French supermarket chains are using a â€œbull cowâ€ label on many fresh foods to show the temperature and time of food storage. This kind of label is like a shooting target. There are many rings of transparent rings. The center ring contains a chemical substance. This chemical will polymerize with time and temperature and change from transparent to black. As the number of bacteria in foods increases with increasing temperature and time, when the inner ring turns black, it means that the food is no longer fresh.
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