As the result of a recall of about 4,000 cartons of Del Monte melons (about 60,000 melons) in March of 2011, we are again reminded of the risks due to Salmonella associated with melons. At this time, the exposure seems limited, and apparently traceability worked; the reported number of ill stand at a little more than two dozen cases. The source of these melons appears to be a single farm in Guatemala, and we have yet to determine what went wrong.  However, in the last several years investigators have learned much about the factors that affect disease transmission through fresh produce.

Investigators have known about the melon’s food safety problem since at least the early 90s, when numerous nationwide outbreaks of Salmonella occurred with melons; tens of thousands of cases were reported in these outbreaks. Cantaloupes (musk melons) are most the most frequently identified melons in outbreaks, although watermelon and honeydew (rarely) have also caused outbreaks.


While other pathogens such as E. coli O157:H7 and Listeria can find their way into many crops, Salmonella seems to be the bacterial pathogen of greatest concern with melons. Salmonellosis can be a severe illness and the infection leads to complications with reactive arthritis and other chronic diseases in about 15 percent of cases. It is therefore of utmost importance for public health that only Salmonella-free agricultural products reach the consumer.

Fresh Produce Contamination Sources

The melon supply chain has numerous hazardous points beginning with growing and harvesting and continuing through packing, storage, transport, distribution, processing and final consumption.

Cantaloupe can be contaminated any time the crop is in the ground, but the contamination problem is likely to start when the fruit is ripe and ready to be picked. Animals such as deer, coyotes, raccoons, rodents, feral pigs, and birds are attracted to the crop at this stage. Animal vectors in the growing environment may also include amphibians, reptiles and domestic animals.

Animals can affect the crop directly or through contamination of the water supply used for irrigation and crop protection. Manure may contain animal wastes, and fertilizers pose their own unique threat.

Infected humans are also a risk anywhere in this chain. Fecal matter, especially human waste in the growing or harvesting area, is a very significant risk factor and farmers must strictly control this potential.

Fresh Produce Food Safety Efforts

Current food safety efforts by the produce industry rely upon guidance documents from the U.S. Food and Drug Administration (FDA) on safe growing and handling of fresh fruits and vegetables; but until recently, industry compliance has been voluntary.


In the absence of government mandates, the private sector has developed its own standards for the safe production of many types of fresh fruits and vegetables. Determining an operation’s conformance with the industry prescribed rules is primarily the task of industry auditors who are contracted by the major retailers around the world. Without regulation, there is no legal basis for compliance with food safety requirements. The buyer-driven rules can be circumvented when the supply is low and no alternative source exists. There is little oversight when local producers sell directly to the public or to restaurants. Foreign producers are not necessarily held to the same production standards as those in the U.S., and several U.S. melon outbreaks have involved Mexico and Honduras.

With the passage of the Food Safety Modernization Act in 2011, we can soon expect better regulation and better compliance on the part of producers, both foreign and domestic.  However, the problem of contaminated melons will likely remain, due to the nature of the agricultural methods used and the nature of the plant itself.

Produce Contamination and the Supply Chain

Once contaminated, a cantaloupe will likely remain contaminated until reaching the consumer. Melons travel through a complex series of supply chain distribution steps involving farmers, growers, harvesters, packers, distributers, wholesalers, retailers, processors and food service operations.


Cantaloupes are not typically washed before packing. If they are packed in a packinghouse, they go through a grading process to remove damaged and diseased fruit; they are then loaded either in bins for further distribution or in retail size boxes. Netted bags bay be used, and packers may also individually wrap melons. Like cantaloupes, watermelon and honey dew may also be packed without a wash step, or even packed for final shipment directly to retailers from the field.

Cantaloupe is protected to an extent by an “inedible” peel and rind. Therefore, one would reason that if contamination occurred on the outside of the fruit, the edible portion would be safe until exposed. Until recently, experts assumed little or no growth of bacteria could occur on the hard outer surfaces of a cantaloupe, but recognized that the netted exterior provided an excellent site for the attachment of bacteria. Thanks to new research, we now know that Salmonella can actually penetrate the exterior of the melon, even when no bruising occurs, and Salmonella may also multiply on the outer portions of a melon after attachment. This means that any spot in the supply chain with extended ambient storage conditions will amplify the Salmonella problem, if it is there.

An explosive situation occurs when the internal portions of the melon are exposed to Salmonella from the external surfaces. Melons provide a rich source of nutrients for bacteria, along with more than adequate moisture. If conditions of temperature are hazardous (above 41° F and below 135° F), bacterial multiplication will occur, and this can be very rapid in the range of 70° F to 120° F. Ingestion of just a few Salmonella may be tolerable to some healthy adults, but many strains are able to infect persons in low numbers. When numbers are high, consumers with underlying medical conditions will experience life-threatening effects, and all exposed persons can become ill to varying degrees if infected.

The Role of the Consumer in Produce Safety

All of us have a role in food safety — it is often said that “food safety is a shared responsibility.” While the fresh produce industry works to control problems in the supply chain, the end user, meaning the final consumer, carries much of the burden. This is unfortunate, as reliance upon the food safety efforts of consumers to protect themselves has many pitfalls.

The exposures begin when consumers physically handle melons at the retail level. In fact, touching, knocking and squeezing melons are the standard methods used by consumers to select a ripe melon. While this seems like an innocent activity, it may be another means by which melons can be cross contaminated. Such handling at the point of sale may even allow Salmonella to infect the consumer directly!

Purchasing criteria at the market:

1. Be aware of product recalls and outbreaks. It’s hard to know in a developing crisis what produce in your local store may be affected. This is why it is so important that the fresh produce industry maintains effective traceability systems and recall procedures. You might elect not to purchase any suspected items during an outbreak period.

2. Do not purchase damaged, diseased, or decayed melons.

3. Select
melons carefully, avoid excessive handling or use hand cleansing arrangements, or gloves if provided.  We cannot expect every retail market to provide gloves for consumers, but many do, and some provide hand gel and other hand cleansing aides.

4. Look for a label on the melon. It will be difficult at first to tell if you are looking at the label used at the register for inventory and pricing (Universal Pricing Code-UPC-barcode) or traceability information. The country of origin must be clearly identified. If the market does not have labels on the melons or you cannot understand them, speak to the manager, but consider shopping where you can identify as much as you can about the individual unit you are buying.

5. Place these products into an individual plastic bag (which should be provided). Do not store them unprotected in a cart or in a cloth shopping bag. Go home as soon as possible; do not leave melons in a warm car.

Once you arrive home, you must take other steps:

1. Before storage at home, wash the melon in water just a little warmer than the fruit. The melon will likely warm up on its way home, so it is best to use warm water, not cold. Colder water will increase the absorption of water into the melon and drive bacteria further into the fruit.

2. You can use a moderately stiff- bristle brush to assist in cleaning.

3. A commercial vegetable wash may be used, or you may soak the whole melon in a dilute household bleach and water solution (7 drops per gallon) for a minute or so. But in reality, if the melon has been exposed to Salmonella, you will likely not be able to remove it completely, whatever the cleaning method.

4. After washing, store the whole melon in the refrigerator in a new plastic bag or wrapper. Discard the old wrapper, but maintain traceability information along with your receipt of purchase. The refrigerator thermostat should be set to maintain the melon at 41° F. Provide a refrigerator thermometer independent of the thermostat and adjust the units cooling settings accordingly.

5. Wash your hands and clean the sink and counter and all utensils after handling the melons. You can use a dilute solution of bleach and water or other household disinfectants designed for the kitchen, as a final step.

Melon prep:

1. When preparing the melon, wash your hands before beginning, use a clean cutting board, and use clean utensils, containers and equipment.  Wash all of these items thoroughly after prep.

2. Consider any peels or rind remaining on product, or materials removed, to be inedible, and dispose so as not contaminate the finished item.

3. After processing, slicing, cubing, etc., place in a clean covered container in the refrigerator.

4. Store at 41° F for up to 7 days and then dispose of the remaining items (some bacteria that cause illness can actually grow in a prepared refrigerated product if you give them enough time).

5. Maintain the cut melons at 41° F during extended periods of service. This may require ice, a cooler or other method when away from home. If melons and dishes containing them are stored at room temperature, dispose the item after four hours. These measures imply you have a clock handy and know when products were removed from refrigeration; you must also have an accurate metal-stem pocket thermometer, calibrated to +/- 2° F, using the ice melt- point method.

Consumers as a group are not well-equipped or knowledgeable enough to handle contaminated foods, especially when they will not cook the food. It is clear that while the consumer is the last line of defense for the safety of foods they prepare, the burden of contamination in ready- to-eat agricultural products falls too heavily on consumers. 

The enlightened consumer with effective tools, equipment and know-how may be better able to control risks with melons (and other problematic foods), but the fresh produce industry must continue to find ways to reduce contamination to levels most consumers are prepared to deal with.


Roy Costa writes about food safety in his Food Safety & Environmental Health Blog. See for more information on Costa’s work and food safety education programs.