Too big of a temperature difference can make a tomato literally suck water inside the fruit through the scar where its stem used to be. If salmonella happens to be lurking on the skin, that is one way it can penetrate and, if the tomato is not eaten right away, have time to multiply.
That does not mean people should not wash their tomatoes - they should, just probably not in cold water.
But as the Food and Drug Administration investigates the outbreak of salmonella in the U.S. from tomatoes, the example shows the farm is not the only place contamination can occur - and checking things like water quality and temperature control in packing houses and other supply stops is one key to safety.
Raw fruits and vegetables are crucial to a healthy diet. But they're also the culprits in a growing list of nasty outbreaks: E. coli in spinach and lettuce. Hepatitis A in green onions. Cyclospora in raspberries. Salmonella in cantaloupe. Shigella in parsley.
This newest salmonella outbreak is the 14th blamed on tomatoes since 1990.
Preventing future illnesses depends on learning how salmonella sneaks onto and inside tomatoes, which might seem to be pretty well protected by their smooth waxy skin. Yet scientists have few answers, prompting the FDA last year to begin a Tomato Safety Initiative that is studying industry practices in the states of Virginia and Florida, origin of several previous outbreaks.
Florida's agriculture department on July 1 begins enforcing so-called "tomato best practices," farming and handling guidelines that leading growers pushed the state to formally adopt, and that many farms voluntarily began following in the past year.
The FDA likewise wants the authority to set mandatory safe-handling rules, what it calls "preventive controls," for growers and suppliers of foods linked to repeated outbreaks of serious illness, such as tomatoes and leafy greens. Congress hasn't yet acted on that request.
"We need them, we've asked for them, and we don't yet have them," says Dr. David Acheson, the agency's food safety chief, who is directing the hunt for the tainted tomatoes.
Further complicating the picture, budget woes mean the FDA's inspections of food-producing facilities have plummeted by 56 percent between 2003 and last year. Acheson says the drop has continued this year, and the FDA plans to hire more inspectors with a pending budget boost from Congress.
But inspections are not the solution to food poisoning, insists Acheson, who also hopes to double or triple the 10 percent of FDA's budget historically devoted to prevention.
FDA "is not arguing that you can inspect your way out of these problems," he says. "The critical point is to build safety upfront, not load up inspection at the end."
There are some common themes when fresh produce sickens, either from salmonella - bacteria that live in the intestinal tracts of humans and numerous animals - or other microbes: Water sources, worker hygiene and wildlife or domestic animals near fields are frequent culprits because they involve points where safety systems can easily break down.
Washing fresh produce under running water is a commonsense consumer defense.
"We know you can wash off some salmonella," says Virginia Tech food microbiologist Robert Williams, who accompanied FDA scientists to Virginia farms as part of the tomato initiative. But, "nobody's ever shown it washes off all salmonella."
Water is an automatic first suspect. Was clean water used to irrigate, mix pesticides sprayed on crops, wash down harvest and processing equipment, and wash field workers' hands?
Then in packing houses, tomatoes often go straight into a dump tank, flumes of chlorinated water for a first wash. To guard against salmonella washed into the water in turn being sucked into the tomatoes, producers often keep wash-water 10 degrees warmer than the incoming crop, says food-safety scientist Keith Schneider of the University of Florida, also part of FDA's tomato initiative.
Beyond packing houses, the industry points to cases where suppliers were shipped unwashed, warm tomatoes and dunked them in ice-water baths to firm them for further processing.
Another question: How often does the water have to be changed? Dirt, leaves and other sediment reduce the chlorine's effectiveness.
Studies never have shown that plant roots can suck salmonella up and inside the tomato, where it can't be washed out, says Virginia Tech's Williams, whose lab is working to confirm that. Still, if contaminated water is sprayed onto the leaves or blooms, or bird droppings fall directly onto the foliage, salmonella might be absorbed internally, he says.
In fact, salmonella may be particularly hard to prevent in a variety of crops because birds, reptiles and amphibians carry it - the same reason children should wash their hands after handling a turtle, iguana or frog. The tomato industry's guidelines already advise surrounding fields with bare soil "buffer zones" to discourage reptiles.
"You're not going to stop a bird going through a field. You're not going to stop a frog," Schneider says.