I use to fry potatoes reusing the same (olive) oil several times (about 5-6 times), and I salt them while frying. But the the oil doesn’t get salty. What’s the reason for that?
In: 2000
It’s about solubility. Some substances, like salt, separate into their ionic components. Na+ and Cl-. These kind of substance easily dissolve in things like water, which has a Oxygen atom that pulls most of the electrons towards itself, making it slightly -ve charged, and the H atoms slightly +charged/ Oil however does not have this slight charge to it’s atoms. It just has carbons and hydrogens which share electrons roughly equally and don’t have +ve or -ve ends. Things that dissolve in water are called hydrophilic (love water) and things that dissolve in oil are called lipophilic (fat loving). Hydrophilic things don’t dissolve in oil. Other oils will, but not salt which is hydrophilic.
Salt is a compound of two atoms, sodium and chloride. The sodium has a positive charge and the chloride a negative charge. Things with opposite charges like to stick together, so they make salt crystals. Water isn’t charged, but it does have a side that’s sort-of negative and sort-of positive. When you add salt to water, the positive side of a bunch of water molecules surrounds the chloride atoms and the negative side of a bunch of other water molecules surround the sodium and pull them apart. There’s no longer a crystal but a bunch of individual atoms (called ions, because they have a charge) floating around in the water, which is called dissolving, and what you taste as “salty” is those ions interacting with your tongue, rather than the crystallized salt. Molecules that can dissolve easily in water are called hydrophilic (water loving) and are either charged like the sodium and chloride, or have the same sort-of positive/negative setup of water.
Oil, on the other hand, is hydrophobic (water hating). Oil molecules have evenly-distributed charges throughout, with no consistent positive(ish) or negative(ish) parts. Without that they can’t pull the sodiums or chlorides off the larger crystal, so it just stays as a chunk. In fact, if you poured the used oil into a narrow enough container and let it sit for a while, you’d probably see a layer of salt accumulate on the bottom.
If you salted the oil heavily enough to start, the potatoes might pick up enough by chance as they fry to be well-seasoned, but most of the salt is probably from when you salt them directly. If you taste the oil, you might get a little salt from whatever crystals you happen to pick up in the spoonful, but they would have to dissolve in your saliva first, unlike when the salt is in water and just has to mix up with your saliva-there’s a whole extra physical/chemical step that has to happen.
Salt doesn’t dissolve in oil.
Salt dissolves in water, because water molecules have a slight charge to them that allows the sodium and chlorine of the salt to pull away from each other a bit and stick to the water molecules. Fats and oils don’t have that.
Salt is a compound of two atoms, sodium and chloride. The sodium has a positive charge and the chloride a negative charge. Things with opposite charges like to stick together, so they make salt crystals. Water isn’t charged, but it does have a side that’s sort-of negative and sort-of positive. When you add salt to water, the positive side of a bunch of water molecules surrounds the chloride atoms and the negative side of a bunch of other water molecules surround the sodium and pull them apart. There’s no longer a crystal but a bunch of individual atoms (called ions, because they have a charge) floating around in the water, which is called dissolving, and what you taste as “salty” is those ions interacting with your tongue, rather than the crystallized salt. Molecules that can dissolve easily in water are called hydrophilic (water loving) and are either charged like the sodium and chloride, or have the same sort-of positive/negative setup of water.
Oil, on the other hand, is hydrophobic (water hating). Oil molecules have evenly-distributed charges throughout, with no consistent positive(ish) or negative(ish) parts. Without that they can’t pull the sodiums or chlorides off the larger crystal, so it just stays as a chunk. In fact, if you poured the used oil into a narrow enough container and let it sit for a while, you’d probably see a layer of salt accumulate on the bottom.
If you salted the oil heavily enough to start, the potatoes might pick up enough by chance as they fry to be well-seasoned, but most of the salt is probably from when you salt them directly. If you taste the oil, you might get a little salt from whatever crystals you happen to pick up in the spoonful, but they would have to dissolve in your saliva first, unlike when the salt is in water and just has to mix up with your saliva-there’s a whole extra physical/chemical step that has to happen.
Salt does not dissolve in oil.
Salt is a *polar* molecule because of its ionic bond. Normal sodium has one more electron than it wants and chlorine has one fewer. Unlike with *covalent* bonds, the two do not share an electron. Instead, the sodium atom gives it up and the chlorine atom takes it. This makes the sodium atom slightly positive and the chlorine atom slightly negative. Opposite charges attract, so they stick together like magnets.
Water is also a polar molecule, although water is made with covalent bonds. The way the electrons are shared between the hydrogen and oxygen makes one side a little bit negative and the other a little bit positive. That makes water very good at dissolving other polar molecules. The positive sodium atom sticks to the negative side of one water molecule, and the negative chlorine atom sticks to the positive side of another water molecule, and the water rips the salt apart, carrying the two atoms away.
Oils are nonpolar. The electrons are very evenly distributed so that there is no positive or negative side. That means there’s nothing to grab onto the two atoms in the salt molecule and the salt won’t dissolve.
In addition to the science behind your question, two things from a culinary perspective:
Don’t fry anything using olive oil. Super low smoke point and it’s expensive. Try peanut oil or canola if you must.
Don’t add salt to oil when frying (to the extent that I’ll keep the salt content lower in a batter that is fried and apply finishing salt instead) as it will effectively spoil it.
Edit: I stand corrected. Olive oil is a good frying oil, albeit expensive ( .84/oz to .12/oz peanut oil with what I have).
I think the real question here is:
How to you know the oil is or isn’t salty?
In addition to the science behind your question, two things from a culinary perspective:
Don’t fry anything using olive oil. Super low smoke point and it’s expensive. Try peanut oil or canola if you must.
Don’t add salt to oil when frying (to the extent that I’ll keep the salt content lower in a batter that is fried and apply finishing salt instead) as it will effectively spoil it.
Edit: I stand corrected. Olive oil is a good frying oil, albeit expensive ( .84/oz to .12/oz peanut oil with what I have).
I think the real question here is:
How to you know the oil is or isn’t salty?
The short and complicated answer is polarity.
The longer and more easily understood answer is magnets! A fundamental property of all molecules is polarity. Think of polarity as how “magnetic” a molecule is. Some molecules act as really strong magnets, like table salt, while others don’t act magnetic at all, like plastic. Every molecule falls on a spectrum between non-magnetic (non-polar) to extremely magnetic (polar). Molecules are like cliquey high schoolers in that they only want to hang out with other molecules who are the same as them: magnets with other magnets, and non-magnets with other non-magnets. The magnets and non-magnets wouldn’t stick together anyway as magnets only stick to other magnets (in this example anyway).
In the case of making fench fries, table salt is extremely magnetic while the cooking oil is completely non-magnetic so they don’t like to mix. Water on the other is a very magnetic so it is happy to hang out with the table salt all day.
Chemists take advantage of this magnetic (polar) property of molecules to do lots of fancy things in labs and industry but there are some real world examples that we deal with everyday besides cooking. If you were to splash your clothes with the hot oil while making those fries, it might be hard to get the stain out in the washing machine. As we have said, oil isn’t magnetic at all and water is, so they don’t like to mix. Soap is used as a intermediary between the two (soap molecules are giant with one side being non-magnetic and the other side very magnetic) but it isn’t a magic bullet as chemistry is complicated. So to get that stubborn stain out, you go to a dry cleaner. Dry cleaners use completely non-magnetic (non-polar) solvents (like tetrachloroethylene) instead of water to dissolve like minded non-magnetic (non-polar) stains from clothes, like the oil.
Hope this helps!
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