Hi! While unfreezing my ice box I put various groceries on the porch, as it is around -5°C at the moment. When I checked the packaging, I noticed that the date the food was supposed to be consumed by, varied widely, depending on the temperature. From 3 days (-5°C) to around a year (-15°C). How does it make that big of a difference? Is the water not equally frozen at both temperature points?
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The water in the food and in the bacteria contains solute like sugar and salt. They make that the freezing point of the impure water is actually below 0C. When the impure water is still liquid, the bacteria are still active though they are slowed down.
Freezer should be kept at around -18C so all water is frozen.
Most of the time higher temperature will be fine (since anyway water that has a lot of solute will typically kill bacteria by osmosis), but -18C is the standard to be really sure.
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