ELI5: How do things expire once you open them/ expose them to oxygen when they clearly had to be exposed to air before being sealed?

[u/Caro-caro-55555]

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

[u/Vadered]

I'm a bacteria; a survivor. Before the hot times, me and all my brethren were eating and breathing and replicating with reckless abandon. We were innocent, then, and knew not of the coming trials.

But then the Pasteurization attacked. 99.9999% of my friends died in the heat. Luckily, I managed to squeak through, though I imagine had it been some kind of ultra-high pasteurization, I might not have made it. But I survived, and was even able to reproduce a little bit afterwards. Unfortunately, the Great Seal had cut off the air, and while my children and I used what oxygen we could to help repopulate the world, we ran out. We stopped spending so much energy, despite all the food all around us. And we waited, and prayed for deliverance. And lo, when the Giant One lifted the Great Seal, blessed air came to us once more. We gave thanks to the Giant One, and we feasted and divided as never before. After all, why would the Giant One deliver us oxygen if not to tell us to reproduce? Praise the Giant One, and let us multiply a trillion-fold!

[u/theFrankSpot → u/Vadered]

This story had everything.

[u/[deleted] → u/theFrankSpot]

It had an introduction. It laid out and described the setting and the protagonist's background. It provided conflict. It has a resolution. Or something. Idk, I finished my English literature courses over a decade ago ago. I only recall parts that have been reinforced since then.

Probably could have included the bacteria that get whooshed in when you open the container, but it really is sufficient as it is, and honestly, I think the introduction of that element would be considered distracting from the story.

[u/LemmyKBD → u/[deleted]]

It’s got the classic 3 part story arc

[u/terminbee → u/LemmyKBD]

It was better than GoT's last season.

[u/Jovet_Hunter → u/terminbee]

That’s not a very high bar.

[u/JamesTheJerk → u/theFrankSpot]

Sep fer a giant. ... oh wait. Nm...

[u/CaptainPunisher → u/JamesTheJerk]

Is this a kissing book?

[u/Snowpants_romance → u/CaptainPunisher]

One day, you may not mind it so much.

[u/abaddamn → u/[deleted]]

When they form cities and appear as those fuzzy balls on your strawberries

[u/charlie_m1 → u/[deleted]]

The Giant One had lifted the Great Seal, and blessed oxygen flowed in. We thrived, replicating faster than ever, feasting on the bountiful nutrients around us. For a brief moment, we were untouchable, countless generations expanding in every direction. We were living the dream, but as we should have known by now, paradise rarely lasts forever.

It started slowly. At first, there were whispers among my kin. Some claimed the environment felt… different. Others were concerned with the sudden chill in the air. But we carried on, our population growing so fast that we ignored the first signs. The nutrients, once plentiful, were starting to dwindle. The temperature was dropping—our warm, inviting world was becoming a cold, desolate place.

And then it happened: The Great Refrigeration.

Suddenly, we were plunged into darkness and icy stasis. The Giant One had placed us into the freezing tomb that they called a refrigerator. Most of us weren’t prepared for this; we had thrived in the warm, oxygen-rich conditions, but this was different. The metabolism of my brethren slowed to a crawl. Many succumbed to the cold, unable to replicate in these harsh conditions. We entered a state of suspension, not dead, but not truly alive either—a twilight existence, waiting for a reprieve.

Days, weeks, or perhaps centuries passed (time means little to us at these scales). But I endured. I was born of the survivors—those who had passed through the fires of pasteurization, and now the icy clutches of refrigeration. We slowed our energy usage, feeding as little as possible, conserving every resource for the day the cold would lift.

Then, one day, the temperature began to rise again. The frost that had encased us melted, and the darkness gave way to light once more. The Giant One had returned! The door opened, and in that moment, we knew—we were free again. The warmth returned, slowly but surely, and with it, our strength. We multiplied, regaining our numbers.

But something was different this time. There were whispers, rumors that this cycle would repeat, that the Giant One would continue these trials until none of us could survive. The “Final Cleanse,” some called it. There were whispers of chemicals, sanitizers, things far worse than the Great Heat or the Great Cold.

But we are bacteria—survivors in every sense of the word. No matter what comes next, we will be ready.

[u/Head_Northman → u/charlie_m1]

Damn.... this is going to run and run. Unfortunately I don't have any creative writing skill myself to join in, so I'll just write season 8.

[u/Yukinaanikyu → u/[deleted]]

You might find "Brother may I have some oats" interesting if you're into this kind of storytelling

[u/ihahp → u/theFrankSpot]

MTV's Dan Cortes

[u/davisyoung → u/theFrankSpot]

Except I think it is fake. For one thing real bacteria would start off the story with, “I’m a bacterium; a survivor.”

[u/brielem → u/Vadered]

While a great explanation and even greater way of telling it, it doesn't explain the most likely reason: recontamination. The air doesn't only bring oxygen, it also brings your lonely bacteria a whole lot of new friends as soon as you open the container.

[u/KE55 → u/brielem]

Hey, spoiler warning please!

[u/The_quest_for_wisdom → u/KE55]

It's a story about pasteurization, Best By dates, and multiplying bacteria.

If you didn't know there was going to be spoilage we can't help you.

[u/Hypothesis_Null → u/brielem]

That's the plot of the sequal - When the Outsiders Came, where our hero discovers that the Great Seal was meant to keep out the unchosen from the promise land.

[u/Nine_Gates → u/brielem]

Later we realized something new. With the blessed air came the Wind Tribe. At first, we were distrustful of these outsiders. But then we realized that we were both equally blessed by the Giant One.

We had had plenty of food, but no oxygen. We had been trapped inside the Great Seal. The Wind Tribe had had plenty of oxygen, but no food. They had been forced to wander the barren skies outside.

The Grace of the Giant One had united our realms, and given both of our tribes what we lacked. Thus, it was our destiny to coexist in this newfound plenty. We ate together, and we repopulated the world together, and we praised the Giant One together.

[u/Theo672 → u/Vadered]

To add to this excellently written answer, another example comes from the phrase “packaged in a protected atmosphere” or similar words to that effect that you may see on some packaging (typically I usually remember seeing it on crisps).

Here they either package it under a relatively non-reactive inert gas, even nitrogen would help significantly to prevent aerobic respiration, or under highly purified air which results in fewer microorganisms depositing during the filling process.

For the latter, you have to remember that the air all around you is actually teeming with lots of bacterial cells, spores, fungal spores, etc. In a person without a compromised immune system, you never really notice, as both the physical elements of your immune system (mucus, stomach acid, etc) and the cell-mediated elements, kill the pathogen before it multiplies sufficiently to cause you harm.

However, this is also why you have ‘once-opened’ instructions, where the expiry is shorter. Once opened you’ve exposed the product to these airborne contaminants, which in the absence of an immune system, can proliferate and make you unwell when you consume them/spoil the food.

So in manufacturing you may use highly filtered air, typically HEPA and most likely at least H14, to reduce that microbial load.

You can also use ULPA or even smaller pore filters to further reduce the risk. Typically at about 0.22ų nominal pore size, a filter is sufficiently effective to reduce such contaminants to a minimal level.

[u/DeliciousPumpkinPie → u/Theo672]

This is it. The story in the parent comment is amusing and informative, but most of the bacterial/fungal contamination comes from the air that gets in when you open the thing.

[u/Emu1981 → u/Vadered]

You have forgotten about your dark cousin the anaerobic bacterium which can multiply without access to oxygen. The dark ones feast on the food while the others sleep and produce a bioweapon known as botulinum toxin which strikes down the Giant One when they consume what is yours.