The Rolling Boil

[Chufi]

warning, stupid questions coming up.

I had a discussion with some friends recently who were boiling a pot of pasta at a very slow boil (the water was bubbling, very calmly) with the lid on the pan. I have always believed (although I do not remember where I got my info from) that you should cook pasta uncovered, at what I believe is called in English a 'rolling boil'.

We then got on to discussing the principles of boiling. My friend said, when the water is bubbling, it´s boiling, and it makes no difference if it´s bubbling hard or slow. While I could not really argue with that, it got me thinking about the difference between boiling and simmering. Is a simmer a boil? If the temperature is the same in both cases, why are we instructed to keep a Bolognese sauce at a very lazy simmer, and should stock never reach a rolling boil?

In the case of the pasta, I could make one good point for the rolling boil: the rapid movement of the water keeps the pasta moving which makes it harder for it to stick together. But this does not answer my basic boiling questions.

Someone less scientifically challenged, please enlighten me...

[sanrensho]

Here's my take on the pasta question. I watched a Japanese show quite a while ago on the subject of Italian food. (Japanese TV is full of this kind of stuff.) The show was filmed in Italy, and the Italian chef gave the specific advice to boil (dried) pasta in a covered pot. His reasoning was that a typical home cooktop does not have the same power as a commercial stove, so covering the pot keeps the water at a higher temperature.

In practice, I don't have have a large enough pot to boil the quantities of pasta I need (350-400 grams), without causing the water to boil over if I cover the pot. However, I will cover the pot if I can get away with using a smaller quantity of water, such as when boiling smaller quantities of dried pasta.

If I had a bigger pot that didn't require bringing the water level so high, I would definitely cover the pot whenever boiling (dried) pasta.

Edited July 5, 2007 by sanrensho (log)

[Chufi]

His reasoning was that a typical home cooktop does not have the same power as a commercial stove, so covering the pot keeps the water at a higher temperature.

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yes, but how can boiling water be at higher temperature with the pot covered? Are there variables in boiling point? Or is that a myth? That´s what I´d like to know...

Unless, ofcourse, you mean that covering the pot after putting the pasta in will help the water to come back to the boil quicker. That I understand. But once it´s boiling - what´s the difference?

[mizducky]

Hi, Klary--

Check out these very informative links from Wikipedia:

This one directly addresses your cookery questions, such as:

A good point to bear in mind in preparing foods by boiling is that slowly boiling water has the same temperature as rapidly boiling water and is therefore able to do exactly the same work

But it also explains a few legit reasons for, say, keeping the lid on a pot of water (it can't effect the final temperature, but it helps your water come to a boil faster).

Meanwhile, this link goes over a bit of the science behind boiling.

[sanrensho]

The regular Wikipedia entry on boiling gives this explanation:

The boiling point is defined as the temperature at which the vapor pressure of the substance equals the pressure above the substance. Increasing the pressure as in a pressure cooker raises the temperature of the contents above the open air boiling point. Adding a water soluble substance, such as salt or sugar also increases the boiling point. This is called boiling-point elevation. However, the effect is very small, and the boiling point will be increased by an insignificant amount.

So it would seem that my source is correct in his logic, but incorrect in terms of having a significant effect once brought to boil. Especially when you consider that even a tight-fitting lid does not act as a pressure cooker.

The point about bringing the water back to boil more quickly is well-taken, however.

Edited July 5, 2007 by sanrensho (log)

[Adam Balic]

The temperature of "boiling" water at sea level is the same (100.C), when at a rolling boil or at simmer. But the amount of energy being put into the system is different. Think about boiling water at a very high heat verses a low heat - the former boils dry (water turns to steam) much quicker.

Haven't quite worked out what the consequences for cooking are, but in theory this should mean that more water molecules interacting with food and passing on their energy.

I boil dried pasta at a rolling boil to help prevent the pasta from sticking together.

Edited July 5, 2007 by Adam Balic (log)

[Hiroyuki]

Keep at a simmer, and you will get clear broth.

Keep at a rolling boil, and you will get cloudy, tonkotsu ramen broth.

It's a matter of convection, right?

[doctortim]

Whether at a simmer or a rolling boil, the temperature is still the same. As far as I understand it, Hiroyuki's right, it's about convection. The higher the temperature of the heat source at the bottom of the pot, the faster the convection currents. That is, the water is moving around more quickly.

In the context of a stock, you want to simmer on a cooler heat source so the water isn't rushing around and brushing off meat and vegetable particles and stirring them up into your stock.

In the context of a meat ragu, you want the ingredients hot and releasing flavours for a longer period of time in order to develop all of the flavours and disperse them throughout the sauce. If it's at a rolling boil the water will be circulating much more quickly and will boil away quicker. Since clarity doesn't matter, conceivably you could keep the sauce at a rolling boil and just be sure to frequently top up the water as it boils away. But your stove would be covered in flecks of ragu, and why do that when you can leave it at a simmer and walk away for 6 hours?

With cooking pasta, as soon as you add the pasta to the water, the pasta is a cooler temperature than the water so the water transfers its heat to the pasta and in the process it cools down. You want to transfer heat as quickly as possible back into the water to bring it back to cooking temperature, so for all other things the same you want the heat source to be at a high temperature. A rolling boil before you add the pasta is a reflection of a high temperature heat source that you want.

It's been a while since I've learned about these things, but I think these are the principles.

[joesan]

Chuffi, Boiling is different from simmering. Boiling always takes place at 100C (actually not strictly true the boiling point will alter minutely depending on altitude) but to all extents and purposes you can count it as 100C.

Here are the other temperatures of common cookery terms -

Poach 71-82C,

Simmer 85-96C,

Boiling 100C

In terms of the pasta boiling is boiling but you would not want to just boil away at maximum temperatures and add water (as mentioned above) with proteins because of the various points when proteins denature. These occur at less than 100C and could result in stringy or oddly textured meat, fish etc. if you choose to boil them in the cookery process.

Edited July 5, 2007 by joesan (log)

[paulraphael]

The temperature of "boiling" water at sea level is the same (100.C), when at a rolling boil or at simmer. But the amount of energy being put into the system is different. Think about boiling water at a very high heat verses a low heat - the former boils dry (water turns to steam) much quicker.

The question is what happens to that additional energy at the rolling boil--does it somehow make it into the food, or does it just escape as steam at a faster rate (hence the rolling boil ... )?

[paulraphael]

Keep at a simmer, and you will get clear broth.

Keep at a rolling boil, and you will get cloudy, tonkotsu ramen broth.

It's a matter of convection, right?

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As far as I know this has to do with the churning action of rapidly boiling water. The vigorous stirring action of the boil emulsifies a lot of the fat and smaller protein particles that would otherwise rise to the top where you could skim them off.

[MarkIsCooking]

Here's what I heard at school recently, just after we had a visit from some Italian food experts visiting from the homeland.

By the way, I have tested water temp with a thermometer and you will start to see small bubbles, which some refer to as a simmer, below the boiling point of 212.

The Italians told us that the goal is to have the water hot enough so that when you add your pasta the water continues to boil. Therefore, you need a pot much larger than what most home cooks use. They said at least 1.5 gallons to a pound of pasta. Also, you want the water to 'taste like sea water' meaning rather salty, however, don't add the salt until after the water boils because it will slow the boil.

When you're boiling a large amount of water, they suggest covering the pot because the water will boil faster (as mentioned above) if covered, thereby conserving heat.

Lastly, as most here know, they always extract their pasta from the water (never pour it out through a colander) because they typically use some of the pasta water to finish their sauces.

-Mark-

[Restorer]

I've been under the impression that the difference between a simmer, a slow boil, and a rolling boil is the temperature gradient in the water. In order for a bubble of water vapor to form at sea level, it must be at or above 100C. Liquid water at sea level must be at or below 100C. Water at 100C can be in either phase, and there's a discrete amount of energy to convert between the two.

Since the heat source is only at the bottom of the pot, what happens is that the water at the bottom obtains the energy to change states, becomes vapor, and begins to bubble upward. As it does so, it's no longer being heated by contact with the pan bottom, and all the liquid surrounding it is no hotter, so it starts to lose heat energy. As it does so, the vapor turns back into liquid.

At any temperature that causes the water to bubble, the water at the bottom of the pot is going to be at 100C. The lower the output power of the heating element (flame, coil, or surface), the greater the gradient of temperature between the bottom and the top, since water is evaporating from the surface, cooling the liquid.

I think the effective difference is in the average temperature of the water. At a rolling boil, you can be sure that the top of the water is as close to the boiling point as it will get. At a low simmer, the surface of the water is just hot enough to assure that the bubbles of steam make it to the top - this could be as low as 85C.

[andrewB]

.  Also, you want the water to 'taste like sea water' meaning rather salty, however, don't add the salt until after the water boils because it will slow the boil.

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on a tangent to water boiling, adding salt will raise the boiling temperature of the water so it will take longer for the water to reach a boil but the temperature would be higher and take less time for the substance to cook. no? wasn't it either Alton or heston who gave a chemical reasoning for adding salt to the water prior to it boiling?

[jackal10]

The physics is rather more complex.

In most cases heat is added at the bottom of the saucepan, and lost through the sides and a the top, and through evaporation at the surface, so there is a temperature gradient.

Bubbles form where the vapour pressure is greater than the local pressure, enough to overcome the surface tension effects, (I'm ignoring the dissolved gas for now). The actual boiling point can be modified slightly by salt etc, but that is a very small effect in normal amounts. A greater effect is the atmospheric pressure, and in high places the boiling point can change by several degrees.

A bubble forms at a nucleation point, maybe a local hot spot, such as a microscopically rough patch on the bottom, and once detached rises and cools, the vapour condensing. If it makes it all the way to the surface the remaining vapour is lost, carrying its heat away.

Thus in a simmer, with the bubble barely breaking the surface the bulk of the liquid will be below boiling point, typically around 90C. The system is stable where heat input is low to match the heat loss, and bubble production moderate

In a rolling boil the bulk of the liquid is hotter close to but still below 100C or it would all turn to vapour, so the cooling effect on the rising bubbles is smaller and most of them break the surface. Many bubbles are produced, since the heat input is high, and the liquid is near boiling point.

In a rolling boil there is much turbulence and mechanical agitation of the liquid by the bubbles, which leads both to a more uniform temperature, and to effects such as emulsification of any fat, which is why fast boiled stock is cloudy.

I imagine the interaction with pasta is complex, and the optimum boil will depend on many variables, such as the size and shape of paste pieces, their initial dryness and composition, their permeability whether there are additional ingredients such as egg to help hold them together and so on. There is a balance between hydration and the effects of heat on the complex protein/starch mix that is pasta.

[slkinsey]

Also, you want the water to 'taste like sea water' meaning rather salty, however, don't add the salt until after the water boils because it will slow the boil.

on a tangent to water boiling, adding salt will raise the boiling temperature of the water so it will take longer for the water to reach a boil but the temperature would be higher and take less time for the substance to cook. no?

Adding salt to water will raise the boiling point less than 0.1 degrees and, in the amounts one is likely to add in the kitchen, shouldn't meaningfully affect the length of time it takes for the water to come to a full boil. Depending on when you add the salt it may, however, help to create the impression that it is taking longer. This is because the salt crystals act as nucleation points and can release a fair amount of dissolved gas if the salt is added at just the right temperature. The result is that it takes longer for bubbles to appear, but the length of time it takes for the water to come up to 100C is not meaningfully changed.

[andrewB]

Also, you want the water to 'taste like sea water' meaning rather salty, however, don't add the salt until after the water boils because it will slow the boil.

on a tangent to water boiling, adding salt will raise the boiling temperature of the water so it will take longer for the water to reach a boil but the temperature would be higher and take less time for the substance to cook. no?

Adding salt to water will raise the boiling point less than 0.1 degrees and, in the amounts one is likely to add in the kitchen, shouldn't meaningfully affect the length of time it takes for the water to come to a full boil. Depending on when you add the salt it may, however, help to create the impression that it is taking longer. This is because the salt crystals act as nucleation points and can release a fair amount of dissolved gas if the salt is added at just the right temperature. The result is that it takes longer for bubbles to appear, but the length of time it takes for the water to come up to 100C is not meaningfully changed.

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so then adding salt to water only after it has boiled is indeed a 'grandmother's clause' in cooking?

[slkinsey]

I can't think of any reason it would make any meaningful difference to add the salt only after the water is boiling. Salt can cause pitting in stainless steel, which might argue for minimizing the amount of time salt contacts the interior of a stainless steel pot, but I can't imagine that this becomes a problem before one has got his/her money's worth out of the cooking vessel (it also doesn't explain why this advice is given with respect to boiling water but yet we still long-simmer stews, sauces, etc. that include salt).

[budrichard]

The physics is rather more complex.

In most cases heat is added at the bottom of the saucepan, and lost through the sides and a the top, and through evaporation at the surface, so there is a temperature gradient.

Bubbles form where the vapour pressure is greater than the local pressure, enough to overcome the surface tension effects, (I'm ignoring the dissolved gas for now). The actual boiling point can be modified slightly by salt etc, but that is a very small effect in normal amounts. A greater effect is the atmospheric pressure, and in high places the boiling point can change by several degrees.

A bubble forms at a nucleation point, maybe a local hot spot, such as a  microscopically rough patch on the bottom, and once detached rises and cools,  the vapour condensing. If it makes it all the way to the surface the remaining vapour is lost, carrying its heat away.

Thus in a simmer, with the bubble barely breaking the surface the bulk of the liquid will be below boiling point, typically around 90C. The system is stable where heat input is low to match the heat loss, and bubble production moderate

In a rolling boil the bulk of the liquid is hotter close to but still below 100C or it would all turn to vapour, so the cooling effect on the rising bubbles is smaller and most of them break the surface. Many bubbles are produced, since the heat input is high, and the liquid is near boiling point.

In a rolling boil there is much turbulence and mechanical agitation of the liquid by the bubbles, which leads both to a more uniform temperature, and to effects such as emulsification of any fat, which is why fast boiled stock is cloudy.

I imagine the interaction with pasta is complex, and the optimum boil will depend on many variables, such as the size and shape of paste pieces, their initial dryness and composition, their permeability whether there are additional ingredients such as egg to help hold them together and so on. There is a balance between hydration and the effects of heat on the complex protein/starch mix that is pasta.

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Couldn't have written it better myself. Must be a Mechanical or Nuclear Engineer!-Dick

[budrichard]

Unless you dump in lots of salt, the effects of normal salting are unmeasurable by most of us, and have absolutely no effect on our SS cooking utensils. -Dick

[Baggy]

Keeping the water boiling has two enormous advantages. First, the turbulence of the boiling water helps to keep the strands of pasta separate and, second, the pasta doesn’t sink to the bottom of the pan where it can get stuck to the base. The end result is the pasta has evenly cooked and separate strands, i.e. not tough, uncooked clumps.

As those who live up mountains know, there is nothing in the temperature itself that is important in cooking pasta – it just takes longer. For example:

Altitude 1,000 metres (approx 3,250 ft) boiling point = 97C (207F) & pasta takes about 15% longer to cook

Altitude 2,000 metres (approx 6,500 ft) boiling point = 94C (201F) & pasta takes about 33% longer to cook

Longer cooking times and less vigorous boiling has a great advantage when cooking fresh pasta, particularly filled pasta like ravioli or ramen as Hiroyuki has experienced. This is because fresh pasta cooks in 2-3 minutes so the difference between al dente and mush is a matter of maybe 30 seconds and, if the centre of a filled pasta heats too quickly there is no time for any trapped air to escape causing the pasta to balloon and, in the worst case, to break open.

If the water temperature is above the 75-80C required to cook the starch, a warm bath of 90C could be a better solution than a rapid boil (at least for fresh pasta).

So, provided you take precautions to stop the pasta from sticking together, there is no reason not to cook at a simmer. To help the process, it is a good idea to stir dried pasta when it is starting to soften so it doesn’t stick to the pan or itself.

With added salt making such a small difference to the temperature, I just need to research what effect it has on the structure of starch – I may be some time…