Picture this: Your fridge is empty, so you  find yourself down at the grocery store, trying to make healthy choices.

You grab a bag of mixed greens from  the produce aisle, and haul it home. Then, if you’re anything like me, it  ends up shoved to the back of the fridge until all the really good  stuff you bought is gone.

So you rediscover it weeks later only  to find that your precious greens are … surprisingly still good enough for a salad! Bagged salads can stay fresh in your  fridge for much longer than seems possible. And you might not think I’ll be  able to talk about a salad bag for the entire length of this video,  but you bet your brassicas I’m gonna!

Because that unassuming bag shoved  to the back of your fridge is a miracle of industrial farming, atmospheric  control, and polymer chemistry. [♪ INTRO] This story doesn’t start in the salad bag. It starts in the refrigerator itself. In many parts of the world, refrigerators  are a commonplace kitchen appliance.

But it wasn’t always that way. The home refrigeration revolution  swept across the US in the 1940s, and caught on in other parts of  the world in the decades to follow. And while this was amazing for the  makers of corny fridge magnets, it was also great for households.

Once upon a time, a home cook would need to trek to the grocer pretty much every  day for fresh ingredients! But with a fridge in the  kitchen, grocery shopping was reduced to a once-a-week chore. And with that chill revolution,  even delicate veggies like lettuce could be shipped from farm to fridge  and live to tell the kale – tale.

Americans across the country  started eating more salad at dinner, and the execs at Big Lettuce  were eager to cash in. It was simple enough to ship an entire  head of lettuce to grocery stores, but pre-chopped leaves were a different story. It was clear that consumers  wanted salad, not just lettuce, so if they could figure out  how to keep cut greens fresh, they knew it would be a major success.

And that’s why there’s been so much research into keeping bagged salads fresher for longer. The secret to everlasting  endive and immortal iceberg came down to two pillars: prep and packaging. To start, let’s talk about how your  lettuce gets from field to fridge.

It’s harvested either by  hand or with a big machine, and then immediately cooled down. These days, most leafy greens are vacuum-cooled. Essentially you suck most of the  air out the lettuce-filled chamber, which causes water to evaporate really fast, which sucks heat from the produce as it dries.

This first step is crucial because  the bacteria and fungi that live on the surface of the veggies  become sluggish in the cold, making them way worse at eating and reproducing. Most of those microbes aren’t  long for this world, though. Because next the leaves take a series of baths.

Leafy greens get washed in a  mixture of water and acids, which do a great job at  killing most of the bacteria the veggies picked up from the machinery. Or, you know, just from  being plants grown outdoors. This high-tech sanitizing process  means that a pre-washed bag of greens is one of the safest ways to prep a salad.

In fact, experts recommend against rewashing greens that are labelled pre-washed. It’s more likely for something icky to  be growing on surfaces in your kitchen than in that factory-sealed bag, so washing them again introduces  a new risk of contamination. After they emerge from their  bath and get all dried off, the leaves will meet the other half of the  long-lived lettuce equation: the packaging.

But before these greens get to our  table, here’s a quick ad break. This episode of SciShow is  supported by Manta Sleep. Manta Sleep makes sleep masks  and sleep accessories designed to help you get the energy  to create your best life.

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You can check out the Silk sleep mask and all that Manta Sleep has to offer at  the link in the description. Use the code SCISHOW for 10% off your order! While our leafy greens are now squeaky clean, bacteria and fungi aren’t the  only threats to your side salad.

Their own metabolic processes can spoil them, too. Because veggies are still alive, even  after they’ve stopped photosynthesizing. Once they’ve been plucked, they start  to consume their stored up nutrients.

And as they do so, they respire, inhaling  oxygen and exhaling carbon dioxide. If you can slow down their breathing,  then you can delay the decay. Because if they can’t breathe, they  can’t break down those nutrients, leaving them in a kind of stasis.

So the key to a longer lifespan is less breathing. At least for lettuce. You can make that happen with a  perfectly designed atmosphere.

And no, I don’t mean by playing some smooth jazz. I’m talking about the atmosphere  as in the gases in the air.   Because every piece of produce  respires at its own rate, finding that perfect atmosphere  takes a lot of experimentation. But it’s worth it, because when coupled with cold, a controlled environment significantly  slows produce’s metabolism, essentially putting it in suspended animation.

They start by tweaking the oxygen. Most storage warehouses contain  a very low amount of oxygen, as low as 2% for lettuces. For reference, normal air is about 21% oxygen!

However, too little oxygen might  cause the produce to ferment, and high levels of carbon dioxide  can cause a slew of other issues. So they tweak the levels of O2 and  CO2, leaving mainly nitrogen behind. And an atmosphere of mostly nitrogen  means no fermentation, and minimal decay.

Amazingly, we still don’t know all the  reasons why this works, just that it does! The final challenge is to fit all this  atmosphere control into your fridge. And those Big Lettuce execs  knew just the thing to do it: differentially permeable membranes!

By the 1960s, scientists had figured  out how to make plastic films that allow molecules to flow through  the membrane at specific rates. Plastic is made of long chains called polymers. They overlap to form a  material that looks continuous, but on a microscale has tons of tiny gaps  between the cross-linked polymer chains.

Those holes can be just the  right size for smaller molecules to squeeze through, like oxygen or carbon dioxide. If the concentration of a gas is  greater on one side of the membrane, it will cause those gas  molecules to diffuse across it from higher to lower  concentration, to even it out. So if you package lettuce  inside these special films, you can control the flows of  oxygen and carbon dioxide.

As veggies use up the oxygen in the bag, then oxygen from the outside  atmosphere can diffuse in. And then as the veggies exhale carbon dioxide, that excess CO2 diffuses across  the membrane out of the bag. The only thing left to do is tune those rates to perfectly suit the produce you need to package.

The first commercial salad bag had five  layers: a middle layer with the labels, two layers to control O2 and CO2, and two  outer layers to seal everything together. It hit grocery store shelves in 1989,  and salad bags have only improved since! Newer films can use fewer layers  of plastic to do the same job, and can even diffuse water  molecules across the membrane to keep your salad from fogging up.

So don’t discount the power of that packaging! There are still a few problems to  solve with transporting produce. For starters, a lot of what we said  about how safe triple-washed veggies are is mainly true when we have really good  oversight to check for contaminants.

And there’s absolutely no reason why we might be worried about a loss of oversight right now! But even in their current state,  premixed salads are a marvel. Well look at that, I told you I could talk  about bagged lettuce for a whole episode! [♪ OUTRO]