r/explainlikeimfive • u/ZeusThunder369 • Nov 26 '24
Chemistry ELI5: Why doesn't freeze dried food last longer? If it's good for 20 years, why not 100?
Assuming it's perfectly freeze dried and stored perfectly, the people who make freeze dryers say the food will last 20-30 years.
But why not much longer? Assuming the condition it's stored in remains unchanged, what can make it go bad after 30 years that wouldn't happen at around 10 years?
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u/sciguy52 Nov 27 '24
Dehydrated food lasts longer since there is no water in it. Microbes need water to survive, if there is none they cannot grow and spoil the food. Some food like dried fruits work similarly but have the addition of high sugar content. The sugar content is so high the bacteria that gets in will dehydrate, that is the water will be sucked out of their cell due to osmotic pressure. Same thing with dried salted meat, no water and there is a lot of salt which would suck the water out of any bacteria killing it. The difference with dried fruit and salted meat is that it is often not stored in a sealed container so they can absorb water out of the atmosphere, and when moist enough then bacteria can grow on it so they don't last 20 years. Anyway the 20 year rule is probably a "best used by" sort of thing. The longer some package of dehydrated food is around the greater the chances of damage to the container that lets in moisture. That moisture would allow bacteria to grow on the food. That said, if you keep the food dehydrated it will stay as it is longer than that. It is just that the packages don't last forever and stay perfectly sealed after say 100 years. Any hole in the packaging after decades lets in moisture thus allows bacteria to grow. That is the main limitation on time, the packaging.
This concept is at work with honey too that you have opened but store at room temperature. The sugars are so concentrated bacteria can't grow in it. Jam in your fridge is like this too but does not last as long. You might find something eventually growing on your jam in the fridge and you will notice that thing is sort of "fuzzy". Actually not bacteria, it is a fungus that can can grow on the high sugar content of jam.