There's something about boiling...
January 22, 2011 5:09 PM   Subscribe

What is it about boiling water that does stuff to things being boiled?

If I'm making pasta or hard-boiled eggs, I need to boil water. Not merely heat water to 100C -- because if you're at a different altitude, the boiling temperature of water might be higher or lower. What makes the magic happen is the phase change, the boiling part. Why?

What is it about the transformation of liquid water into steam (at whatever temperature) that is necessary to alter foods? And why isn't it just a function of temperature, but of phase change? Why does a pressure cooker, which raises the boiling point of water, cook foods faster? Is it just that it lets the cooking liquid become hotter?

If I were to try to make pasta in a low-pressure chamber, would it "cook" as long as the water boiled, even if the temperature were, say, 30C?

Physicists and food scientists, please tell me everything about boiling.
posted by overeducated_alligator to Science & Nature (25 answers total) 5 users marked this as a favorite
 
Um, are you sure about this? Because my understanding is that cooking things by boiling at high altitudes does indeed take much longer. I'm pretty sure it's the "heating things up" aspect - i.e. high temperature - that does the cooking. Which is why you can cook things without boiling, i.e poaching or sous-vide etc.
posted by smoke at 5:12 PM on January 22, 2011


Additionally, a pressure cooker cooks things faster - not because of raising boiling temperature per se, but because it allows the temperature to get higher than it otherwise would (e.g 120 degrees celsius). The reason for this is because without pressure, too much heat is lost through boiling - which is transferring the heat from the water into vapor into the atmosphere.

So again, it's all about heat.
posted by smoke at 5:14 PM on January 22, 2011


Yes, you just need the right temp for the right time. And remember that a pressure cooker is pressurizing everything, including the water that's in the food itself.
posted by Cool Papa Bell at 5:17 PM on January 22, 2011


If I'm making...hard-boiled eggs, I need to boil water.

This is not the case. Using boiling water makes the process faster, and it's easier to time a boiled egg in boiling water than it is to mess with lower temperatures, but there's nothing magic about cooking a hard-boiled egg that requires boiling water.

An egg will become hard-boiled once the yolk reaches 158ºF / 70ºC. At that point all of the proteins in the white and yolk have denatured and firmed up. So you can cook an egg in water as 'cool' as 158ºF, it'll just take a lot longer than if you use boiling water.

Even cooking an egg until the point that the yolk becomes crumbly only requires that the yolk get up to 194ºF / 90ºC.
posted by jedicus at 5:22 PM on January 22, 2011 [4 favorites]


You're overthinking it.

The boiling water makes things hotter. Cooking things in boiling water takes notably longer at altitude (trust me, I've tried).

Normally the variations in boiling point aren't enough to matter for recipes or directions since it's all approximate anyway.
posted by Diplodocus at 5:22 PM on January 22, 2011


P.S. to the original question -- I have brewed beer for years; my dad and I have wondered whether you really need to BOIL the work to extract the resin from the hops, or just heat the wort to 100C. Beer wort with malt extract has a higher boiling temperature than plain water. I have always figured that it's OK to just heat the work to 100C and it will extract from the hops; my dad had read that you have to bring it to a rolling boil.
posted by overeducated_alligator at 5:25 PM on January 22, 2011


~"work" should read "wort" in several places...
posted by overeducated_alligator at 5:26 PM on January 22, 2011


Yep, it's the heat, not the boiling. It's difficult to raise a liquid to a higher temperature than its boiling point—unless you add additional pressure, it will just boil faster, not get hotter. Boiling just represents the highest practical temperature you can raise a given liquid at a given pressure.
posted by dephlogisticated at 5:33 PM on January 22, 2011


It's a well-trodden anecdote that it takes longer to cook rice at high altitude because the boiling point of water is lower, so you're cooking it at a lower temperature.

For most things, you use boiling water because that's the hottest you can get water, and it's easy to keep it at a constant temperature.
posted by auto-correct at 5:47 PM on January 22, 2011 [1 favorite]


I had a physics teacher once who demonstrated the water-boiling-at-room-temp-in-a-vacuum, and he specifically said that you would not be able to cook anything in that situation. The phase change of the water has nothing to do with the cooking process. Heating water to a boil, under typical conditions, just means that you're getting the water as hot as it's going to get.
posted by hwyengr at 6:04 PM on January 22, 2011 [5 favorites]


Boiling is simply an easy way to keep water at a constant temperature, because otherwise you'd have to constantly adjust the amount of flame or electricity to maintain a constant temperature and that is not easy to pull off without special gear. It doesn't really matter what that temperature is, just that it's constant, because you can adjust the time to compensate for temperature. If you had some other means of controlling temperature, say an immersion circulator with an PID controller, then you wouldn't have to use boiling, and you could sous vide chicken or make 64C eggs. It's only the combination of heat and time that matters.
posted by Rhomboid at 6:12 PM on January 22, 2011 [1 favorite]


And why isn't it just a function of temperature, but of phase change? Why does a pressure cooker, which raises the boiling point of water, cook foods faster? Is it just that it lets the cooking liquid become hotter?

It is just a function of temperature. A pressure cooker just allows the water to get hotter. There is nothing special about boiling water. Its just the hottest water and its a convenient reference temperature you don't need a thermometer or any kind of monitoring to achieve and maintain.

Most food is 'cooked' when it reaches a given internal temperature. Sous vide cooking uses water held at the target internal temperature (below boiling) to cook food to ensure that the food doesn't get cooked beyond that temperature. The fancy way to do this is with an immersion circulator, but you can also cook the things you mentioned by heating a large volume of water (large so that its more temperature stable) to the desired final temperature of your food and submerging your vacuum-bagged food in that water. With a thermometer and careful watching you can keep your water within a couple degrees of your target temperature and you can cook fine.

If I were to try to make pasta in a low-pressure chamber, would it "cook" as long as the water boiled, even if the temperature were, say, 30C?

No. Because 30C is too low. I'm not sure what temperature pasta is cooked at, but its definitely higher than 30C. If it was 30C, all the pasta in the store would eventually cook on hot days.

A steak, though, will be nicely medium-rare cooked at an internal temperature of say 60C. So you absolutely can cook a steak in a 60C water bath (in an airless plastic bag to keep the steak from being super water logged) for a few hours until the entire piece of meat has reached 60C.

There are plenty of chemical changes that occur in food that will not happen below 100C: the Maillard reaction, for one. Carmelization is an example, as is browning of meat. A broad array of chemical reactions takes place, leaving behind all sorts of tasty flavonoids and such.

This is why boiled beef in something like a New England Boiled Dinner is bland, compared to a grilled steak, or why sauteed vegetables taste so much different from boiled ones. You can make boiled potatoes in boiling water, but never french fries, because water can't be heated to a temperature sufficient to carmelize the sugars on the outside of the fries. So what do you do? Immerse the fries in heated oil, which can go to a higher temperature. If you built a crazy pressure cooker that would allow you to heat water to 200C, you could make french fries in boiling water. Although they'd immediately get soggy, so that would really be a crap way to make fries.

Recipes use boiling because it's hot, it's relatively consistent and it's a reference point. If you say to boil something, you generally know its cooking at around 100C, so you can say "do so for X minutes", knowing the heat will transfer into the food at a more-or-less consistent rate (but not perfectly consistent, which is why you still need to check for doneness) and will achieve the desired internal temperature somewhere around the time you suggest. You can also say, "cook this at 90C for Y minutes", but how do you keep your water at 90C? This is what sous vide recipes say, keeping water at 90C is what the immersion circulator does.

Ideal boiled eggs, though, are not cooked to 100C, this is why most people's egg-boiling recipe includes a step like "heat the water to boiling, then turn off the heat and leave it off for X minutes with the lid on". You are riding both the egg-heating and water-cooling functions, knowing that the water is also losing heat, and hoping the egg gets to the perfect internal temperature without getting overdone. Check out this awesome article for a way more detailed explanation.

Also, many people feel that coffee and certain teas are actually optimally extracted at temperatures slightly below boiling, but still, its easy to say "boil water, pour over tea", so that's what most people do.

If you find yourself wondering about other things like this, you should pick up a copy of Harold McGee's On Food and Cooking, which explains the science of cooking in wonderful and fascinating detail, without being unapproachably complex.
posted by jeb at 6:14 PM on January 22, 2011 [15 favorites]


One of the reasons recipes tell you to wait until water is actually boiling to put stuff in is that boiling is a really clear indication that the water is at a warm-enough temperature to cook with. It might have been warm enough ten minutes ago at 70C, but you don't know for sure. And you want consistency in recipes and cooking.

Another reason is that the temperature of the water will drop once you add cold stuff to it, like pasta. And if it drops from boiling, it might only drop to a temperature that is still effective for cooking; if it's at the minimum cooking temperature already, it will fall to lower than that, and you'll have to wait for it to heat up again before your food starts to cook.
posted by lollusc at 6:19 PM on January 22, 2011


It's all about temperature. I've cooked dried pasta at over 20,000 feet and it takes more than half an hour. Even though there is a very vigorous, steaming boil, the temperature is only 170 degrees. In fact it boils so rapidly that you will lose all the water to steam before your food is cooked if you aren't careful. So boiling alone isn't the answer. It's temperature, and at high altitude/low pressure water boils at a low temperature.

In contrast to low pressure at altitude, a pressure cooker raises the boiling point so the food cooks hotter. Boiling isn't really the point. A boil just ensures that the water is as hot as it can get at a given altitude/pressure.
posted by JackFlash at 6:29 PM on January 22, 2011


(And people will sometimes use a pressure cooker at altitude in order to raise the boiling point of their water back up to 100C, so that it's easier to cook.)

There are some situations, I'm sure, where the phase change is actually the significant thing. Popcorn is the only example that comes to mind immediately but I bet there are a few others. They're the exception, though.
posted by hattifattener at 6:47 PM on January 22, 2011


Boyle's Law
posted by Cuspidx at 6:47 PM on January 22, 2011


Basically you just want the hottest water you can get so you can cook as quickly as possible. The hottest water can be is just before the phase change from a liquid to a gas.

You can get other liquids way hotter, for example oil, and cook objects very quickly without being at the oil's boiling point.
posted by 2bucksplus at 6:50 PM on January 22, 2011


If you are talking about heating a sealed container of stuff (bag-o-soup, or egg), then it is just about heat.

But if you are talking about soup or meat or stew, the process of boiling does have an effect. It coagulates proteins, because all the jostling causes the protein strands to knock into each other.

The energetic molecules knock into the food and tear it apart. (That noise the pan makes when it is about to boil is tiny bits of water vaporizing and re-condensing against the metal pan.)

The reason you use a pressure cooker also, is that you can cook things quickly *without* the water vigorously boiling and tearing up your chili. There is a LOT more energy in a system of water boiling at 100c than there is in water not boiling at 110c.

Also, as the water vapor molecules in the bubbles that are rising to the top, they bump into the not-boiling food and condense, releasing their energy into the food. This is why the beer wort needs to be boiled if you want to extract the nasty resins out of the hops.

It isn't just about temperature.
posted by gjc at 7:02 PM on January 22, 2011


Notwithstanding all of the above (heat is what cooks, not phase change), I find that pasta sticks to itself more if I put it in before the water actually starts bubbling. So I think the rolling boiling helps pasta cook *better*--not more thoroughly, just better.

I wonder if there are other foods that are similarly affected by rolling boil.
posted by galadriel at 7:27 PM on January 22, 2011


But if you are talking about soup or meat or stew, the process of boiling does have an effect. It coagulates proteins, because all the jostling causes the protein strands to knock into each other. The energetic molecules knock into the food and tear it apart.

No offense, gjc, but I'm pretty sure this is nonsense. On an atomic scale, the individual molecules in boiling water at 100°C aren't moving around any faster than in water that's not boiling at 100°C. It only appears otherwise because, on a larger scale, water vapor is fighting it out with atmospheric pressure, causing the liquid to bubble and cavitate.

If you put soup, meat, or stew in a sealed container and shake it around, it's not going to have any real effect on the food. Likewise, if you lower the pressure in the sealed container, causing the liquid to boil at room temperature, the food still will not be affected. It's heat that denatures proteins, not boiling.
posted by dephlogisticated at 7:57 PM on January 22, 2011 [3 favorites]


The biggest advantage of boiling water is that the temperature is consistent -- always the same from beginning to end during the process, and always the same from batch to batch. If you try to use a temperature somewhat lower, you're going to have consistency issues, and you're more likely to undercook or overcook.
posted by Chocolate Pickle at 8:25 PM on January 22, 2011


This is why recipes where you are instructed not to let the mixture boil (e.g., custards and sauces made with eggs) are notoriously more difficult.
posted by bad grammar at 8:33 PM on January 22, 2011


The energetic molecules knock into the food and tear it apart.

The most probable velocity (which is slightly less than the mean velocity) of water molecules at 100C is ~500-600 m/s (from our friend the Maxwell-Boltzmann distribution). The bulk velocity of boiling water is, roughly, no greater than about a meter per second. (you can just judge that for yourself; if my pot is 5cm tall, do bubbles rise in 1/20th of a second?) Even if we assume that the water is moving at 10 m/s, the kinetic energy of the bulk motion is orders of magnitude less than the energy in the thermal motion of the particles.

Also, as the water vapor molecules in the bubbles that are rising to the top, they bump into the not-boiling food and condense, releasing their energy into the food.

Those gas atoms may do so, but the gas in the bubbles is less dense than the liquid water, so the gas bumps into the food less frequently than the liquid would. Any air bubbles that attach to the food are thus slowing down the heat transfer. This is why steaming your food takes longer than boiling it, and why defrosting in air is slower than defrosting in room-temperature water.

There is no more energy in "boiling water" than there is in non-boiling water at the same temperature. A basic thermal physics textbook will explain this in detail (Kittel and Kroemer has been the canonical text for the past 30 years), but a far more exciting and enjoyable read is Harold McGee's "On Food and Cooking", as mentioned by jeb.
posted by kiltedtaco at 9:06 PM on January 22, 2011


Well, when you're making cheese the heating of the milk causes the protiens to bind together and shrink which pushes out the moisture (whey). My understanding is that all proteins act this way - steaks shrink, bacon curls up, eggs get firm. It wouldn't surprise me if your wort behaves similarly, squeezing out the good stuff. Optimum temp for best results depends on what you're making.

But, I'm no hot water scientist....
posted by amanda at 5:26 AM on January 23, 2011


I have brewed beer for years; my dad and I have wondered whether you really need to BOIL the work to extract the resin from the hops, or just heat the wort to 100C. Beer wort with malt extract has a higher boiling temperature than plain water. I have always figured that it's OK to just heat the work to 100C and it will extract from the hops; my dad had read that you have to bring it to a rolling boil.

The importance of a rolling boil in beer brewing is more than just extracting hop oils. Boiling is important for driving off off-flavors like DMS, for coagulation of hot-break compounds, and for evaporation to concentrate your wort (important in all-grain brewing, probably not as important for most extract brewing). Here is a nice explanation that goes into the science in very readable terms. Failure to boil, and more specifically, failure to reach and sustain a vigorous, rolling boil, will significantly affect your beer. (Unless you want to make sour beers, in which case you sometimes specifically don't want to boil. Probably the most successful beer I ever made was a no-boil Berliner Weisse recipe that I got from a friend, but that is a pretty specific style.)

So, the reason for full boils in beer brewing are slightly different than for reasons in cooking (and there are some great explanations above for that). Like others have said, if you are interested in this topic for cooking, Harold McGee's "On Food and Cooking" is a beautiful tome full of interesting food science facts.
posted by wondercow at 9:54 AM on January 23, 2011 [3 favorites]


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