Tuesday, April 28, 2009

Science of Salt

Last month, my oldest sister emailed me with the subject line "settle an argument":

"What does salt do to water?"

She went on to say that she and our mom were disagreeing on what happens to the boiling point of water that's salted, and whether the amount of energy it takes to reach that new boiling point increases or decreases.

"So we googled it," she wrote, "and found that salt increases the boiling point and that cooks add it to water make something cook more quickly. To me, this means: adding salt means you have to heat the water more for it to boil, taking longer for it to boil. But since the boiling point is raised, once you put pasta or rice or whatever in, you can cook it for less time than normal because the temp is higher."

Salt and Water
I replied as best I could, dodging the boiling point question, but trying to be as practical about it as possible:
"Cooks add salt to water to add saltiness to the food being made in that water. Pasta soaks in the saltiness from the water while it's cooking, for example. Same with rice.

There is/was a 'rule' in cooking that beans should never be cooked in salty water. The theory was the salt disrupted the cooking process, as the heated water slowly got into the different layers of the bean, and made the outermost layer tough. This theory has since been proven untrue, but some people still abide by the rule anyway and only salt their beans after they're finished cooking.

I also know that most cooks don't add salt to a pot of water until it's at the boil or nearly at the boil. This has nothing to do with boiling point, but rather is a way to keep your pots untarnished. If you put the salt in when the water is cold, it will settle to the bottom until the water boils, and for some kinds of metal, direct and prolonged exposure to salt can cause tarnishing.

When I drop salt into a nearly-boiling pot of water, there is usually a little explosion of more rapidly boiling water where the salt went it.

Does that help? I can look up the answer about the boiling point if you want, but this is what I know off the top of my head.

Can I blog about this?

Do you know there is a non-fiction book Salt about the history of salt?"


A few days later, I picked up the Salt book. Right in the introduction, it quotes the Diamond Crystal Salt Company's booklet called "One Hundred and One Uses for Diamond Crystal Salt," citing one of the uses as "getting more heat out of boiled water."

To recapitulate, when salt is added to water, the boiling point rises, meaning it takes more energy to reach the boiling point, but food cooks faster because the water is hotter than 212F.

Salt and Oil
Another neat fact I read not long ago has to do with salt and oil, specifically frying oil (from How to Read a French Fry and Other Stories of Intriguing Kitchen Science). When frying food, it's a bad idea to put salt into the fry batter. When it hits the hot oil, the salt will drop out of the batter and cause the oil to break down faster. Ideally, food should be salted before being dipped in batter, and often a second time after it comes out of the oil.

Salt and Beans
To return to the issue of cooking beans in salted water, here's what Russ Parsons says in How to Ready a French Fry: "One common myth is that beans should never be salted before cooking because that toughens the skins. Not only is there no scientific evidence for this, but practical experience says otherwise as well. Actually, salting before cooking has no effect on cooking, it is seasoned through, allowing you to use less salt than if it were added afterward. Furthermore, many of the same recipes that claim it's necessary to hold back on salt until the beans are soft call for cooking beans with salt pork or bacon: an obvious contradiction" (pp. 158-9).

However, Parsons also says that beans cook best when one starts with cool water and leaves the pot in the oven rather than on the range top so that a the heat is steadier and more easily controlled. Maybe it's possible that cooking the beans in salted water is a problem because the water can reach too high a temperature. Maybe that's how the old wives' tale originated.

4 comments:

Leigh said...

So my main concern: I was right?

I asked my chef friend, John, about this and he said the only reasons cooks use salt is for seasoning - they do not worry at all about making things cook more quickly or water boil more quickly.

Jill Duffy said...

Yes, you were and are right. And I agree with John: Cooks add salt primarily to water to add saltiness ("seasoning") to food.

But there are other chemical uses in cooking, too, such as using salt to create the proper temperature to freeze ice cream, or adding a pinch to egg whites when beating to help them stabilize (cream of tartar or a drip of citrus juice works, too).

ba said...

i dunno if it's ONLY for seasoning. many pasta chefs say they oversalt their water because they have to make lots of pasta in a very short period of time. if you haven't read it yet, check out "heat," by bill buford. great read.

so do you recommend "salt," the book?

Jill Duffy said...

I did read Heat. I thought it was a very good read, too.

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