Washington ? Pick a tomato in the blazing sun and plunge it straight into cold water. If that happened on the way to market, it might be contaminated.

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’s one way it can penetrate and, if the tomato isn’t eaten right away, have time to multiply.

That doesn’t mean people shouldn’t wash their tomatoes – they should, just probably not in cold water.

But as the Food and Drug Administration investigates the nation’s outbreak of salmonella from tomatoes, the example shows the farm isn’t 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.

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 Virginia and Florida, origin of several previous outbreaks.