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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Sure, water is frozen at both of those temperatures. But then water isn’t really what spoils food, is it?
Bacteria is one of the major causes of food spoiling, tiny organisms that consume the food and poop out things which are often toxic to humans. Bacteria can still grow at both -5°C and even -15°C, although of course quite slowly. The shift from -5 to -15 apparently inhibits growth enough to dramatically lengthen the time the food can be safely stored but neither is indefinite.
What country do you live in? I’ve never seen food packaging list freezing temperatures for expiration.
Regular kitchen freezers usually go through a regular defrost cycle. That means the freezer shuts off and is allowed to warm to just above freezing for short periods of time. This reduces frost buildup, but it comes at the cost of food not lasting as long due to the slight thawing and refreezing. The food never completely thaws, it never comes even close to that…but the cycle takes a toll.
Deep freezers both achieve a colder temperature (better at reducing bacterial growth) and they hold that constant cold temperature forever…so no wear and tear on the food due to temperature cycling.
Part of the answer that’s not yet mentioned is sublimation. Water freezes at zero, but it also **still evaporates** below zero.
Sublimation is why wet laundry hung out to dry on a winter clothesline will still eventually dry even below zero, why older icicles start looking “eroded” after a few days of very-cold breezy weather, and (the most relevant example) why icecubes shrink in your freezer over time. Some of the water molecules in below-zero ice translate directly from solid to vapor.
The more energy in the air, the more sublimation will occur. And in your fridge or freezer, a tiny bit of moisture still evaporates from the surface or near the surface of your food unless it’s vacuum-sealed and all air has been removed.
At -5 it happens a little faster than -15 because it’s still… warmer.
Now, *losing that moisture changes your food.* That leached-away moisture doesn’t go back INTO the food when it condenses back into ice. It’s either removed from the freezer and replaced with drier air, or it resettles as ice crystals on a surface.
If any part of your food is exposed, even IN a sealed package that has some air-space, it can get “freezer-burned” and change appearance or texture or flavour. And this happens faster at -5 than -15.
Bacteria grows quicker at -5 degrees than at -15 degrees. Cells are made of more than just water so they can still be functional below the freezing temperature of water
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.
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.
If you slowly freeze something, ice crystals form very slowly in it, causing the crystals to grow and poke little holes in the structure of the food. This happens on a cellular level. You can particularly notice this in something leafy like lettuce, after thawing the leaf becomes very floppy and soft.
If you quickly freeze something, the ice crystals don’t get to grow so much, so the ice crystals can’t damage the structure of the food, so it prezerved it better.
The colder the freezer, the faster things freeze.
Once the food has been frozen, storing it at -15 or -5 won’t make much of a difference.
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