"Best Practices for a Pathogen-free Picnic", with Angela Fraser. Ms. Fraser began by deflating the "mayo myth" explaining that mayonnaise has such a low (high acid) pH, it does not readily support pathogens. Foods with a pH above 4.6, however, are considered "hazardous" when it comes to food poisoning, and if the mayo is mixed with less acidic foods the mix can become more problematic. Then she really caught my attention. She explained how the high, less acidic, pH level [~6.1 - 6.6] of melon makes melon particularly susceptible to food pathogens and admonished melon should be refrigerated after cutting and any cut melon left at room temperature for 4 hours or longer should be discarded. I thought of the melon I had sliced for an early breakfast that morning and then left on the table for mid-morning snacks. I put the melon in the refrigerator at lunch time, well over 4 hours later. When I returned home I consulted On Food and Cooking and found a chart: "Disease Outbreaks Caused by Raw Fruits and Vegetables".* Harold McGee lists several harmful microbes, and melon under both E. coli and Salmonella. McGee also advises "Once fruits and vegetables have been cut up, they should be kept refrigerated and used as soon as possible."** Lesson learned. The remainder of the melon that had been left on the table all morning went directly to the compost bucket.
In retrospect, this makes perfect sense. I have long known that melon flesh can be contaminated by microbes from the rind and am careful to wash the exterior as well as to keep the flesh away from the surface which has been exposed to the rind. McGee goes a step further specifying a "hot soapy" wash: "The melon surface can become contaminated with microbes in the field and cause food poisoning when the microbes are introduced into the flesh during cutting; it's now recommended that melons be thoroughly washed in hot soapy water before preparing them".*** Despite these precautions, if microbes from the rind should find their way via the cutting surface, knife or hands to the melon flesh and then the cut fruit then sits in a warm place, the potential for food poisoning greatly increases.
* On Food and Cooking, page 260
** On Food and Cooking, page 261
*** On Food and Cooking, page 368