On a package of ground beef, milk, eggs, etc., there would be a small patch of a substance that decayed and turned a different color over time, but was slowed by refrigeration. There would be an indicator on the package saying, "If patch is <color>, then food has expired.". It would work by getting the
amount and ratios just right, so it accuratley represented the X axis of food decay, time. The second part, or the Y axis, of my theoretical food decay graph, would be to make it temperature-sensitive. If you left the milk out on the counter overnight, it would cause the decay of the substance to speed up as a reaction to the warmer temperatures. Another example would be if you put the ground beef in the refrigerator to defrost. It would go bad faster than it would in the freezer, and the decay of the substance would reflect that. That way, you *really* know if the meat in the fridge is bad.
"FreshTags® are patented color indicators that sense the production of gases such as the fishy odor common to seafood. The odor-causing chemicals react with the patented indicator inside the tag and gradually produce a color reaction. The color change indicates that the seafood is past the point of useable freshness." [-alx, Aug 05 2002]