Jump to content
  • Welcome to the eG Forums, a service of the eGullet Society for Culinary Arts & Letters. The Society is a 501(c)3 not-for-profit organization dedicated to the advancement of the culinary arts. These advertising-free forums are provided free of charge through donations from Society members. Anyone may read the forums, but to post you must create a free account.

When the water returns to the boil


Fat Guy
 Share

Recommended Posts

Dozens of cookbooks on my shelf instruct that "when the water returns to the boil" you're supposed to remove the vegetables you're blanching, or you're supposed to start timing the pasta, or you're done cooking a particular item. But does this advice really make sense? Some stoves being so much more powerful than others, and depending on the quantity of what you're cooking, won't some pots of water return to the boil in seconds while others take minutes? And won't that affect results?

Steven A. Shaw aka "Fat Guy"
Co-founder, Society for Culinary Arts & Letters, sshaw@egstaff.org
Proud signatory to the eG Ethics code
Director, New Media Studies, International Culinary Center (take my food-blogging course)

Link to comment
Share on other sites

Dozens of cookbooks on my shelf instruct that "when the water returns to the boil" you're supposed to remove the vegetables you're blanching, or you're supposed to start timing the pasta, or you're done cooking a particular item. But does this advice really make sense? Some stoves being so much more powerful than others, and depending on the quantity of what you're cooking, won't some pots of water return to the boil in seconds while others take minutes? And won't that affect results?

Salt also makes a difference in how quickly the water returns to a boil. Heavily salted water boils at like 213º to 214ºF. So blanching which is typically highly salted would cook faster than say, Pasta or dumplings. you also have to consider the ratio of water to product. I add enough water to just cover my potatoes, but when I blanch veggies or cook pasta I like to fill the pot most of the way so that there is a lot of water and adding things doesn't drop the temperature very far.

Another thing I have found interesting was the instructions on a package of pork dumplings I bought after sampling them at the local Super H-mart. It had me bring the dumplings to a boil, add a cup of cold water, and bring to a boil again. I had to repeat the process two more times, but in the end the dumplings came out excellent and I probably would have let them go to mush trying to get them done in a rolling boil.

Edited by RAHiggins1 (log)
Veni Vidi Vino - I came, I saw, I drank.
Link to comment
Share on other sites

You're right, it doesn't make sense. In a perfect world you'd have a huge pot of water and a lot of BTUs, so the water would come back to a boil almost instantly. More often it takes a while ... but the food is still cooking at the reduced temperature, just not as quickly, and in most cases not as well.

I start timing the minute the food goes in, and no matter what, use time as the roughest of guidelines. Cooking actual time has to be based on taste and texture.

Salt will have no meaninful effect on temperature (water at the salinity of sea water will boil at 1 degree F hotter than unsalted water; this represents a 0.15% increase in thermal energy). Most vegetables will cook faster in salt water, at least on the surface, but this is because salt softens the cell walls ... not because of temperature. If you do an experiment at home, you'll find that the difference in boiling time between salted and unsalted water is probably too small to measure.

Edited by paulraphael (log)

Notes from the underbelly

Link to comment
Share on other sites

In the context of home cooking, adding salt does not have much affect on boiling point. As a general rule of thumb, every time you add about 58 grams of salt to a liter of water, you raise the boiling point by one-half a degree Celsius. This means, for example, that if you have an 8 liter cooking vessel, you would need to add 928 grams of salt (almost a kilogram of salt!) to raise the boiling point up to 101C/214F.

--

Link to comment
Share on other sites

I too have often wondered about the "return to a boil....' wondering when to start timing. I chucked worrying about it too much and just go by the Marcella Hazan rule, cook it till its done. While it is a good guideline for many things, I think after we've done something once or twice we have a pretty good feel for the rest of it.

Charles a food and wine addict - "Just as magic can be black or white, so can addictions be good, bad or neither. As long as a habit enslaves it makes the grade, it need not be sinful as well." - Victor Mollo

Link to comment
Share on other sites

I usually blanch in batches. I have never timed myself, but two or three batches seems just as fast as waiting for the water to boil when I thow in everything at once.

A Chinese-style "chicken wire" strainer makes the whole process fairly efficient.

The thing I blanch most ofteen is string beans for stir-frying. The change to bright green is pretty clear.

BB

Food is all about history and geography.

Link to comment
Share on other sites

You're right, it doesn't make sense. In a perfect world you'd have a huge pot of water and a lot of BTUs, so the water would come back to a boil almost instantly. More often it takes a while ... but the food is still cooking at the reduced temperature, just not as quickly, and in most cases not as well.

I have heard that in Thomas Keller's kitchens, if the water drops below a boil when you drop the blanching vegetables in, you are expected to pull them out and start over with another, smaller, batch.

But he probably has access to BTU's and pots that would be impractical in, say, a typical Manhattan kitchen.

I almost always go by visuals or tasting, so the time the water takes to return to a boil is largely irrelevant. I think that the "return to a boil" benchmark is imprecise not only for the reasons Fat Guy mentioned, but because no two brands of pasta, batches of green beans or whatever are exactly the same.

I'm on the pavement

Thinking about the government.

Link to comment
Share on other sites

Just as a point of physics, if I recall correctly a larger pot of water will not return to the boil any more quickly than a smaller pot. All other things being equal, while the volume of water in the large pot may cause the temperature to drop less, it should take a correspondingly longer amount of time for the water to heat back up because it takes more energy to heat a larger pot. So, a more powerful burner will help with recovery speed but not a larger pot unless there are other factors at play.

Steven A. Shaw aka "Fat Guy"
Co-founder, Society for Culinary Arts & Letters, sshaw@egstaff.org
Proud signatory to the eG Ethics code
Director, New Media Studies, International Culinary Center (take my food-blogging course)

Link to comment
Share on other sites

Just as a point of physics, if I recall correctly a larger pot of water will not return to the boil any more quickly than a smaller pot. All other things being equal, while the volume of water in the large pot may cause the temperature to drop less, it should take a correspondingly longer amount of time for the water to heat back up because it takes more energy to heat a larger pot. So, a more powerful burner will help with recovery speed but not a larger pot unless there are other factors at play.

This is true, since the energy you took out by adding something cold needs to be replaced by the same amount of energy regardless of how much water is in the pot, but from a practical cooking standpoint, it matters.

If your water temp is only a couple degrees below boiling your food will probably cook very similar to the way it cooks in fully boiling water, whereas if you drop the temp 50° it will certainly not. So, for practical purposes, if you have enough water there is no need to wait until the water returns to a boil to start timing. (Note that I don't intend this to be authoritative, I'm just speculating).

Chris Hennes
Director of Operations
chennes@egullet.org

Link to comment
Share on other sites

Just as a point of physics, if I recall correctly a larger pot of water will not return to the boil any more quickly than a smaller pot. All other things being equal, while the volume of water in the large pot may cause the temperature to drop less, it should take a correspondingly longer amount of time for the water to heat back up ...

Yup. That's why the ideal situation is a lot of water (so the temp doesn't drop much) and a lot of stove (so the temp recovers quickly).

But regardless of your stove's abilities, it's generally better to have a lot of water. All that thermal mass will keep the water temperature from dropping too low. In the time it takes to cook most pasta or green vegetables, an anemic stove won't make much progress raising the temperature of even a small amount of water. So it's best to keep the water temp from dropping too much in the first place. If you've ever had pasta or green vegetables cooked in tiny amounts of water, you know how this plays out.

Edited by paulraphael (log)

Notes from the underbelly

Link to comment
Share on other sites

Right. If you're, say, blanching vegetables and the temperature of your water drops only 5-10 degrees, that's not such a big deal. The way the vegetables are cooking won't be all that different. If the temperature of the water drops by 50 degrees, you've got another situation on your hands. So you definitely want a large thermal mass (in the form of a body of water) relative to the thermal mass of the food you wish to cook. In this sense, it is no different from putting food into a frypan.

--

Link to comment
Share on other sites

Dozens of cookbooks on my shelf instruct that "when the water returns to the boil" you're supposed to remove the vegetables you're blanching, or you're supposed to start timing the pasta, or you're done cooking a particular item. But does this advice really make sense? Some stoves being so much more powerful than others, and depending on the quantity of what you're cooking, won't some pots of water return to the boil in seconds while others take minutes? And won't that affect results?

I was always taught to not "overwhelm" the water with so much product that you never loose the boil in the 1st place. And the timiing has always been to the texture of the product your looking for - it _will_ cook in non-boiling water (albiet slower) so testing texture with fingers is the best solution.

Salted water will always speed up the cooking process as the outside cell walls willl break down faster.

I would say its more important to go by taste/texture than a timer.

Cheers

Gregory Bastow

Link to comment
Share on other sites

There's another point:

The old first-cut rule in chemistry was that the rate of a chemical reaction doubles with each 10 C = 18 F increase in temperature.

If regard cooling in hot water as such a 'chemical reaction', then, when the temperature is 10 C = 18 F less than boiling, aren't cooking very fast. So, are really cooking at a decent rate compared with boiling only when the temperature gets back within 10 C = 18 F of boiling. So, first-cut, start timing the actual cooking at a decent rate when the temperature gets back to boiling.

What would be the right food and wine to go with

R. Strauss's 'Ein Heldenleben'?

Link to comment
Share on other sites

I'm not sure it's an accurate analogy to equate chemical reaction kinetics and heat transfer.

I think the equation we want is Newton's law of cooling:

dQ/dt = h*A(Tobject-Tenvironment)

or: change in kinetic energy over time is = heat transfer constant times the area times the difference in the two bodies.

Given this model, then you'd have to halve the temperature difference between the two bodies to half the rate of temperature transfer to the food. This would equate to room temperature asparagus in 100degC water vs. room temperature asparagus in 60degC water.

However, this is only taking into account the increase in temperature of the object, when what is more important in blanching is the quick denaturation of enzymes that will break down desirable compounds. I'm still thinking in terms of asparagus, and the difference between bright green, and a beige-grey. What we want in this instance is the rapid denaturation of the enzymes acting on chlorophyll, particularly near the surface.

I understand that most proteins denature around 60-70degC, but I'm also aware that there is variation in succeptability to denaturation (think hours of simmering stock to break collagen into gelatin). Also, chlorophyll leaches out into cooking water over time.

Given these two things, I imagine that raising the temperature of the vegetable to ~60-70degC as quickly as possible, and accomplishing other tasks (eg. breaking down cell wall components) as quickly as possible would leave you with the brightest vegetables.

I don't think "when it comes back to the boil" has anything to do with it.

Unless perhaps, the boiling temperature itself is important. Eg. the enzymes involved did not denature until 100degC.

Link to comment
Share on other sites

gregnz is quite right that the heat flow rate is just dQ/dt = hA(T1-T2), where T1 is the temperature of the food's surface, T2 is the temperature of the heat source, and A is the foods cross-sectional area. It is incorrect, however, to assume that h, the surface heat transfer coefficient, is the same in water below boil and boiling water. Indeed, h is about 5 times larger for boiling water as it is for simmering water. So, the initial heat transfer rate to the food's surface is about 1000 kW/m^2 for boiling water and 160 kW/m^2 for water at 90C/200F (where the food started at 10C) [McGee 1999]. (This is also the reason why forced convection is used in sous vide cooking.)

Edit: Fixed Typo.

Edited by DouglasBaldwin (log)

My Guide: A Practical Guide to Sous Vide Cooking, which Harold McGee described as "a wonderful contribution."

My Book: Sous Vide for the Home Cook US EU/UK

My YouTube channel — a new work in progress.

Link to comment
Share on other sites

Wow - that changes the interpretation hugely! Googling convection.

A change in a few degrees of temperature changes the heat transfer from being free convective (ie. density of water changes with temperature, so a current is set up, continually bringing hot water in contact with the food) to being forced convective (ie. the current is not based on gravity).

Ok so:

i. what is the cause of the difference in the heat transfer?

ii. what is the current based on?

iii. what happens in a pressure cooker with respect to heat transfer? boiling at higher temperature?

iv. is this why braises/stews should be simmered, not boiled?

v. is there something special about the conversion of collagen to gelatin that is time dependent, as well as heat dependent?

Link to comment
Share on other sites

One other thought that comes to mind is the pre-blanching temp of the vegetables. had never really given it any thought but room temp must help some in reducing the time it takes to come back to a boil. What about a quick rinse just before hand in luke warm water as well?

Charles a food and wine addict - "Just as magic can be black or white, so can addictions be good, bad or neither. As long as a habit enslaves it makes the grade, it need not be sinful as well." - Victor Mollo

Link to comment
Share on other sites

Yes, anything you can do to reduce the temperature differential between the food and the water will help the water return to the boil faster. That said, there are certain circumstances where you would like for the vegetables to be cold (e.g., blanching tomatoes to remove the skin) because you want to mostly cook the outside and not the inside.

--

Link to comment
Share on other sites

For home cooking, the practical question is how little water you can get away with and still get great results.

Restaurant chefs will often tell you to bring gallons of salted water at a raging boil, either for blanching or for pasta making. This is practical for them, because they're cycling batch after batch of food through that water. If you're making just one batch, there's a big (and probably unnecessary) cost in energy and time to bring the giant pot of water to temperature.

When I'm cooking a meal that doesn't have to be amazing, I'll typically use a 5qt pot, mostly full, water at a rolling boil, for up to a pound of vegetables or pasta. This is less then half of what's ideal, but the results are good. Asparagus will be bright green and lightly crisp; pasta will be good (saucing well and eating right away will make a bigger difference than boiling improvements at this point).

If it's a special meal, I'll fill the 12qt stockpot for similar quantities. But the improvements are pretty small.

Next time I blanch some veggies I'll put a probe thermometer in the water and see just how much temperature is lost when the food goes in.

Notes from the underbelly

Link to comment
Share on other sites

The specifc heat of water is 1 Cal/degree Celsius per gram of water i.e it takes 1 Calory of heat(not a food calories) to raise the temperature of one gram of water, one degree Celsius.

The heat is supplied through the walls of the cooking vessel by conductivity. The greater the amount of heat your burner can put out usually discussed in But/hr and the greater the conductivity of the material of your vessel, the faster and more importanly, the more heat that can be transferred to your water.

So you cannot make a simple comparison of small pots to large or whatever because its not that simple but not that difficult either. If one uses the same heat source(burner) and same pot, obviously the more water in the pot, the longer it will take the bulk temperature of the water to raise the same amount.

If you use the same burner, different size pots and different amounts of water, you either have to calculate or measure, the rates of temperature rise, a simple I think it is not correct.

Heat is transferred most effectively into the bulk fluid by what is termed nucleate boiling. There are sites in the vessel material that allow steam bubbles to form which can transfer heat much faster than simple conductivity and then convection. This is what you see when water starts to boil as little streams of bubbles. These gradually increase in size and number and eventually result in what is referred to as 'rolling boil'.

As well discussed, it all depends on your volume of material put into the pot, amount of water in the pot, size and material in the pot and the rate at which your heat source can deliver energy to the outside of the pot as to how low the temperature will drop and how long to recover.

The reason that cooking times are then referred to from 'time to boil' is because this is a standard and is usually 212F at sea level. The boiling temperature does not depend on material volume, pot size and material or heat source thereby elimnating all these variables.-Dick

Edited by budrichard (log)
Link to comment
Share on other sites

gregnz:

We are discussing cases there the cooking instructions ask that the cooking water get back to a boil. In particular, we are not discussing cases of rapid blanching or poaching where the cooking time is so short that the temperature of both the food and the water, with different parts of the food at different temperatures, are increasing fairly rapidly during the cooking.

You wrote:

"I'm not sure it's an accurate analogy to equate chemical reaction kinetics and heat transfer."

Right: Chemical reactions and heat transfer are very different things.

For some mathematics, let

t -- time, with the cooking starting at t = 0.

T(t) -- temperature at some test point inside the food

r(T) -- rate of cooking at temperature T

C(t) -- amount of cooking by time t

Then in TeX notation:

C(s) = int_0^s r(T(t)) dt

So the amount of cooking C(s) at time s is a Riemann integral from 0 to s.

Intuitively the point is that, since

r(T + 10 C) = 2 r(T)

the function r increases so quickly with increasing T that it is essentially an exponential and nearly all the cooking is just from the time at boiling or near boiling.

Or, letting

B = { T | 200 F <= T <= 212 F }

essentially

C(s) = int_B r(T(t)) dt

That is, start timing when T gets back up to 200 F.

Basically, for a first-cut view, I was neglecting heat transfer essentially by assuming that the food would quickly be at essentially the same temperature as the water. For small pieces of food, e.g., broccoli flowers, chunks of beef and vegetables in a stew, with a lot of water that should be an okay approximation.

Then I was assuming that the 'cooking' was more than just getting the food up to a specific temperature and, instead, also needed time at some such temperature. This cooking by a time at 'cooking temperature', then, I was assuming was essentially a chemical reaction of some sort where another 10 C = 18 F in temperature doubles the rate of the reaction (cooking).

This view of 'cooking' as a chemical reaction may be appropriate for, say, breaking down collagen, enzymes, or cell walls, killing bacteria, etc. E.g., can't make BBQ just by heating small chunks of pork shoulder to 180 F; instead, need some time at 180 F or some such.

With this approach that the cooking proceeds much like in a chemical reaction, and assuming that the cooking is to take place in boiling water, then for a first-cut rule in practice starting the timing when the water gets back to the boil should be okay if only because at 10 C = 18 F less than the boil the rate of cooking is so much slower.

What would be the right food and wine to go with

R. Strauss's 'Ein Heldenleben'?

Link to comment
Share on other sites

nope, no sense. Just another piece of historical assumption. There is no need to go over the science behind it because the degree of change from on piece to the next is infinite.

I guess the best advice for anyone cooking anything is a qoute from Marc Haymon

"you have to get in there and look for yourself".

Dean Anthony Anderson

"If all you have to eat is an egg, you had better know how to cook it properly" ~ Herve This

Pastry Chef: One If By Land Two If By Sea

Link to comment
Share on other sites

 Share

  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...