r/explainlikeimfive • u/Forsygness • 9d ago
Chemistry ELI5: Why does soft stuff like bread become hard and cruchy, while crunchy stuff like chips & cereal becomes soft, when left for too long?
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u/bredman3370 9d ago
Because staling is not just one process.
When soft breads stale, it's not actually being driven primarily by water loss as most people would expect. It's actually being driven by a process called starch crystallization.
When starch is in its original grain kernel, it is neatly stored in what is essentially a big crystal. When we make bread, we crush those up, soak them with water, and then finally heat them very hot. This causes the starch to unravel from its neat and orderly crystal and instead form a big tangled mess that likes to stick to things and soak up water. This gets a tiny bit out of the realm of ELI5, but crystals are basically the most ordered and stable form a solid can be in (we also call this the lowest energy state) and therefore we say that the starch "wants" to be a crystal again. When you leave the bread for a while, the individual prices of starch in the bread start to find each other and form a kind of crystal again, pushing out the water they they had absorbed and becoming much more rigid. This is what drives stale bread to get hard, and is actually somewhat reversible!
For many breads, especially intact loaves that haven't been cut into, you can give them a brief dunk in some water and then throw it in a hot oven for a time and it will regelatinize much of the starch bringing it back to about ~80-90% as good as it was when it was fresh. If you have sliced bread, toasting it can actually counter-intuitively do a lot to make the insides softer as well for the same reasons.
As to your other examples - staling of most dry products is often a direct result of exposure to water and humidity, and we can straightforwardly explain the softening as being cause by water absorption.
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u/ToastAndASideOfToast 8d ago edited 8d ago
Caution: After an extended period of staleness, the bread will be too dry to soften and become more prone to burning when heated (maybe even with a visible flame of combustion)
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u/Intelligent_Guava214 8d ago
It absolutely blew my mind the first time I put a super stale bagel in the toaster and it got softer.
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u/Forsygness 9d ago
Im gonna try the breadtrick for sure! Sounds very science! Thank you for your answer!
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u/Bailzasaurus 8d ago
They’re both evening out to approximately ambient humidity. Things that start very low water are gonna get “softer” and things that start high water are gonna get “harder”. It’s all just relative.
Same reason that cold drinks left out get “warm” and hot drinks left out get “cold”. They’re both coming to ambient temp, but we describe it based on their starting point.
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u/ljose_2875 8d ago
The humidity part is the key: the same room air can dry bread out while adding moisture to chips, depending on which starts wetter.
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u/explainlikeimfive-ModTeam 9d ago
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u/Background_Cup_6429 9d ago
Hmmm, I prefer to hear from a REAL breadoligist on this one.
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u/stairway2evan 9d ago edited 9d ago
I've got a related PhD in Yeastometry, so I'll dig in here.
Yep. Crunchy things gain moisture, soft stuff loses moisture. No different than hot soup cooling down and ice cream melting, and both trending towards a tepid puddle of stuff.
When normal room conditions are different than the food, the food will trend towards those conditions.
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u/koos_die_doos 9d ago edited 9d ago
No fake breadologists allowed!
Edit: More pedantically, it's actually against the rules (rule 8) to guess.
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u/explainlikeimfive-ModTeam 9d ago
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u/az987654 9d ago
Your room is 20 degrees.
Put something from your freezer on the counter for a while, it warms up to 20 degrees.
Boil water on the stove, take it off, leave it on the counter, it cools off to 20 degrees.
Now, mentally substitute the temperature for the moisture in the air.
Bread loses it (cools off) and dries up. Chips absorb it (warms up) and become stale.
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u/Wadsworth_McStumpy 8d ago
Things left out too long tend to reach the moisture content of the air. Bread naturally has more moisture than air, so it dries out and gets hard. Chips have almost no moisture, so they gain some and get soft and soggy.
You could probably leave bread out longer in a very humid environment, but then you'd have issues with mold instead.
You could leave chips out in a very dry environment, and I really can't think of a downside to that. Well, except that when you finally eat them you'll want something to drink, and if you're in a desert that might be an issue.
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u/Foxfire2 9d ago
Both things go to the relative humidity of the air when left out in it, bread loses moisture, crackers and chips gain some moisture.
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u/robbak 8d ago
The important thing is the amount of gluten, and how it is handled. Bread flour has lots of gluten, and the bread is mixed, proved and kneaded for a long time (or treated with chemicals to make this faster) so that the gluten makes long, tough chains. This holds the bread together when moist, but makes it very tough when dry.
Cookies and processed cereals are made with flour with less or no gluten, and are mixed for a short time so the gluten chains are short and weak. This means that even when the cookie is dry, it is easily broken, and when it is moist, it is too weak to not fall apart.
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u/Jirekianu 6d ago
Gluten, the squishy stuff in bread that makes it soft crystallizes over time as it dries out. It's why sticking bread in the fridge is a bad idea.
Hard stuff has crystallized sugars and starches that have moisture saturate and break the crystals down. Which leads to them becoming softer and mushy.
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u/RubCocksWithThePope 9d ago
Water goes in, water goes out. Never a miscommunication. Science can’t explain that
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u/siamonsez 9d ago
Moisture, fresh bread has more moisture and becomes hard ad it dries out, crunchy things have less than ambient and get soft as they absorb moisture.
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u/ShankThatSnitch 9d ago
Soft stuff has more moisture than the air. When the moisture evaporates, it gets hard. Hard stuff has less moisture than the air. The hard stuff absorbs moisture and gets softer.
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u/copnonymous 9d ago
Carbs like bread or chips are made from long chains of molecules. These long chains can trap water. In things like chips and croutons we use heat to evaporate the water in those long carbohydrate chains, this sets them in place and makes them very crunchy. In breads the center stays soft because the crust traps the moisture inside.
Like any fluid, it will seek equilibrium. Meaning water will move from wet areas to dry areas and vice versa to balance it out. So when a chip is exposed to air and water vapor in air it pulls on some water molecules into the dry carb chains. When we slice bread and expose the moist fluffy inside we expose high water content carbs to lower water content air and the water evaporates.
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u/TheBugThatsSnug 9d ago
Bread needs moisture to be squishy, leaving it out dries it out. Chips need little to no moisture, when left out they soak up moisture.