Warning: Red Means `Refrozen'

SUPERMARKETS DON'T HAVE SPECIAL SECTIONS marked "refrozen," but they probably should. Accidents happen. Manufacturers don't always keep fridges cold enough. Goods being delivered sometimes sit on a curb too long. Either way, consumers may not know about it until they eat the product.

A temperature sensor from Sandia National Laboratories may have a solution. Invented by researchers David Martinez and Mo Shahinpoor, it consists of a shape-memory alloy of nickel and titanium on a piece of paper the size of a postage stamp. One end of the wire is anchored to the paper. The other is tethered to a green, lentil-size shutter that covers a red dot. When the temperature rises above 32F, the wire shrinks, pulling the green shutter into a tiny pocket and exposing the red dot underneath (picture). Refreezing will not bring the green shutter back out.

Food processors are leery about the new sensors. Jenny Scott, a senior director of food safety programs at the National Food Processors Assn. in Washington, says refreezing poses few health hazards because most cases occur in too short a time frame to allow bacteria to proliferate. But Scott admits there are "quality issues." And that's exactly what the sensor will address. Sandia's Shahinpoor says that the sensors, produced in volume, will cost just a few cents each. So for pennies, manufacturers who want to reassure finicky customers will be able to put proof right on the label.

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