How does a sealed container preserve food safely at room temperature for months/years, but a sealed contained of leftovers in the fridge spoils in less than a week?



For dinner today, I had a liquid precooked curry mix (sauce, cooked veggies etc..) that comes in a little foil pouch and doesn’t have to be refrigerated- just heat the pouch in hot water for 5 minutes and pour over cooked rice. All the food is precooked, just chilling at room temperature in that pouch for months, but perfectly safe to eat.

How can it be so safe and not-spoiled like that, when the same ingredients sitting in a Tupperware in the fridge will go bad in just a few days?

In: Chemistry

a lot of it is other elements mixed with the item (curry) and the elements exposed to it. In this case where the curry is already cooked in a pouch, it’s the air and particles in the air that spoil the food.

Canning/jarring involves removing the oxygen and heating the contents,while sealed, to high heat to kill bacteria.

More complicated then slapping a Tupperware lid and putting it in the fridge.

This is also why most things must be used immediately after opening or at least refrigerated because it’ll go bad.

Back in the 1880s a French microbiologist named Louis Pasteur showed that heat could be used to kill/deactivate microbes that spoil food. This coupled with the concept that microbes don’t appear out of nowhere meant that food treated with heat then sealed in a container that prevented entry of microbes would be able to stop the food from spoiling. This process is called “pasteurization” and is the underlying concept behind many food packaging today.

Your Tupperware container of food has microbes in it already, and isn’t sufficient seal against their entry either. Cool temperatures can slow down the action of the microbes and delay the spoiling, but it will happen eventually.

Your pre-cooked meal pouch was made in a place that was very, very clean (aseptic) compared to the conditions in your kitchen. The food maker follows important rules to keep grocery store food safe (government and industry regulations about hygiene and manufacturing conditions). There are probably some few germs in there, but it usually takes a long time before they grow enough to ruin the food or make you sick.

Food that has been in the clean — but not very, very clean — area of your kitchen picks up germs that grow even faster with the bit of heat that comes from a cooked meal. So, it takes much less time for them to grow enough to ruin the food or make you sick.

Edit: as /u/nadalcameron points out, you can preserve food very well in your kitchen. “Canning” is a time-tested way of using pressure and high temperature to get the pasteurization or sterilization of industrial production at home. Pickling is another accessible way to keep germs at bay and preserve food.

The food is sealed and then cooked so it kills off any pathogens and sterilizes the food. It’s sealed off from any elements getting inside there after that so it stays good. Refrigeration slows down the process of food spoiling, usually just gives you enough time to eat your leftovers, but since they are exposed to the air for some time, they are inoculated with bacterial and it starts to grow. If you can keep every cell of bacteria and mold out of things they will stay good for quite some time.

It depends, but the main factor is sterility. For something to spoil dangerously or just unpleasantly, it has to have growth of bacteria or fungi (viruses don’t really apply here), which eat the food you want to eat.

One way to avoid that is to process and package everything as clean as possible. The less contamination you start out with, the less likely it is to spoil. You can also package the food in vacuum sealed containers, as a lot of pathogens need oxygen to survive. Similarly, you can package food with a different gas to displace all the oxygen (meat is packaged with CO2). Then the final option is to sterilize the food, usually with radiation, to kill all the organisms living in it. This sterilization can be performed with the food already sealed in it’s packaging, so nothing new can be introduced.

When you buy a jar or can of food at the grocery store, it has been vacuum sealed and heated to a temp that kills any bacteria inside. So there is no live bacteria inside and no way for any to enter, meaning the food is preserved for a long time even when not refrigerated.

The sealed container you buy is packaged in a sterile facility so there’s nothing in the container to make the food go bad. When you put leftovers in the fridge, even if they’re in an airtight container, there are already bacteria and mold spores inside the container with the food to make it go bad.