On boiling
April 20, 2018 1:52 PM   Subscribe

Does boiling water actually do anything special besides evaporate? Or is it just a convenient eyeballing metric for temperature?

For example, "boil, then reduce to a simmer."
posted by aniola to Science & Nature (19 answers total) 3 users marked this as a favorite
 
Could you expand on your question a little? It's kind of unclear.
posted by zamboni at 1:58 PM on April 20, 2018 [1 favorite]


"Boil then simmer" often just means "simmer, but since you don't want to take an hour and a half for simmering to start, spend a few minutes pushing it up to boil and then back off."

However, if you're boiling something other than mostly-water - soup, sauce, candy, etc. - boiling can mean "bring to a temperature that causes chemical changes in the non-water ingredients." Details vary a great deal depending on what foods are involved.
posted by ErisLordFreedom at 2:05 PM on April 20, 2018 [20 favorites]


Technically simmering is boiling, but simmering implies just enough heat to keep the liquid at the boiling point, whereas in your example "boiling" implies adding heat as fast as possible. It doesn't do anything special to the water but it does save time.

On preview, what ErisLordFreedom wrote.
posted by jon1270 at 2:06 PM on April 20, 2018 [3 favorites]


Liquid water doesn't get any hotter than about 212 degrees Fahrenheit. It turns into a gas after that. So whether your pot is simmering or at a rapid boil, the water in your pot is the same temperature.

You turn up your stove to high to get the water up to cooking temperature quickly. After it's at boiling, any extra energy your stove puts into the pan is just turning the water to steam. It's not going into your food, so it is a waste. Reducing the pot to a simmer just uses the least amount of energy you need to keep the pot at 212.

If you want to cook water (or mostly water) on the stove at any temperature above 212, you have to use a pressure cooker.
posted by Quonab at 2:07 PM on April 20, 2018 [4 favorites]


In the context of cooking, I think the relevant thing about boiling water is that it's as hot as it's possible to make water get...hotter than that and you get steam. So, "boil, then reduce to a simmer" is essentially saying, get the water as hot as you can get it, then back off a bit.
posted by Ipsifendus at 2:07 PM on April 20, 2018


I'd have to think there was food involved, who simmers plain water?
posted by rhizome at 2:10 PM on April 20, 2018


While we often think about heat and temperature as describing the same thing, the energy of a system, they aren't identical. Heat is the total energy of a system, temperature is the average energy.

While a liquid solution is boiling, its temperature remains unchanged. In the case of water, that temperature is 212°F / 100°C. That's true whether it is at a rolling boil or a simmer. The difference between those two isn't the temperature, it's the heat - putting more heat in the system (turning up the flame) = boiling faster, but BECAUSE it's boiling faster that water vapor being created carries away all that extra heat, hence why the average temperature the (remaining) liquid water remains constant.

So why boil then simmer? Two main reasons:
1. Good mixing. If you only bring your water up to what you see as a simmer, chances are the average temperature in your whole pot of water hasn't gotten to 100°C - the sides are hotter (because metal is conductive and capable of getting much hotter than water is before it hits any phase change), and the water next to them is getting enough heat to simmer, but the center is still a bit cooler. By coming all the way up to a rolling boil first, you know the whole pot is all at 100°C.
2. To reheat stuff being added. Other than boiling, your water has other mechanisms to lose heat - the main one being to transfer it to the other things you add while cooking. That will lower the temperature of the water; it will stop boiling. By bringing it up to a rolling boil before adding that stuff, you know you've got your burner pumping in a lot of extra heat so the temperature can quickly come all the way back up to boiling. Then you can lower it back to simmer and be sure everything in your pot is sitting at 100°C.
posted by solotoro at 2:15 PM on April 20, 2018 [18 favorites]


Water doesn't get any hotter than 212 F, but the pot definitely does. Boiling gets everything hot quickly, but backing it off keeps whatever is in the pot from scorching along the bottom.
posted by supercres at 2:16 PM on April 20, 2018 [1 favorite]


I think I understand your question, which is - why do most recipes center around boiling and simmering (212° food), as oppossed to, say, 185° food. I've thought about this a lot, like with pasta for example. What temperature below boiling could you cook pasta and still have the noodles cooked thoroughly and it take about the same time.

My guess is that there are temperatures (probably different ones for different foods) that will totally work as well as 212°, but because of the lack of reliable thermometers in almost all houses, we've just gotten into the habit of using the boiling point, because you can tell the temperature without any tools.
posted by FirstMateKate at 2:18 PM on April 20, 2018 [2 favorites]


"reduce to a simmer" is usually called out when it's substantially more than just water. I can think of two reasons -
1 - nothing is a perfect conductor, so the part that's next to the heat will get hotter than the rest... in the case of a creamy soup, the layer just next to the pan burns if you kept the heat high
2 - simmering is normally called out for a long time period. If whatever you're cooking is left on high, you'll boil all the water off.
(that and simmering means less splatter and mess all over your stove!)
posted by cfraenkel at 2:22 PM on April 20, 2018 [1 favorite]


I'd have to think there was food involved, who simmers plain water?

People who make tea from it. Simmering is a lower temp than boiling - the water at the bottom of the pan, touching the heat, is boiling; the rest of the water is just below that. Simmered water cools to drinking temperature faster.

And some foods are closer to water than others. Some don't have the water mixed in, which changes how they react to the heat - double boiler things like chocolate and cooking hardboiled eggs both involve the water as heat-carrier rather than ingredient, so the time spent reaching the correct heat level may be important.
posted by ErisLordFreedom at 2:25 PM on April 20, 2018


Does boiling water actually do anything special besides evaporate?

In terms of cooking 100C isn't that significant a temperature.

Meat and fish proteins cook to various levels of doneness between 50C and 70 C.
Connective tissues don't break down until about 80C+ (what you want for ribs, say)

Egg whites start to cook at 60C, the yolks at around 63C.

Vegetables need to be 80 to 83C as well for the cell walls to start to cook properly.

Millard reactions, browning/toasting, happens at 140 to 160 C (and needs to be dry).

Probably most significantly, bacteria and other microorganisms generally don't survive much above 60C.

In terms of the chemistry of what heat does to food, it's not a threshold temperature for deciding whether most foods are cooked or not. Most boiled food will be cooked, however, and that is an important guarantee. It guarantees "doneness" but also safety, by ensuring reasonable kill levels of the growies.
posted by bonehead at 2:35 PM on April 20, 2018 [7 favorites]


I've seen it (somewhere that I can't find right now) explained as about timing.

Scenarios:
1. Bring to boil, reduce to simmer, cook for x minutes. You know when it's boiling. You know when you've reduced to simmer. You can count x.
2. Heat directly to simmer, cook for y minutes. OK, when does that simmer start? First bubble? A few bubbles? Just on the edge? When it starts to bubble, then you stir, and it actually bubbles again?
posted by Huffy Puffy at 3:53 PM on April 20, 2018 [3 favorites]


My guess is that there are temperatures (probably different ones for different foods) that will totally work as well as 212°, but because of the lack of reliable thermometers in almost all houses, we've just gotten into the habit of using the boiling point, because you can tell the temperature without any tools.

This is why people use sous vide, which relies on precise water temperatures well below the boiling point. Sous vide definitely produces different results than boiling/simmering, especially in terms of texture. But if all you have is a stovetop, then you are mostly limited to the regular warm/simmer/boil like in most recipes.
posted by Dip Flash at 4:26 PM on April 20, 2018


Cooking at a simmer provides a fairly precise repeatable recipe for cooking because the temperature will always be 212 F (varies somewhat with altitude) and doesn't require a thermometer to measure.

So you could have a recipe that says cook for 10 minutes at 180 degrees, or cook for 8 minutes at 190 degrees or cook for 5 minutes at 200 degrees.

It much easier to just have one recipe that says cook for 4 minutes at a simmer, which is 212 degrees. This is easily repeatable and you get the same results every time without needing a thermometer.
posted by JackFlash at 4:37 PM on April 20, 2018



I'd have to think there was food involved, who simmers plain water?


I simmered plain water just the other day as an ad-hoc humidifier. Works great!
posted by Autumnheart at 5:08 PM on April 20, 2018


Best answer: I think boiling does do something very different as long as the surface temperature of the food remains more than a couple of degrees below 100C.

Simply because boiling makes the food come into contact with a significantly greater amount of water vapor than simmering does, and water vapor carries more than five times as much energy for a given mass of water than it takes to heat that same mass of water from 0C all the way to 100C (9717 cal/mole as opposed to 1800 cal/mole), and when that vapor condenses on the food it delivers that energy to the food, whereas liquid water coming into contact with the food can only deliver the energy corresponding to the calories it would take to heat that amount of water from the temperature the water has when it moves away from the food to the general temperature of the water bath.

This effect is diminished by the fact that water vapor is much less dense than liquid water, but as anybody who's gotten a steam burn can attest, despite its lower density, running ones hand through a jet of steam results in a far worse burn than even plunging ones hand briefly into a pot of simmering water.

So boiling something first and then turning the heat down to a simmer results in a brief period of very rapid cooking as the surface temp of the food rises toward 100C where condensation is less efficient, followed by a longer period of more gradual cooking as the food gets energy from slightly hotter water by conduction during simmering.
posted by jamjam at 5:16 PM on April 20, 2018


It's much easier to hold water at 100°C than pretty much any other temperature, because rather than getting hotter it will just boil more vigorously. It's also easy to tell if it's cooling down, because it will stop boiling. This was especially true in pre-thermometer times, which is when almost all basic recipes were invented. If your water is boiling, you can be sure (unless you're at high elevation which few people are, especially back then) that it's going to do what boiling water does and your food will cook as expected. It's a very simple and reliable way of maintaining a constant cooking temperature.

As for simmering, that's mostly just about not wasting energy (because if it's simmering, it's as hot as it's going to get) and not burning the food on the bottom of the pot (because the bottom of the pot will get hotter than 100°C).
posted by Anticipation Of A New Lover's Arrival, The at 5:16 PM on April 20, 2018 [1 favorite]


The water in the pot is not all at the same temperature. One easily sees that the water turns to steam at the bottom where heat is supplied by the burner. Water at the sides of the pot is radiating heat to the pot wall and then the air, so it's cooler. it a big pot at a very low simmer, it could be quite a bit cooler.

I saw a TV chef (Sara Moulton, I think) describe the nominal temperatures associated with various visible effects, and I think the lowest simmer might have been associated with an average temperature in the pot of 150F. (Don't quote me.) I imagine this to be cooking school lore.

In answer to your specific question of whether boiling is different in a particular way, there are certainly times when the production of steam is required for a good result. The obvious is boiling a bit of water in the bottom of a pot in order to steam vegetables. Another is that steam is what causes popovers (and equivalently Yorkshire pudding) to rise. Possible also crepes (pretty much the same batter).

Steam also plays a part in braising, cooking the food that's above the water level. So braising has to be hot enough to keep the production of steam going.
posted by SemiSalt at 6:24 AM on April 21, 2018


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