For this reason, you should always put a coin in your freezer

How Science Ensures Food Safety in Freezers

Inside a freezer, the bacteria remain dormant at a temperature of 0°F (-18°C). But once the food is thawed, the environment becomes conducive to bacterial multiplication. Meat, fish, dairy, and all other perishable food can become spoiled if there is minor temperature fluctuation. Also, partial thawing of a food product can activate dangerous bacteria like Salmonella or Listeria, which will multiply and thus make it unsafe for human consumption.

When food thaws and then refreezes, bacteria do not die. They just go dormant, waiting to resume their activity when the food thaws again. That’s why it’s important to know if your freezer contents were ever compromised, and the coin-in-the-freezer trick offers a low-tech way to detect those events.