We've all heard the expression: "He's such a bad cook, he can't even boil water." But how often do you actually think about the hidden complexities behind throwing a pot full of water on top of a burner?
Earlier this week, after writing over 7,000 words on the subject of boiling water, I discovered that the average length of my Food Lab posts is directly proportional to my waistline down to the third decimal place. Unfortunately for you, my readers, and for my wife who has to look at me every day, both are expanding at quite a disturbing rate. Rather than expose you to the horrors of an hours' worth of reading on the kitchen's simplest subject, instead, here's my attempt at self-editing down to a more-reasonable-but-still-thorough-attempt. Let's begin.
Up, Up, and Away
First things first: What exactly is boiling? The technical definition is what occurs when the vapor pressure of a liquid is greater than or equal to the atmospheric pressure.
Basically, even though liquid water molecules tend to like each other and stick together, give them enough energy (in the form of heat), and they'll get so hyperactive that they'll attempt to jump up and off into the atmosphere. At the same time, molecules of air (mostly nitrogen and oxygen) are bumping down onto the surface of the water, trying to keep the little guys in line. At reasonable temperatures, the air does a pretty good job of keeping the water in check, allowing only a few molecules to jump up and away. But, give enough heat, the outward pressure of the water vapor trying to escape will exceed that of the air pressing it down. The floodgates open, and water molecules rapidly jump from liquid state to gas.
Ah, the sweet smell of freedom, they seem to say.
This conversion of liquid water to water vapor (steam) is what you see when you're looking at a pot of boiling water.
As we all know, for pure water at standard pressure (the air pressure that exists at sea level), the temperature at which this occurs is 212°F (100°C). But what kind of things can effect this temperature, and what's it all mean for your cooking?
Let's find out.
Quiver, Quiver, Bubble, and Simmer
Recipes often call for things like "simmer," "quiver," and "boil" without offering much by way of technical definition. Here's a quick timeline of what happens when you bring a pot of water to a boil:
- 140 to 170°F: Beginning of "quiver" phase. At this stage, tiny bubbles of water vapor will being forming at nucleation sites (more on those later) along the bottom and sides of the pan. They won't be large enough to actually jump and rise to the surface of the water, though their formation will cause the top surface to vibrate a bit, hence the "quiver." The temperature range between 140 and 170°F is ideal for gently poaching meats, fish, and eggs (around 160°F is standard if you don't want to wait hours for your proteins to cook)
- 170 to 195°F: Sub-simmer. The bubbles from the sides and bottom of the pot have begun to rise to the surface. Usually, you'll see a couple of streams of tiny, champagne-like bubbles rising from the bottom of the pot. For the most part, however, the liquid is still relatively still. This is the temperature range you're looking for in things like making stock or slow-cooking gentle braises and stews. Much lower, and they'll take too long to cook. Much higher, and you run the risk of drying out your meat.
- 195 to 212°F: Full simmer. Bubbles break the surface of the pot regularly, and from all points—not just a few individual streams as in a sub-simmer. This is the temperature to use when using a steamer basket above the water, melting chocolate, or making things like hollandaise in a double boiler.
- 212°F: Full rolling boil. You know the drill. Blanching vegetables, cooking pasta (the traditional way, not our new and improved method), throwing over enemies, etc.
Altitude and Boiling Point
A couple years ago, I was visiting my future in-laws in Bogotá, Colombia. Intent as I was on demonstrating exactly how well-fed their daughter would be in my care, I decided to wake up extra early to make breakfast for the whole family. Mangoes were freshly squeezed, coffee beans were lovingly hand-selected and roasted, fresh milk was gently coaxed from ripe udders, and pandebono was crisped in the oven.
With everything in order and my hosts seated at the kitchen table, I gently slipped a half dozen freshly laid huevos into a pan of water heated to a gentle quiver and waited for them to transform into ethereally tender poached eggs—a transformation I've successfully effected hundreds, if not thousands of times.
Of course, this time nothing happened, and we ended up eating omelets.
The problem is that because of gravity, the higher you go, the less air molecules there are in a given space—the air is less dense. Lower density means lower atmospheric pressure. Lower atmospheric pressure means that the water molecules need less energy to escape into the air. All of this means that everything that happens to our precious water timeline at sea level occurs at much lower temperatures at higher altitudes.
In Bogotá, for example, which is a good 8,000 feet above sea level, water that appears to me to be around 165°F is in reality a good 14 or 15 degrees cooler. In fact, go up high enough, and it becomes nearly impossible to poach eggs—the water comes to a full boil long before appropriate poaching temperatures can be reached).
This graph charts the boiling temperature of water as you go into higher altitudes.
This altitude effect can wreak all sorts of havoc on recipes. Beans don't cook right. Pasta never softens. Stews take longer to braise. Pancakes can over-rise and deflate, just to name a few. Go high enough, and you won't even be able to cook vegetables, which need to be heated to at least 183°F to break down.
For some of these problems, most notably stews, dry beans, and root vegetables, a pressure cooker can be a lifesaver. It works by creating a vapor-tight seal around your food. As the water inside heats up and converts to steam, the pressure inside the pot increases (because steam takes up more space than water). This increased pressure keeps the water from boiling, allowing you to bring it to a much higher temperature than you would in the open air. Most pressure cookers will allow you to cook at temperatures between 240 and 250°F (122°C), no matter what altitude you are at. This is why pressure cookers are so popular throughout the Andes—no self-respecting Colombian home is without one.
As for the other effects of altitude (poached eggs, pancakes, and the like), there are unfortunately no hard and fast solutions to apply across the board. Sometimes, the best you can do is pat your elevationally-inclined friends on the back and say "tough luck. Perhaps next time you won't think of yourself so highly."
Cold Taps, Previously Frozen Water, and Other Myths
Let's sidetrack a bit to dispel a few common water-boiling myths.
- Cold water boils faster than hot water. False.This one makes no sense, and that's because it's completely untrue, and really really easy to prove. It's a wonder it persists. There is, however, a good reason to use cold water instead of hot for cooking: hot water will contain more dissolved minerals from your pipes, which can give your food an off-flavor, particularly if you reduce the water a lot.
- Water that's been frozen or previously boiled will boil faster. False. This one has a little bit more scientific backing. Boiling or freezing water removes dissolved gases (mostly oxygen), which can slightly affect the boiling temperature. So slight, in fact, that neither my timer or thermometer could detect any difference.
- Salt raises the boiling point of water. True... sort of. Dissolved solids like salt and sugar will in fact increase the boiling point of water, causing it to come to a boil more slowly, but the effect is minimal (the amounts normally used in cooking effect less than a 1 degree change). For it to make any significant difference, you need to add it in really vast quantities. So for the most part, you can ignore this one.
- A watched pot never boils. True. Also, my dog isn't cute.
- Alcohol completely boils off when cooking. False. It seems to make sense. Water boils at 212°F and alcohol boils at around 173°F, so surely the alcohol will completely vaporize before you've even made a dent in the water, right? Nope. Even after three hours of simmering, a good 5% of the initial alcohol in your stew will remain. Cook it with the lid on, and that number jumps up by up to ten times higher. It's not enough booze for most people to worry about, but something a teetotaler might want to keep in mind.
On Salt and Nucleation
"But wait!" I hear you cry. "I've seen it myself: Throw a handful of salt in a pot of nearly boiling water, and it will suddenly and rapidly come to a full rolling boil. Surely salt has some significant effect on boiling temperature?"
Adding a handful of salt to simmering or boiling water certainly appears to make it rapidly boil. This is because of little things called nucleation sites, which are, essentially, the birthplace of bubbles. In order for bubbles of steam to form, there needs to be some sort of irregularity within the volume of water—microscopic scratches on the inside surface of the pot will do, as will tiny bits of dust or the pores of a wooden spoon. A handful of salt rapidly introduces thousands of nucleation sites, making it very easy for bubbles to form and escape.
Ever notice how in a glass of champagne the bubbles rise in distinct streams from single points? It's a good bet that there's a microscopic scratch or dust particle right at that point.
On a much grander scale, entire galaxies were formed when matter started to collect in gravity wells formed initially by tiny nucleation sites in the early universe. This baffles scientists (if there was nothing before the big bang, what then were these primordial nucleation sites?). But that's neither here nor there (or perhaps it's everywhere?)
A model of the universe in a pot of boiling water. Whoda thunk it, right?
As we know, water is composed of individual molecules (each with two hydrogen atoms and an oxygen atom; H2O). The faster these molecules move around, the higher the temperature of the water. Now, these molecules have a magnetic charge, meaning that they are affected by electro-magnetic radiation (which, by the way, is not as nefarious as it sounds—the light you see with your eyes and the heat you feel on your skin are both forms of electro-magnetic radiation). Microwaves take advantage of this fact by shooting waves that cause water molecules to rapidly flip back and forth. This motion in turn heats your food.
Because microwaves allow so little energy to be lost to the outside environment (the way, for example a gas burner will heat up the room), they are extremely efficient at heating water. They're great for boiling water quickly without heating up the apartment. An electric kettle is also extremely efficient on this front.
But there's one thing to be aware of. It's called superheating, and it really is as cool as it sounds. Heat up water in a blemish-free container with minimal disturbance (like in the microwave, for example), and because of a lack of nucleation points, it's possible to heat it well beyond its boiling point without it ever boiling.
As soon as some turbulence is introduced—a little wobble from the turntable, for example—bubbles burst forth, sending hot water all over the inside of your microwave. This doesn't happen on the stovetop, since heating from the bottom of the pot creates lots of convection currents (the movement that occurs between relatively hot and cool regions of liquid or gas).
It's a lot like my wife, who will quietly suppress tiny annoyances until the slightest disturbance will send her into an all-out rage. In both cases, the results aren't pretty. It's best to avoid these violent outcomes by commenting on how nice your water's hair looks today or by sticking a wooden spoon in your wife before microwaving her.
Here's an interesting one. Say I'm making a stew in the oven. I put my heavy Dutch oven in there, set the temperature to a moderate 275 degrees, and walk away. Eventually, the water should come to a 212-degree boil, right?
Actually, no. Because of the cooling effect of evaporation (it takes a significant amount of energy for those water molecules to jump from the surface of the liquid—energy that they steal from the liquid itself, cooling it down), an open pot of stew in a 275 degree oven will max out at around 185 degrees. Good news for you, because that's right in the optimal sub-simmer stewing temperature zone.
Pop the lid on, however, and you cut the amount of evaporation that takes place. Less evaporation means higher max temperature. In my quick test at home, putting on the lid increased temperatures in the pot by almost 25 degrees!
For this reason, I generally braise or stew with the lid to my pot slightly ajar. This allows enough evaporation to keep the temperature down, but not so much that the top surface of the stew dehydrates or browns.
Pop quiz: I've got two identical pans. One is maintained at 300°F on a burner, and the other is maintained at 400°F. I then add a half ounce of water to each pan and time how long it takes for the water to evaporate. How much faster does the water in the 400°F pan evaporate than the 300°F pan?
- A. About ten times as fast.
- B. At 4/3rds the rate.
- C. At almost the same rate.
- D. None-of-the-above-and-actually-the-exact-opposite-of-what-you'd-expect-because-the-universe-enjoys-being-confusing.
You got it. The water in the 400°F pan will actually take longer to evaporate. In fact, when I performed this very test at home, it took nearly ten times as long for the water in the hot pan to vaporize. This seems contrary to pretty much everything we've learned so far, doesn't it? I mean, hotter pan = more energy, and more energy = faster evaporation, right?
The principal was first observed by Johann Gottlob Leidenfrost, an 18th century German doctor. The epic coolness of his observation is matched only by the epic coolness of his hairdo. Turns out that if you give a drop of water on a pan enough energy, the steam that it produces will press out so forcefully that it will actually lift the water droplet clear off the surface of the pan. No longer in direct contact with the pan and insulated by this layer of steam, the transfer of energy between the pan and the water becomes quite inefficient, thus the water takes a long time to evaporate.
This effect can be quite useful in the kitchen.
Drop a bead of water on a pan while heating it. If it stays on the surface and evaporates rapidly, your pan is under 350°F or so—a sub-optimal temperature for most sauteing and searing. If, on the other hand, the pan is hot enough for the Leidenfrost effect to kick in, the water will form distinct drops that skid and scoot over the surface of the metal, taking quite a while to evaporate. Congratulations: Your pan is hot enough to cook in.
Put cold milk in a pot and heat it up slowly, you end up with a layer of browned proteins stuck to the bottom of the pot. But, preheat the pot before adding the milk and the Leidenfrost effect will prevent the milk from coming in direct contact with the pan during the initial heating phase, effectively preventing your milk from scorching.
Even cooler: you can actually pour small amounts of liquid nitrogen on your tongue to no ill-effect. The gaseous nitrogen evaporating from he super-cold liquid forms a protective layer, insulating your tongue. I don't recommend trying that one at home.
So. To summarize: things are really only as simple or complicated as you want them to be. You can worry about all this, or you can just pull out the fun facts in casual conversation when you want to sound smart and continue to just throw the pot on the stove when you're really cooking. Most of the time, things will work themselves out just fine.
I think I've managed to cover all the bases, but please feel free to chime in with any further questions on this fascinating subject in the comments!