You stroll into the local grocery store for a quart of milk, squint at the ''expiration'' date on the top of the carton, and leave thinking you have at least five days before it goes bad. But next morning, when you peel open the carton, it smells like an old shoe.
What happened? Somewhere between the cow and the dairy case, the milk was probably left unrefrigerated for a time.
A number of new products entering the market are designed to cut down on the amount of goods that spoil before they make it to your pantry.
Called time-temperature tags, the devices are being used on packing cartons to keep a constant check for damage to refrigerated and frozen foods. They're also being developed to monitor other perishable items, from photographic film to paints.
You won't see these tags on individual items in the grocery or hardware store , but they are slowly popping up on some crates in warehouses and back rooms.
Essentially warning signs for products, these tags use chemicals or biochemicals to simulate the deterioration of a product. They gradually turn color in response to warmer temperatures. If, say, a case of frozen peas were moved from a refrigerated truck to a warm warehouse, the tag would go through a series of chameleonlike color changes as deterioration of the peas progressed.
The tags don't actually measure the deterioration of the product itself. Instead, they indicate what its condition might be by measuring changes in the environment. The color guards are seen as a more reliable way of detecting spoilage in perishables than the current standard dating method, which assumes a product has been properly handled before it pops up in the market.
After nearly a decade of research and mixed success in development, at least three companies are coming out with such labels. All look slightly different - and work in slightly different ways. St. Paul-based Minnesota Mining and Manufacturing (3M), for instance, is test marketing a Band-Aid-size label. It contains a blotter of colored chemicals that gradually move down a wick in response to temperature changes. I-Point Technologies Ltd. of Washington has developed tags that, when activated, turn from green to yellow to red. Allied Chemical Corporation is testing sensing chemicals that can be mixed with ink and printed on labels.
''The potential for these is certainly significant, but for all practical purposes it is an uninhabited market right now,'' says Wendell Manske, a development specialist at 3M.
One big market for the companies could be the chemical and pharmaceutical industries, which need tight temperature control over many of their products. Tags are already being used on vaccines by some international relief agencies.
The Navy is working with time-temperature labels to monitor the shelf life of heat-sensitive resins. Other areas for potential use are photographic supplies and fresh-cut flowers.
Far and away the biggest potential market - and probably the toughest to crack - will be perishable foods. A few major food processors and supermarket chains are beginning to use the tags to monitor the transport of frozen bread dough and other goods with a short shelf life. So are a number of fish processors and frozen-food handlers.
Still, widespread use of the tags in the food industry remains a few years off. Some food handlers are reluctant to use a device that tells them, in living color, that a product has gone bad. It raises the question of blame. A few truckers have refused to carry goods with time-temperature tags on them. ''They can be perfectly reliable, but who wants to hear bad news?'' says Hugh Symons, a researcher at the American Frozen Food Institute. ''They don't solve problems. They only tell you you've got them.''
The tags may never be put on individual consumer goods on supermarket shelves. For one thing, they are too costly. They range in price from a few cents up to more than a dollar. Instead, they are usually put on the outside of shipping cases and used to monitor the flow of goods. ''It is more of a quality-control tool,'' says Robert Rose, I-Point vice-president for product development.