During packaging, a temperature sensitive sticker is added
to the product; when the food reaches a temperature
outside the "safe zone," it changes color (e.g., from white
[safe] to red [danger]).
Different color codes could be used for frozen (-32°F, 0°C)
or refrigerated (45°F, 7.2°C); e.g.,
blue/red for frozen,
white/red for refrigerated).
The purpose would be to alert the grocer if the food has
gotten too warm during transport, or the consumer if it
has been too warm in the store.
This is prompted by an experience I had some years ago,
when I discovered that the owner of a convenient store
was turning off his refrigerated storage at night, to