia guilliermondii, and Pseudozyma sp. were antagonistic to Salmonella Poona in cantaloupe juice stored at 20 degrees C for 48 h. Wounds in cantaloupe rinds were inoculated with yeast and mold or yeast, mold, and Salmonella Poona, and cantaloupes were stored at 4 degrees C for 14 days or 20 degrees C for 7 days. The pH of rind tissue inoculated with C. cladosporioides and yeasts increased significantly (P < or = 0.05) at 20 degrees C. Wounds that were inoculated with P. guilliermondii, together with C. cladosporioides or G. candidum, did not show mold growth at 4 and 20 degrees C. Populations of Salmonella Poona (6.40, 7.26, and 7.98 log CFU per sample) were lower in wounds coinoculated with G. candidum and three of the test yeasts (D. hansenii, P. guilliermondii, and Cryptococcus albidus, respectively) compared to coinoculation with G. candidum or the other seven yeasts. Candida oleophila and Rhodotorula glutinis showed the most promise in reducing the population of Salmonella Poona in wounds in rinds of cantaloupes coinoculated with G. candidum and stored at 4 degrees C.     

Effects of cell surface charge and hydrophobicity on attachment of 16 Salmonella serovars to cantaloupe rind and decontamination with sanitizers

Adherence of bacteria to cantaloupe rind is favored by surface irregularities such as roughness, crevices, and pits, thus reducing the ability of washing or sanitizer treatments to remove or inactivate attached cells. In this study, we compared the surface charge and hydrophobicity of two cantaloupe-related outbreak strains of Salmonella Poona (RM2350 and G-91-1595) to those of 14 additional Salmonella strains using electrostatic and hydrophobic interaction chromatography. The relative abilities of the 16 strains to attach to cantaloupe surfaces and resist removal by washing with water, chlorine (200 ppm), or hydrogen peroxide (2.5%) for 5 min after a storage period of up to 7 days at 5 to 20 degrees C also were determined. Whole cantaloupes were inoculated with each pathogen at 8.36 log CFU/ml, dried for 1 h inside a biosafety cabinet, stored, and then subjected to the washing treatments. Only the positive surface charge of the two cantaloupe-related strains of Salmonella Poona was significantly higher (P < 0.05) than that of the other strains. Initial bacterial attachment to cantaloupe surfaces ranged from 3.68 to 4.56 log CFU/cm2 (highest values for Salmonella Michigan, Newport, Oranienburg, and Mbandaka). The average percentage of the total bacterial population strongly attached to the cantaloupe surface for the Salmonella serovars studied ranged from 0.893 to 0.946 at 5 degrees C and from 0.987 to 0.999 at 25 degrees C. Washing inoculated melons with water did not produce a significant reduction in the concentration of the pathogens (P > 0.05). Chlorine and hydrogen peroxide treatments caused an average 3-log reduction when applied 20 to 40 min postinoculation. However, sanitizer treatments applied 60 min or more postinoculation were less effective (approximately 2.5-log reduction). No significant differences were noted in sanitizer efficacy against the individual strains (P > 0.05). The two cantaloupe-related outbreak Salmonella Poona strains did not significantly differ from the other Salmonella strains tested in negative cell surface charge or hydrophobicity, were not more effective in attaching to whole melon surfaces, and were not more resistant to the various washing treatments when present on rinds.     

Effect of nisin in combination with EDTA, sodium lactate, and potassium sorbate for reducing Salmonella on whole and fresh-cut cantaloupet

Nisin (50 microg/ml), EDTA (0.02 M, disodium salt), sodium lactate (NaL, 2%), and potassium sorbate (KS, 0.02%) were tested individually and in various combinations as sanitizer treatments for reducing Salmonella on whole and fresh-cut cantaloupe. Whole cantaloupe and fresh-cut pieces were inoculated with a five-strain cocktail of Salmonella to give 4.76 +/- 0.23 log CFU/cm2 and 3.42 +/- 0.13 log CFU/g, respectively. Inoculated whole melons and fresh-cut pieces were stored at 5 degrees C for 7 days. Washing treatments were applied to inoculated whole melons at days 0, 3, and 7 of storage, and surviving bacterial populations were determined. The effect of the washing treatments on transfer of Salmonella to fresh-cut pieces prepared immediately after treatment was also determined. Directly inoculated fresh-cut pieces were treated at day 0, and surviving bacteria were enumerated at days 0, 3, and 7 of storage. The combination treatments of nisin-EDTA, nisin-NaL, nisin-KS, NaL-KS, and nisin-NaL-KS all resulted in reductions of approximately 3 log CFU/cm2 at day 0 for whole melons. When tested alone, all compounds, along with water washes, were ineffective. After 3 and 7 days of storage, the five combination washing treatments were less effective, resulting in reductions of approximately 2 log CFU/cm2. None of the combination treatments completely eliminated transfer of pathogen survivors to fresh-cut pieces. The combination treatments nisin-NaL, nisin-KS, NaL-KS, and nisin-NaL-KS, but not nisin-EDTA, gave significant (P < 0.05) reductions of Salmonella directly inoculated onto fresh-cut pieces. Washing with nisin-NaL-KS was significantly (P < 0.05) more effective than the other three combination treatments, resulting in a reduction of 1.4 CFU/g. Inhibition by the four effective treatments carried over from day 0 through day 7 of storage, with no increase in the population of Salmonella on the stored fresh-cut pieces. Sensory evaluations indicated that treatment of fresh-cut pieces with nisin-NaL and NaL-KS, but not nisin-KS or nisin-NaL-KS, were acceptable in terms of appearance, odor, and overall acceptability. After the required regulatory approval, treatment of whole cantaloupe with nisin in combination with EDTA, NaL, KS, or NaL and KS and of fresh-cut pieces with nisin-NaL or NaL-KS could help ensure the microbiological safety of fresh-cut cantaloupe.     

Effect of sanitizing treatments on removal of bacteria from cantaloupe surface, and re-contamination with Salmonella

There are many reports of disease due to consumption of cantaloupes contaminated at the surface with enteric pathogens. Salmonella is among the most frequently reported cause of foodborne outbreaks of gastroenteritis in the United States. Research was undertaken to determine the effects of sanitizer and hot water treatments on microbial populations on cantaloupe surfaces and to determine whether prior decontamination of melons by sanitizer treatment affects vulnerability to recontamination by Salmonella. Cantaloupes were sanitized with 200 ppm chlorine or 2.5% hydrogen peroxide solution for 2 min, or hot water (96 degrees C) for 2 min and were held at 5 degrees C for 24 h. Hot water treatments reduced the microbial populations on cantaloupe surface by 4.9 log reduction while H2O2 or chlorine caused approximately 2.6 log unit reduction on cantaloupe surfaces. When sanitized or hot water treated whole cantaloupes were re-inoculated with Salmonella. Higher populations of Salmonella were recovered from sanitized cantaloupes than from the untreated controls; recovery was greater from hot water treated cantaloupes than from cantaloupes treated with chlorine or hydrogen peroxide. The results of this study clearly show that sanitized cantaloupes are susceptible to recontamination if exposed to a human bacterial pathogen during subsequent handling.     

Surface pasteurization of whole fresh cantaloupes inoculated with Salmonella poona or Escherichia coli

Numerous outbreaks of salmonellosis by Salmonella Poona have been associated with the consumption of cantaloupe. Commercial washing processes for cantaloupe are limited in their ability to inactivate or remove this human pathogen. Our objective was to develop a commercial-scale surface pasteurization process to enhance the microbiological safety of cantaloupe. Populations of indigenous bacteria recovered from cantaloupes that were surface pasteurized at 96, 86, or 76 degrees C for 2 to 3 min were significantly (P < 0.05) lower than those of the controls. Whole cantaloupes, surface inoculated with Salmonella Poona RM 2350 or Escherichia coli ATCC 25922 to a final cell concentration of ca. 5 log CFU/cm2 were stored at 4 degrees C or room temperature (RT = 19+/-1 degrees C) for up to 72 h before processing. Treatments at 76 degrees C for 2 to 3 min at 24 h postinoculation resulted in a reduction in excess of 5 log CFU/cm2 of Salmonella Poona and E. coli populations. Cantaloupes that were surface pasteurized and stored at 4 degrees C for 21 days retained their firmness qualities and had no visible mold growth compared with the controls, which became soft and moldy. These results indicate that surface pasteurization will enhance the microbiological safety of cantaloupes and will extend the shelf life of this commodity as well. Storage of untreated inoculated cantaloupes at RT for 24 to 72 h postinoculation caused a significant (P < 0.05) increase in Salmonella Poona and E. coli populations compared with storage at 4 degrees C. This indicates that cantaloupes should be refrigerated as soon as possible following harvest to suppress the growth of any possible contaminant on the rind.     

Attachment of Salmonella Poona to cantaloupe rind and stem scar tissues as affected by temperature of fruit and inoculum

A negative temperature differential between fruits or vegetables and the water in which they are immersed theoretically enhances infiltration of water and any microorganisms it might contain into tissues. The effect of temperature differentials between cantaloupes and wash water, each at 4 and 30 degrees C, on changes in cantaloupe weight and populations of Salmonella enterica Poona recovered from rinds and stem scar tissues of Eastern and Western (shipper) types of cantaloupes was assessed. The percent weight increase in Western cantaloupes was significantly greater (P < or = 0.05) than that in Eastern cantaloupes for all cantaloupe and inoculum temperature combinations. Salmonella Poona attachment to or infiltration of Eastern but not Western cantaloupe rind is enhanced when the fruit is at 4 degrees C, compared with 30 degrees C, regardless of the temperature of the immersion suspension. The number of Salmonella Poona cells recovered from rind tissue of Western cantaloupes at 30 degrees C immersed in inoculum at 30 degrees C was significantly less (P < or = 0.05) than that recovered from rind tissues of cantaloupes at 4 or 30 degrees C that were immersed in inoculum at 4 degrees C. Salmonella Poona in immersion water can adhere to or infiltrate surface tissues of cantaloupes. The populations of Salmonella Poona recovered from stem scar tissues of Eastern and Western types of cantaloupes were not significantly (P > 0.05) affected by cantaloupe and inoculum temperature combinations. Populations of cells adhering to or infiltrating various cantaloupe tissues is not dictated entirely by temperature differentials between fruits and immersion suspensions: rather, it also apparently is influenced by structures unique to surface tissues.     

INFLUENCE OF WASHING TREATMENT ON NATIVE MICROFLORA AND ESCHERICHIA COLI POPULATION OF INOCULATED CANTALOUPES

The influence of chlorine or hydrogen peroxide treatment on populations of Escherichia coli 25922 on the external surface of inoculated cantaloupe was investigated. Surface treatment with 70% EtOH, followed by immersion in 10⁸ CFU/mL E. coli inoculum deposited an average of 4.4 log₁₀CFU/cm² cell population on the cantaloupe surface. The efficncy of washing inoculated cantaloupe was dependent on storage interval between inoculation and treatment. Dipping the cantaloupes in solutions containing 1000 mg/L chlorine or 5% peroxide for 5 min, within 24 h of inoculation, caused a 2 log₁₀ CFU/cm² reduction of the indigenous surface microflora and a 3–4.0 log₁₀ CFU/cm² reduction in E. coli. The efficacy was less when the interval between inoculation and treatment exceeded 24 h. Chlorine appeared in be a better antimicrobial agent than hydrogen peroxide against F. coli ATCC 25922 inoculated on cantaloupe surfaces while hydrogen peroxide was better in reducing surface microflora of cantaloupe.     

Reduction by gaseous ozone of Salmonella and microbial flora associated with fresh-cut cantaloupe

This research investigates the efficacy of gaseous ozone, applied under partial vacuum in a controlled reaction chamber, for the elimination of Salmonella inoculated on melon rind. The performance of high dose, short duration treatment with gaseous ozone, in this pilot system, on the microbial and sensory quality of fresh-cut cantaloupes was also evaluated. Gaseous ozone (10,000 ppm for 30 min under vacuum) reduced viable, recoverable Salmonella from inoculated physiologically mature non-ripe and ripe melons with a maximum reduction of 4.2 and 2.8 log CFU/rind-disk (12.6 cm(2)), respectively. The efficacy of ozone exposure was influenced by carrier matrix. Salmonella adhering to cantaloupe was more resistant to ozone treatment when suspended in skim-milk powder before aqueous inoculation to the rind. This indicated that organic matter interferes with the contact efficiency and resultant antimicrobial activity of gaseous ozone applied as a surface disinfectant. Conversely, in the absence of an organic carrier, Salmonella viability loss was greater on dry exocarp surfaces than in the wetted surfaces, during ozone treatment, achieving reductions of 2.8 and 1.4 initial log CFU/rind-disk, respectively. Gaseous ozone treatment of 5000 and 20,000 ppm for 30 min reduced total coliforms, Pseudomonas fluorescens, yeast and lactic acid bacteria recovery from fresh-cut cantaloupe. A dose Ct-value (concentration x exposure time) of 600,000 ppm min achieved maximal log CFU/melon-cube reduction, under the test conditions. Finally, fresh-cut cantaloupe treated with gaseous ozone, maintained an acceptable visual quality, aroma and firmness during 7-day storage at 5 degrees C. Conclusions derived from this study illustrate that gaseous ozone is an effective option to risk reduction and spoilage control of fresh and fresh-cut melon. Moreover, depending on the timing of contamination and post-contamination conditions, rapid drying combined with gaseous ozone exposure may be successful as combined or sequential disinfection steps to minimize persistence of Salmonella on the surface of cantaloupe melons and transference during fresh-cut processing of home preparation. Based on these results, greater efficacy would be anticipated with mature but non-ripe melons while ripe tissues reduce the efficacy of these gaseous ozone treatments, potentially by oxidative reaction with soluble refractive solids.     

Reducing Salmonella on cantaloupes and honeydew melons using wash practices applicable to postharvest handling, foodservice, and consumer preparation

Washing conditions that included a soak or brush scrub were evaluated for removal of Salmonella from the surface of smooth (honeydew) or complex (cantaloupe) melon rinds. Melon rinds were spot-inoculated onto a 2.5 cm2 area of rind (squares) with approximately 6.0 log(10) CFU/square of an avirulent nalidixic acid-resistant strain of Salmonella typhimurium. Melons were washed by immersion in 1500 ml of water or 200 ppm total chlorine and allowed to soak or were scrubbed over the entire melon surface with a sterile vegetable brush for 60 s. Inoculated sites, uninoculated sites ("next to" sites) that were adjacent to inoculated sites, and sites on the side of the melon opposite (remote sites) the inoculated site were excised and pummeled in a stomacher for 2 min prior to plating onto tryptic soy or bismuth sulfite agar supplemented with 50 microg/ml nalidixic acid. S. typhimurium was reduced on the rind of cantaloupe by 1.8 log CFU/melon after soaking for 60 s in 200 ppm total chlorine, which was significantly better than the 0.7 log CFU/melon achieved with soaking in water. For both water and 200 ppm total chlorine, scrubbing with a vegetable brush was shown to be significantly (0.9 log CFU/cantaloupe) more effective than soaking alone. When honeydew melons were soaked or scrubbed in water, reductions of 2.8 log CFU/melon or >4.6 log CFU/melon (four of five samples), respectively, were observed. However, when water treatments were used, the presence of Salmonella-positive "next to" and remote sites indicated that bacteria were spread from inoculated site on the rind to uninoculated sites either through the rinse water (40-70 CFU/ml of Salmonella) or scrub brush (400-500 CFU/brush). Transfer to other sites occurred more often with cantaloupe than honeydew melons. This transfer was eliminated when 200 ppm total chlorine was used. When 200 ppm total chlorine was used, Salmonella could not be detected in the water or on the scrub brush. For optimal microbial removal in food service and home settings, melons should be scrubbed with a clean brush under running water. However, to ensure the benefits of brushing, instructions for cleaning and sanitizing brushes must also be emphasized. For food service settings where concentration and pH can be adequately measured, the use of chlorinated water may provide additional benefit.     

Use of green fluorescent protein expressing Salmonella Stanley to investigate survival, spatial location, and control on alfalfa sprouts.

Laser scanning confocal microscopy (LSCM) was used to observe the interaction of Salmonella Stanley with alfalfa sprouts. The green fluorescent protein (gfp) gene was integrated into the chromosome of Salmonella Stanley for constitutive expression, thereby eliminating problems of plasmid stability and loss of signal. Alfalfa seeds were inoculated by immersion in a suspension of Salmonella Stanley (ca. 10(7) CFU/ml) for 5 min at 22 degrees C. Epifluorescence microscopy demonstrated the presence of target bacteria on the surface of sprouts. LSCM demonstrated bacteria present at a depth of 12 microm within intact sprout tissue. An initial population of ca. 10(4) CFU/g seed increased to 7.0 log CFU/g during a 24-h germination period and then decreased to 4.9 log CFU/g during a 144-h sprouting period. Populations of Salmonella Stanley on alfalfa seeds decreased from 5.2 to 4.1 log CFU/g and from 5.2 to 2.8 log CFU/g for seeds stored 60 days at 5 and 22 degrees C, respectively. The efficacy of 100, 200, 500, or 2,000 ppm chlorine in killing Salmonella Stanley associated with sprouts was determined. Treatment of sprouts in 2,000 ppm chlorine for 2 or 5 min caused a significant reduction in populations of Salmonella Stanley. Influence of storage on Salmonella Stanley populations was investigated by storing sprouts 4 days at 4 degrees C. The initial population (7.76 log CFU/g) of Salmonella Stanley on mature sprouts decreased (7.67 log CFU/g) only slightly. Cross-contamination during harvest was investigated by harvesting contaminated sprouts, then directly harvesting noncontaminated sprouts. This process resulted in the transfer of ca. 10(5) CFU/g Salmonella Stanley to the noncontaminated sprouts.     

INHIBITION OF LISTERIA MONOCYTOGENES BY NATIVE MICROFLORA OF WHOLE CANTALOUPE1

Fresh‐cut cantalcupe has been recalled due to the possible presence of Listeria monocytogenes. Several studies have reported that naturally occurring microflora of vegetable surfaces may be antagonistic to pathogen attachment, growth or survival. To test this possibility for L. monocytogenes and cantaloupes, whole melon were treated with water, ethanol (70%) or chlorine (200 ppm) to reduce the native microflora on the melon surfaces. Treated or untreated melons were immersed in a six strain cocktail of L. monocytogenes (10⁷ CFU/mL) for 10 min and then allowed to dry for 1 h inside a biosafety cabinet followed by storage at 5, 10 and 20C for 15 days. Fresh‐cut pieces prepared from the treated or untreated melons and directly inoculated with L. monocytogenes (3.48 log CFU/g) were stored under the same conditions listed above. Populations of L. monocytogenes and five classes of native microflora were investigated. Growth of L. monocytogenes in sterile or nonsterile rind and fresh‐cut homogenates was also studied. The population of L. monocytogenes recovered from inoculated (10³ to 10⁸ CFU/mL) whole melons given no disinfection treatment or washed with water was significantly less (P < 0.05) than that recovered from melons treated with chlorine or EtOH. In general, populations of L. monocytogenes declined on the surface of treated and untreated whole melons and on fresh‐cut pieces over the 15 days storage period at the temperatures tested. However, the decline in pathogen populations was less rapid in the presence of reduced populations of native microflora. Higher populations of L. monocytogenes were attained in sterile tissue homogenates than in nonsterile homogenates. Addition of yeast and mold to sterile rind homogenates was highly inhibitory to growth and survival of the pathogen. The results of this study indicate that native microflora of whole cantaloupe inhibited attachment to rind surfaces as well as survival and growth of L. monocytogenes on cantaloupe surfaces and homogenized fresh‐cut pieces. Thus, L. monocytogenes recontamination of melons having a reduced level of native microflora following application of a disinfection treatment may be a food safety concern.     

Effect of vacuum-steam-vacuum treatment on microbial quality of whole and fresh-cut cantaloupe

Minimally processed fruits and vegetables have a limited shelf life because of deterioration caused by spoilage microflora and physiological processes. Cutting may increase microbial spoilage of fruits through transfer of microflora on the outer surfaces to the interior tissue. The objectives of this study were to use the vacuum-steam-vacuum (VSV) process to reduce indigenous spoilage microflora on the surface of cantaloupes and to investigate the effects of such treatments on transfer of spoilage microflora from the cantaloupe surface to the fresh-cut melon during rind removal and cutting. Whole cantaloupes were treated in the VSV processor, and fresh-cut pieces prepared from treated and control samples were stored at 5 and 10 degrees C for up to 9 days. Presence and growth of mesophilic bacteria, yeasts and molds, and Pseudomonas spp. were determined in fresh-cut samples during storage. Texture and color (CIE L*, a*, and b*) also were measured during storage. VSV treatment resulted in a 1.0-log reduction of aerobic mesophilic bacteria, a 2.0-log reduction of yeasts and molds, and a 1.5-log reduction of Pseudomonas spp. on cantaloupe surfaces. VSV treatment significantly reduced transfer of yeasts and molds and Pseudomonas spp. from whole cantaloupe surface to fresh-cut pieces during preparation (P < 0.05). Texture and color of the fresh-cut pieces prepared from the VSV-treated whole melons were similar to those of the controls. The results of this study indicate that the use of the VSV process to reduce the surface populations of yeasts and molds and Pseudomonas spp. on whole cantaloupes will reduce subsequent transfer of these microbes to fresh-cut pieces and enhance the microbial quality of the fresh-cut product.     

Metabiotic associations of molds and Salmonella Poona on intact and wounded cantaloupe rind

Salmonella Poona, a serotype rarely implicated in human infections, has recently caused several cantaloupe-associated outbreaks of salmonellosis. Metabiotic associations of molds and foodborne pathogens on produce have been reported. We tested proteolytic activity and measured changes in the pH of cantaloupe rind caused by growth of Alternaria alternata, Cladosporium cladosporioides, Epicoccum nigrum, Geotrichum candidum, and Penicillium expansum. Survival and growth characteristics of Salmonella Poona co-infected with each mold on the surface rind and in wounded rind tissue as affected by temperature were determined. C. cladosporioides, G. candidum, and P. expansum, but not A. alternata and E. nigrum, showed proteolytic activity on agar media containing gelatin and/or casein, with concurrent increases in pH, thus favoring survival and growth of salmonellae. Intact and mechanically wounded tissue of cantaloupe rinds were inoculated with a five-strain mixture of S. Poona and/or test mold. Five inoculation schemes were used: mold only, S. Poona only, mold and S. Poona simultaneously, mold then S. Poona 3 days later, and S. Poona then mold 3 days later. The pH of cantaloupe rinds inoculated with molds and stored at 20 degrees C for 14 days was significantly higher (P < or =0.05) than on day 0. Only the pH of rinds inoculated with C. cladosporioides or G. candidum was significantly higher (P < or =0.05) on day 21 than on day 0, when cantaloupes were stored at 4 degrees C. An initial population of S. Poona increased from 3.3 log(10) cfu/sample (ca. 7 cm(2)) of cantaloupe rind to populations as high as 9.5 log(10) cfu/sample during storage at 20 degrees C for up 14 days, regardless of co-inoculation with molds. Populations of S. Poona decreased or remained constant at 4 degrees C for up to 21 days. Results demonstrate that persistence and growth of S. Poona on intact, wounded, and decaying cantaloupe rind are not markedly affected by the presence of molds.     

Method of applying sanitizers and sample preparation affects recovery of native microflora and Salmonella on whole cantaloupe surfaces

Standardized methods for applying sanitizer treatments to cantaloupes and for recovering surviving native microflora or Salmonella on inoculated cantaloupe after sanitizing are lacking. Accordingly, the objectives of this study were to compare four methods for applying sanitizers (dipping, dipping with rotation, dipping with agitation, and dipping with rubbing) using 200 ppm of chlorine or 5% H2O2, two recovery methods (homogenization of rind plugs in a stomacher or blender), and five selective recovery media for Salmonella. Whole cantaloupes were submerged in a cocktail of five strains of Salmonella (each at approximately 2 x 10(8) CFU/ml) for 10 min and allowed to dry for 1 h inside a biosafety cabinet and stored at 20 degrees C for approximately 23 h before sanitizing. The recovery of Salmonella from whole cantaloupe without sanitizing averaged 5.09 log CFU/cm2 by blending and 4.30 log CFU/cm2 by homogenization in a stomacher for the five selective agar media. Microbial populations (Salmonella or the indigenous aerobic mesophilic bacteria, gram-negative bacteria, lactic acid bacteria, Pseudomonas spp., and yeast and mold) were not significantly (P > 0.05) reduced by treating with water regardless of the treatment method used. Sanitizing with chlorine or H2O2 by dipping, with or without rotation for 2 min, also did not reduce microbial populations. However, populations of all classes of native microflora and Salmonella were significantly (P < 0.05) reduced by sanitizer treatments (2 min) applied with agitation or by rubbing. In general, sanitizer treatments applied by rubbing resulted in greater log reductions (by up to 1.7 log unit) than for treatments applied with agitation. Populations of native microflora and Salmonella recovered from cantaloupe were higher (by up to 1.8 log unit) by blending compared to homogenization in a stomacher. In most instances, selective media used did not differ significantly (P > 0.05) for recovery of Salmonella after washing treatments.