Like milk goes bad a week or two after opening it but if you don’t open it, it will stay good until the expiration date? Like yogurt, sour cream, shredded cheese. All those things. I’m confused
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Its not just exposure to oxygen, its continued exposure, Bacteria needs 5 things to live and breed, Oxygen, Moisture, Warmth, Time and Food. Obviously you can’t deny them Time or Food and Moisture is kinda hard as most food items will naturally have some degree of Moisture, so Warmth and Oxygen are the main we ways we can control it.
Which is why Food is either kept in the Fridge or Freezer to inhibit the growth and many items are sealed to prevent more oxygen getting in. You can go further by either Vacumn sealing the items by sucking out all the air so there will be no more oxygen or Nitrogen sealing which I’m not super familiar with but I think it works by sealing the item in a Nitrogen rich environment for the same purpose.
I work in a Kitchen and we have a Vacumn sealer machine and it puts so much life onto our items, especially meat and Fish
Food spoils when bacteria eats it. The bacteria (or at least most types in question here) require oxygen. A sealed gallon of milk only has a very small amount of free air in the bottle which only allows for bacteria to grow for a very short time period of time before depleting the oxygen supply. There’s also very little bacteria initially in the bottle because the milk has been pasteurized. So limited oxygen, minimal microbes to start a colony. When you open the milk, you introduce fresh air and it reinvigorates whatever bacteria is in the milk. You’re also pouring milk out so when you reseal the lid, there’s more oxygen in the bottle than when it was sealed.
Pasteurization is how a lot of that works. That’s how beer, wine, canned goods, milk, and so on last so long.
Some dry goods, however, like potato chips will fill the bag with low humidity nitrogen instead of air. That keeps them fresh for months on the shelf even though they go stale in days if you leave them open.
There are a lot of good answers already, but I want to add something about canned food seaming you asked about oxygen.
A lot of canned food has the air replaced with steam just as the lid is sealed on the can. This isn’t to kill any bacteria but to reduce spoilage from oxygen. That steam will condensed and create a vacuum, so when you open a can, you should hear a hiss as the air rushes in. If you don’t, the seal has failed.
And as others have said, they heat the product up to kill the bacteria and microbes the main one is Clostridium botulinum(Botulism), and they also cool it down in a controlled way to ensure that any bacteria that can survive the the high heat(there are some but are harmless) has as little time to reproduce at the temperature they like to reproduce at.
Food, medicine, etc can oxidize when exposed to air, becoming less attractive, less flavorful, or less nutritious. (Though rarely if ever does oxidization make it dangerous.) It’s different from rotting or becoming contaminated; the chemicals in the product are reacting with oxygen and being changed.
For example, when you cut an apple in half and let it sit for several minutes, the exposed flesh starts to turn brown. This is the apple’s tissue oxidizing.
You need a supply of oxygen for this process to continue. So something that is prepared, and then vacuum sealed, will run out of oxygen to react with and the process will stop. If you cut an apple and then immediately seal it in shrinkwrap, some oxidation will have started to happen, but not enough to be noticeable before you removed the supply of oxygen.
There is no “expiration date” on many perishable foods like dairy products, only “sell by” dates. This is so you basically know the store is following the rules and not holding on to these items indefinitely when they go past these dates because they haven’t sold them yet. It just keeps the stores honest.
As to why they expire when exposed to oxygen: The Bacteria that can hurt you need oxygen to grow. When you let in the oxygen, you let them grow faster. Eventually, they are in large enough numbers that they can hurt you. The chemicals they release are toxic to you and they smell bad so don’t eat food that smells bad.
Doesn’t yogurt go good after the expiration date?
Spoilage can classified as safe spoilage and unsafe spoilage. Spoilage that is still considered food safe is that which does not make you sick, for example colour, texture, taste. For example stale crackers, or soured yoghurt. Oxygen can cause discoloration for example.
The other kind of spoilage that makes you sick (or dead!) is usually though microbiological growth, or chemical means. An off-gas is usually produced or some breakdown and souring. Fungal growth can also take place if the food is exposed.
When the food is just produced processed bacteria count is low. They will naturally grow with temperature. And more types of bacteria can go into the food if you expose it to the open air (not necessarily oxygen) or other contamination. Same for fungi as mentiioned previously.
If you find your milk going bad/shredded cheese going moldy/bad before you consume all of it, try Ultra High Pasteurized (uht) milk from a warehouse store and if the shredded cheese is used in a cooked dish, freeze the unused portion, the texture will be off when defrosted but the flavor is identical in cooked dishes.
Worked as an engineer for a packaging machinery manufacturer. Almost everything you purchase has the oxygen flushed out with a big puff of nitrogen. Residual oxygen is 1-4% depending on how much the company wants to spend on nitrogen, how fast you package and the “fluffiness” of the product to get flushed with N2.
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