Cooking for Engineers

7/28/2019 Cooking for Engineers

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tchen Notes

Heat Transfer and Cooking by Burr ZimmermanPrinter-frienNormal view

Cooking For Engineers

aving a fundamental understanding of what is going on in the kitchen can not only help you avoid disasters but also a

n making the right decision the first time you try out a recipe or wing it. Understanding how heat transfer affects your

ooking is a first step in realizing whywe'd choose a particular cooking implement or specific heating method (steamin

s. baking, frying vs. boiling) for one dish but not another. In this article, Burr Zimmerman describes how heat transfe

works as it relates to cooking.

magine you are on the bank of a river. It's raining, hard, so you and some other workers have started to build a wall of

and bags. You each have a shovel, sand, and bags. The harder you work, the faster you can build the wall. The more

eople you have, the faster you can build the wall. In a way, building a sandbag wall can illustrate how heat moves into

ood, called heat transfer. We'll come back to this sandbag wall example in moment.

he Basics of Heat

ooking, ultimately, is about heat, how heat enters the food and what happens to the food when it enters. This article

ocuses on heat transferin cooking, or how heat is applied to and enters food. I won't spend much time on the chemica

eactions that occur in food during cooking.

n cooking, typically there is a heating element (such as a fire), a heat transfer medium (oil, water, air, a pan, etc.), a

he food itself. The heat moves from the element through the medium to the food. 'Temperature' and 'heat' are often u

nterchangeably, but they are not the same thing! Temperature is the driving force for heat transfer. Like gravity move

masses, temperature moves heat. Heat moves from hotter materials to colder materials (a temperature difference cau

he heat to move).

emperature measures, roughly speaking, how much the molecules in a material are vibrating. Temperature is a prop

f a material independent of how much of a material there is.

eat, or thermal energy, is a measure of the amount of energy that is contained in a material (this is a bit simplified

here are lots of different measures and forms of energy). Heat depends on how much of the material you have: if yououble the amount of a material, you double the heat.

When you 'heat' something, it means you are transferring energy into it, or adding thermal energy to it. As you increase

he thermal energy in a material, it often increases in temperature (but not always!). An example is boiling water. As y

dd heat to water, its temperature increases... until you reach the boiling point. Then, as you add heat, the temperatu

tays constant until the water is completely boiled off.

n cooking, there are three general ways that heat can be transferred from one material to another. Most engineers ha

aken courses in heat transfer and have heard of the big three: 'conduction', 'convection', and 'radiation'. All three play

ole in cooking, but depending on the cooking method, only one or two may be important. Before looking at how they a

o cooking, let me briefly define each:

onduction - this is heat transfer due to contact of molecules. Thermal energy, which can be thought of as the vibratio

molecules in place, is transferred directly from one material into another in contact with it. If you touch a hot pan, you

and gets hot too (don't do that!). A temperature gradient forms from hot to cold - the bigger the temperature differe

he faster the conduction. The kinds of materials matter too - some materials (e.g. metals) conduct heat better than

thers (e.g. air).

onvection - this is heat transfer due to the bulk movement of molecules. Molecules move - changing places, not just

ibrating in place - and take their heat with them. When heating a pot of water, before it boils, conduction will make t

water nearer the heat source will be warmer than water far away. When you stir the pot, the hotter molecules move aw

rom the heating source, taking their heat with them, and are replaced with colder ones.
http://www.cookingforengineers.com/http://www.cookingforengineers.com/http://www.cookingforengineers.com/article/224/Heat-Transfer-and-Cooking


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adiation - this is heat transfer due to energy waves emitted by another object. Energy in the form of electromagneti

adiation (as distinguished from 'nuclear radiation', which is completely different) is absorbed by food. The two most

ommon types of radiative energy in cooking are infrared waves ('heat waves') and microwaves. Unlike conduction and

onvection, radiation doesn't require a medium to be between the heat source and the food (in fact, the medium just g

n the way). The energy is literally 'beamed' directly to the food.

also want to define a couple other terms that are critical to understanding heat transfer.

hermal Conductivity- this describes how readily a material will give or take heat through conduction. A material wit

igh thermal conductivity will transfer heat quickly, while a low conductivity material will transfer heat more slowly. Beareful - moving heat rapidly does not necessarily mean rapid temperature change.

eat Capacity- the other important aspect of moving heat is how much the temperature of a material changes when

move a certain amount of heat into it. Heat capacity refers to how quickly a material's temperature changes with the

ddition of heat. As you add heat to a material with high heat capacity, it will increase temperature more slowly than a

material with lower heat capacity.

bsorbance - The way in which a material absorbs radiation is called its absorbance (more specifically, the fraction of

adiation at a given wavelength that is absorbed). Absorbance here is a specific term related to radiation, and should n

e confused with 'absorption'. In order for a food to absorb heat from radiation, it must be able to absorb the radiation

aterials absorb different kinds of radiation differently. For instance water does not absorb light strongly, but it does

bsorb microwave radiation readily.

o armed with some definitions, let's re-examine the sandbag wall. The analogy is a little bit loose, so bear with me.

the amount of heat transferred is like the amount of the wall built, then heat capacity and thermal conductivity can b

hought to relate to the endurance and the work rate, respectively, of the workers. A high heat capacity material can k

iv ing heat without losing (much) temperature - that's like a worker who can keep working without getting tired. A hig

onductive material transfers heat quickly, something like a worker who can fill bags very quickly -- although that does

mean it can do so for very long.

emperature is like the height or reach of the wall workers - with heat transfer, once two objects are the same

emperature, they no longer transfer heat. Similarly, as the wall gets higher, it becomes harder to add bags to it.

ventually the wall will be tall enough that the workers can't reach high enough to add any more bags. If food is uniform

he same temperature as its surroundings, no heat transfer will occur (although chemical reactions will still be occurrin

o 'cooking' may still be happening.

onduction

ontinuing the sandbag wall example, consider conduction of heat. Conduction is like the direct laying of bags on the w

y workers. Heat capacity is like the endurance of each worker - a high-endurance worker can keep working

without slowing down effort or reaching his endurance limit, and a high heat capacity material can deliver a lot of heat

without changing temperature as much. Thermal conductivity is like the worker's speed in filling and laying bags. Amaterial with high thermal conductivity moves bags quickly. However, highly conductive but low heat capacity materia

ot make for good heat transfer (think aluminum foil). Like a worker who works fast, but tires quickly, aluminum foil

eats and cools quickly, so heat transfer likewise ends quickly.

et's compare the properties of common cooking media: air, steam, liquid water, vegetable oil, steel, and aluminum.

able 1: Thermal Properties of Common Cooking Media

ll values approximate; many are functions of temperature

MaterialTypeHeat Capacity

(J/g-K)Thermal Conductivity

(J/sec-m-K)Effective Temp.Range

(F/C)And therefore, it's good for... (applications

Air 1* 0.02 Virtually no limit High temp, radiative cooking, deep browning

Steam 2* 0.02 212F / **100C** Gentle, low temp, "drier" than boiling


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Water 4.2 0.6 32-212F / 0-100C Low temperature, faster than steam, add watefoods (e.g. pasta)

CookingOils

2 0.2 40-450F / 5-230C Moderate temperature, moderate browning

Aluminum 0.9 250Melts at > 1100F /600C

To distribute heat evenly across a surface, highand low heat

Remember, the heat capacities are listed in terms of mass, not volume. It would take roughly 1000 times the volume

as (temperature dependent) to have the same mass as water or oil.

Higher temperatures possible with pressure cookers

s an aside, heat also conducts within food. A grilled steak cut open is a great illustration of conduction within food. T

utside is charred and yummy. The inside is cool, red (and yummy, if you like a good rare tenderloin like I do!). With

onduction, a temperature gradient forms from the hot outside to the cool inside. The color of the meat, transitioning

rom brown to pink to red shows the temperature gradient!

onduction also explains the phenomenon of "carryover", or how the internal temperature of a food continues to rise a

ou remove it from heat (e.g. a roast from the oven). When the outside of food is very hot and the inside is much cool

ven after you remove it from the heat source, heat in the hot outside of the food will continue to transfer to the cool

nterior. The hot outside of the steak transfers heat to the cool center even after you remove it from the grill; a

hermometer in the center will register a temperature increase.

onvection

ontinuing our example of building a sandbag wall, now consider convection. Convection is like moving in fresh worker

he wall and giving tired workers a break. Recall that convection is the movement of heat due to bulk movement of a

medium. In terms of heat transfer, food is generally a closed system; the cooking medium (air, water, oil) transfers he

o the food by conduction, but does not itself move into the food very much. Sometimes medium gets in (a good thing

asta, bad in frying), but not enough to make a big difference in heat transfer. Thus, in cooking, convection's job isn't

eat the food directly, but to make sure that heating (conduction) happens efficiently.

emember that a temperature difference causes heat transfer, and a larger temperature difference means more heat

ransfer. So the way that convection can contribute to heat transfer is by moving cooking medium around, replacing th

old medium (next to the food, where it has transferred its heat to the food already) with hot medium, and exchanging

or cold next to the heating element. Moving material, taking its heat with it, transfers more heat within the medium

would conduction alone. And, because the average temperature of the medium close to the food stays higher, more he

ransfer occurs into the food, and the food cooks faster. The larger the distance from the heating element to the food

more convection matters. In frying or baking, convection makes a big difference. In sauting, where the distance from

ot pan to the food is small, convection is much less important (and convection is negligible in solid materials anyway)

adiation

astly, consider radiation. Radiative heat transfer is like having a second group of workers separated from the wall

eaving sandbags on top of it. Some bags don't make it and bounce off or go past the wall. It's pretty hard to have them

and on the wall, especially if the wall is tall or far away. In real life, radiative heat transfer involves a source oflectromagnetic radiation that beams energy to food. The radiation particles (yes, they behave like particles in this

nstance) are absorbed sometimes by the food. Each food - each part of the food in fact - has its own characteristic wa

nteracting with the radiation, known as its absorbance. When food absorbs radiation, the energy of the radiation parti

converted to heat. The food can also reflect the radiation, or simply let it pass through. I'm not going to discuss

microwave absorption in too much detail, but compare identical containers of oil and water when heated in the microw

espite that water's heat capacity is higher than oil, its temperature will rise faster than the oil. This is because it is

bsorbing more heat from the microwave radiation than the oil is.

nother consideration in radiative heating is the cooking medium. Radiative cooking is done almost exclusively in air

ecause water, oil, and other liquids and solids absorb radiation strongly. Using the sandbags as an example, the secon

wave of workers far from the wall are throwing bags at the wall. With air, the row of workers near the wall are spaced part, and nearly all the bags hit the wall (they may not land on the wall, but at least they get there). With water or oil,


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workers at the wall are packed much more densely. Instead of the bags landing on the wall, they hit the workers. Infrar

adiation, the glowing heat you feel from hot coals, is absorbed strongly by foods, but it does not penetrate food deepl

When you cook under a broiler, infrared radiation is absorbed by the food's surface, and then conducted into the food.

ontrast, microwaves can penetrate food more deeply; the interior of the food can be heated directly.

adiation is also convenient in that it can largely be controlled independently of conduction and convection. For examp

ou can change the setting on your microwave, move food away from the coals, cover food, etc. Thus radiation is an

dded, controllable element to ensure you get the results you want.

ne last note - you do not need to have a glowing red material for radiation to occur. All things - food, ice, you, my do

mit infrared radiation. Infrared detection is how some night vision goggles work. In your oven, radiation is importantoo. Some heating occurs through conduction and convection of hot air, but some also is due to radiation. Radiation is

mportant especially for browning of food during roasting. Putting foil over your casserole not only prevents convection

ut also radiation.

ngineering Heat Transfer

ow that you have the knowledge, let's compare different cooking methods' reliance on heat transfer.

able 2: Common cooking methods and how they cause heat transfer

Method ConductionConvection Radiation

Steaming High High Low

Boiling High Moderate Low

Deep frying High Moderate Low

Sauting High Low Low

Broiling Moderate Low High

Baking High High Moderate

Grilling M oderate M oderate High

Microwaving Low Low High

able 3: Qualitative comparison of maximum temperature and rate of heat transfer using various cooking method

he higher the heat transfer, the lower the cooking time. Temperatures are qualitative illustrations only - most cookin

methods span a broad temperature range.Temperature is a property of matter, not radiation, and doesn't apply directly to radiative heat transfer. Since we are

iscussing temperature as it relates to heat transfer, microwaves and infrared waves are listed as 'high temperature'

ecause they can transfer heat into even very hot foods.

o how is this useful in cooking? You can use this information to achieve the exact doneness that you want for your foolong with the precise amount of browning you desire.


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Written by Burr Zimmerman Published on June 01, 2007 at 08:10

or instance, do you want a rare steak with beautiful char lines? Crank up the heat, put the steaks directly over the coa

nd oil the grill grates. The direct radiation will char the outside faster than the inside can cook, leaving a rare ins ide.

il on the grates will improve conduction of heat from the grates to the steak. Cast iron has tiny pores and pockets fill

with air; heat will conduct through the pores filled with oil faster than with air. So now you too can wow your friends wi

eautiful sear lines.

nother example: beautiful golden brown fried turkey. Do you want to wait 4 hours for your bird? Deep frying transfers

eat much faster than baking, so your bird will be done in under an hour. And, the moderate temperatures of frying

compared to roasting) mean it will stay golden brown, not burned, on the outside.

ow about that shrimp? If you want delicate, sweet, tender shrimp, nothing beats a slow poach (like boiling, but lower

emperature). The low temperature and high transfer rate (high transfer rate means faster cooking time) are just want

ou want. Conversely, if you want to char the shell slightly on the outside to develop a smoky, spicy taste, rub 'em with

nd pepper and throw 'em under the broiler!

n order to achieve the results you want, you have to consider both temperature *and* rate of heat transfer. From pota

yonnaise to broiled asparagus tips, your food demands a full range of temperatures and heat transfer rates. Since no

method can do it all - and since no one likes burned or undercooked food - it helps to know a little about each of them.

onclusion

rmed with some fundamental understanding of cooking and heat transfer, anyone can select the perfect cooking meth

or the dish they want to make. By taking into account the temperature and the rate of heat transfer, you can achieve

xact brownness and doneness you desire!

urr Zimmerman has 11 years of (amateur) cooking experience, 9 years of chemical engineering experience, and 7 of

rying to combine them.

ide comments

24 comments on Heat Transfer and Cooking:(Post a comment)

On June 04, 2007 at 11:36 PM, BlackGriffen (guest) said...

Subject: Temperature IS a Property of Radiation

Yes, radiation has a temperature, too. The radiation in your oven is actually in thermal equilibrium with the

walls of the oven.

Just look up a black body spectrum - it comes from assuming that the radiation of a black body in an oven is in

thermal equilibrium with the radiation in the oven.

The thing about the microwave is that the radiation being used isn't from a thermal source, like the walls of an

oven. So it's not that it doesn't have a temperature, it's just that the temperature of the microwaves

themselves is actually pretty low. Kind of like how a fast enough blender can boil water.

On June 05, 2007 at 08:00 PM, GrimmyTea (guest) said...

Subject: blackbody radiation

I thought the spectrum of blackbody radiation was a function of the temperature of the *blackbody* -- not the

radiation itself.

On June 05, 2007 at 10:32 PM, GaryProtein said...

Subject: Re: blackbody radiation

GrimmyTea wrote:
http://www.cookingforengineers.com/forums/posting.php?mode=reply&t=1153http://www.cookingforengineers.com/article/224/Heat-Transfer-and-Cooking/print#


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I thought the spectrum of blackbody radiation was a function of the temperature of the *blackbody* --

not the radiation itself.

I thought the same thing. The black body radiation exists because of the temperature of the object.

Furthermore, the radiation of the "black body" walls of an oven is due to the fact that the oven walls have been

heated by radiation, conduction and convection from the heat source of the oven, (gas, electric, wood or

nuclear [just kidding about the nuclear]) and the walls are then radiating it back into the oven. That's why when

you want to brown something, you should put it in a preheated oven-so the walls can radiate heat all the way

around what you want browned.

A radiation in and of itself does not have a temperature. It has energy capable of heating other objects. DON'T

confuse the heat of a flame with radiation. A flame has a temperature, and depending on the type of flame, a

certain amount of heat that is also radiated.

Microwave radiation isn't a black body radiation in a microwave oven. It is a radiation at a frequency that is

absorbed by molecules of water and to a lesser degree, fats and sugars, so they heat up. A microwave oven is

simply a 2.45-2.50 GHz radio transmitter with its broadcasting antenna inside the box you place your food into

to heat it up.

On June 06, 2007 at 06:36 PM, BurrZimmerman (guest) said...

Subject: combination

I think what everyone is posting here is basically correct; maybe just the way it's being phrased is at issue.

Every body acts like a black body in the sense that the body's temperature results in a characteristic emiss ion

spectrum. So there are definitely certain bands of radiation associated with materials of a given temperature.

That said, "Temperature" is a measure of molecular vibrations, and so it doesn't really make sense to say

radiation has a temperature itself. I prefer the way GaryProtein wrote it.

To say that radiation is 'in thermal equilibrium' with a warm body is somewhat correct in the sense that there is

a relationship between temperature and radiation, but it is probably not strictly accurate in a thermodynamics

sense.

Burr

On June 11, 2007 at 11:07 PM, BlackGriffen (guest) said...

Subject: Temp Not Just About Molecular Vibrations

Temperature is not just about molecular vibrations. There are two characteristics of any system that determine

its temperature- the energy it stores internally at rest and the number of ways it can store that energy (this is

tied in to the entropy). So, beyond vibrations temperature also ties in to rotations and unfettered random

movement.

The simplest way to see that radiation has temperature is to think of it as a photon gas. The temperature is

related to the random movement of the photons in the gas. This makes it, in fact, an ideal gas that happens to

be ultra relativistic. It even obeys PV = nRT, if I recall correctly.

Even without quantum mechanics, though, if you get in to the nitty gritty of temperature's definition you'll find

that radiation does have temperature because it presents another way for a system to store energy.

So, bottom line, radiation has a temperature like all other things. The cosmic microwave background, for

instance, which does not have any known molecular origin in s ight, has a temperature of 2.3 K. Not to mention

the fact that you can derive the temperature of the black body spectrum by finding the temperature of a body in

thermal equilibrium with it without regards to any walls or cavity containing the photon gas.
http://map.gsfc.nasa.gov/m_uni/uni_101bbtest3.html


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Now, back to the article. The radiation in a microwave is fairly cold because the photons aren't really moving

about very randomly, so they don't have a lot of different ways to store their energy. So, why does a microwave

heat up food? Same reason the blade of a blender can boil water - friction, in a broad sense. Basically, when you

use a microwave you're forcing the molecules to v ibrate harder and they, by bouncing against each other and

whatnot, turn that energy in to thermal energy.

On June 12, 2007 at 03:54 PM, Burr Zimmerman (guest) said...

Subject: post an article?

I guess there's a lot about temperature -- especially regarding radiation and quantum effects -- that I don't

know. Maybe Black Griffen would be willing to post an article about this -- or maybe you have a good link? Thissite requires authors to simplify somewhat so that articles are accessible to a broad range of readers, so maybe

such an article isn't appropriate for this site. In any event, can you suggest some more reading on temperature?

Regarding radiation, there's some debate about microwaves and the back-and-forth motion and viscous heat

generation versus excitation of vibrational modes. I have read from many sources what you have written, but

others contradict it as well. Perhaps for this subject also you could suggest an authoritative source and post a

link here? I'm not sure if there would be any peer-reviewed article on this, but that would be best if it existed.

Thanks,

Burr

On June 13, 2007 at 06:52 AM, BlackGriffen (guest) said...

Subject: Temperature

Lesse, I don't know that a more detailed picture of temperature than what you give is necessary - I only

provided more detail to justify what I felt was a necessary, but small, correction.

I would have liked to be able to link wikipedia's temperature article, but it contains the same error. I could

recommend a few statistical mechanics/thermodynamics textbooks - on the undergrad level the only book I've

ever used is Bowley and Sanchez (p 166) and at the graduate level the two main standbys, Reif(p 373 and

following) and Landau & Lifshitz (p 183 and following) have both served me well. This site has a pretty good

summary and mentions the phenomenon of negative absolute temperature, too. Having actually looked at thesebooks, I have to retract a photon gas obeying PV = nRT - it obeys P = 4sigma * T^4 / 3 / c. Landau also shows

that a gas of extremely fast moving electrons obeys the same law.

No need for quantum effects, actually, I chose to describe it as a photon gas because I figured that would be the

fastest way to make my point apparent. This can all be done completely class ically as long as one doesn't mind a

few infinities popping up in the energy stored in short wavelength radiation.

Perhaps the best way to explain temperature is the classical equipartition theorem - the thermal energy stored

in a body is equal to kT/2 times the number of places it can store energy (strictly speaking, this is only true for

"harmonic degrees of freedom" but just about everything is to some approximation). This includes random

vibrations, movement in each dimension, energy stored in atomic and molecular bonds, etc. Reversed,

temperature is a measure of how much random energy is available to each way a thing can store it. The

definition of heat follows naturally from that - it is just a transfer of thermal energy. And heat is driven from

hot things to cold things for no other reason than that the random movements inside of a thermal body tend to

equalize the energy shared by the different energy stores, on average. Perhaps that's more technical than this

site really needs... I dunno.

Re: the way microwaves heat things up. I haven't read any peer reviewed articles on the matter. Honestly, I

can't rule out microwaves coupling to any particular degree of freedom over another, and I would guess it

probably just depends on which molecule you're talking about. Regardless, when I talked about "friction" (I put in

the scare quotes intentionally) I was just talking about how the molecules turn otherwise cold radiation intoheat. The details could have included bumping, rubbing, or just good old fashioned re-emitting the radiation at
http://www.pbs.org/transistor/science/info/quantum.htmlhttp://math.ucr.edu/home/baez/physics/ParticleAndNuclear/negativeTemperature.htmlhttp://www.amazon.com/Statistical-Physics-Course-Theoretical/dp/0750633727/ref=pd_bbs_sr_2/104-1332195-6623145?ie=UTF8&s=books&qid=1181709534&sr=1-2http://www.amazon.com/Fundamentals-Statistical-Thermal-Physics-McGraw-Hill/dp/0070518009/ref=pd_bbs_sr_1/104-1332195-6623145?ie=UTF8&s=books&qid=1181709489&sr=1-1http://www.amazon.com/Introductory-Statistical-Mechanics-Roger-Bowley/dp/0198505760/ref=sr_1_1/104-1332195-6623145?ie=UTF8&s=books&qid=1181709279&sr=8-1


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a lower wavelength.

On June 13, 2007 at 10:12 AM, BlackGriffen (guest) said...

Subject: Sorry for the Lengthy Post

I went back to reread and potentially correct the wikipedia article. On closer examination, it seems to be fine.

The entropy stuff under the second law definition of temperature is a little off, but overall it seems to be fine.

Temperature

We now return you to your regularly scheduled cooking blog. :)

On July 13, 2007 at 10:50 AM, Horst said...

Subject: Heat transfer 99 (=even less than 101)

Both articles, the original Heat Transfer and Browning Foods and this one, omit one important thing. Something

every American says every year in his Porsche dealership when buying 6 litre SUV for this year: "I only want

something to get me from A to B".

In heat transfer, the heat must be also transferred from somewhere ("A") to somewhere else ("B"). For

example, in the oven the "A" may be its walls for the radiation to the bun but the air surrounding the bun for

the convection to it. And as regards the bun, "B" will be firstly just the bun surface where all those nice

browning reactions are taking place. But in the moment we are curious about setting of the dough in the middle

of the bun, the "B" shifts into the middle of the bun and the path of the heat gets one more step which works

somehow in combination with the preceding ones. Etc. etc.So, in my opinion matters are a bit more complicated (and different) than presented in both articles. Even the

nice comparison to the baglayers crew can not help much. It definitely did not help me.

I have also enjoyed the stubbornness of second author about the "conduction by convection" or "convection

being just conduction anyway" as if the convection was not the long decades accepted term for the heat transfer

combined from the flow of the fluid in the vicinity of the wall and conduction from the fluid through the

boundary layer to the wall.

On July 21, 2007 at 04:43 AM, Brillat-Savarin (guest) said...

Subject: Pressure Cookers

Not to nitpick, but the point of a pressure cooker is to NOT steam food - or, rather, to not boil your water off.

By increasing the pressure, water can be maintained in a liquid state at higher temperature than would be

possible at atmospheric pressure. This means that you can cook things more quickly - higher temperature and

greater heat transfer between your fluid and your food. If you've ever noted the high-altitude directions on

some food packages, it's because water boils at less than 212 deg F, due to the low pressure. Of course, as a

flatlander, it's only something I've read about...

On August 27, 2007 at 02:13 PM, Chef Harry Otto (guest) said...

Subject: Steam 2* 0.02 212F / **100C** Gentle, low temp?

Just stumbled upon your writing doing some research for a lecture.

Steam temperature effective range to 212F?

Your standard pressure cooker at 15psi uses a steam temp of 257F. In commercial units, I've cooked with steam

at over 300F.

With regard to your simplified definition of cooking. Cooking does not have to use heat. It can also rely on an

enzymatic process, or such of that with ceviche (acid).

All in all though, nice article.

Harry Otto, New York
http://en.wikipedia.org/wiki/Temperature


9/15

On August 27, 2007 at 05:59 PM, Michael Chu said...

Subject: Re: Steam 2* 0.02 212F / **100C** Gentle, low temp?

Chef Harry Otto wrote:

Just stumbled upon your writing doing some research for a lecture.

Steam temperature effective range to 212F?

Your standard pressure cooker at 15psi uses a steam temp of 257F. In commercial units, I've cooked

with steam at over 300F.

With regard to your simplified definition of cooking. Cooking does not have to use heat. It can also rely

on an enzymatic process, or such of that with ceviche (acid).

Burr put the ** after the 212F to signify that the temperature is higher when pressure cooking. Sorry that this

wasn't clear.

Clearly Burr doesn't think it's cooking if no heat is applied. I tend to agree with him on this as well. I've always

referred to ceviche as "cooking" (always with quotes) because you're enzymatically tightening the proteins to

produce a texture similar to that if you cooked it. Not the same thing. Now, since the scope of the website

definitely includes food such as ceviche, sushi, and whipped cream toppings (I haven't written these articles yet,

but I might!) we'll have to say there are at least two definitions to the word cooking that I use - 1. the act orpreparing food for consumption, and 2. the act of physically altering the properties of food through the

introduction of heat.

On June 12, 2008 at 08:09 PM, an anonymous reader said...

I rather liked both of these articles, though I think that the "Cooking Materials of Cookware" article did a bit

better job of explaining thermal transfer, at least as it pertains to cookware.

Personally I think the sandbags analogy got stretched a bit thin and seemed to lose consistency as the article

went on. I will admit that it's not the easiest thing to describe as an analogy. Most of the ones I can think of

break down at some point.

The one I like best is to picture an object as a pool. The water in the pool is heat and the level of the water in

the pool is temperature. The pool's width defines it's heat capacity. The wider the pool the more water it takes

to fill it to a certain temperature. Radiation is easily described as s imply shooting water into the pool from a

hose, or even the pool filling from rain. Conduction requires connecting two pools adjacent to each other with

pipes at the bottom of each pool. The thermal conductivity of a pool is basically how big the holes are to the

pipes. Bigger holes allow water (heat) to travel at a faster rate into and out of a g iven pool (object). When two

pools are connected if they are filled to different levels (temperature) water will flow from the higher pool to the

lower pool until they are the same level (equilibrium). The water pressure is also higher depending on how much

higher the high pool is, and thus will push water through the pipes even faster, slowing down as the two near

equilibrium. In thermal conduction the mechanism is not the same but more heat is definitely transferred froma hotter object.

Experiment: Get two identical pans. Leave one pan at room temperature and heat another one. Place an

identical ice cube in each pan and measure how long each one takes to melt. Intuitively the hotter pan will melt

the ice faster even though both pans are significantly warmer than the ice. Conclusion, heat was able to

transfer more quickly into the ice from the heated pan by conduction. Note: This experiment depends on neither

pan actually reaching equilibrium with the ice before the end of the experiment. It would be more scientific to

expose the ice cubes for exactly the same amount of time and then measure how much each melted by

measuring either the cube (by weight) or melted water (weight or volume), but that also complicates it. heh

Obviously I'm not trying to describe a perfect mathematical analog, just making an analogy to explain the

concepts.


10/15

BTW all the pools technically have the same depth... Absolute zero. I should also note that the width of the pool

can be different at different levels as the properties of materials can change greatly depending on temperature

(and pressure, which is not represented in this analogy along with many other things). Doing so is pushing the

limits of this analogy, though.

That takes care of radiation and conduction. Convection is a bit more difficult and where the analogy tends to

lose cohesion. To describe convection here you have to put lots of small "pools" on wheels. heh These pools can

then move around in groups, transporting their contained water (heat) with them. That's a hell of an image... ;-

/

Anyway...

I actually did like the author's stubbornly pointing out that the final stage of heat transfer into the food does not

happen by convection.

It's theoretically possible... but it requires a food where your definition of what is the food and what is not are

overlapped by an amorphous medium. Something like a stew where most of the liquid is actually strained off

and not considered part of the food, maybe. Though I suppose some of this inevitably happens in deep frying or

boiling anyway unless you keep a significant portion of the medium as your food the effect becomes negligible.

The author did make the point that he was referring to the final heat transfer into the food, not the entire

system. Obviously a convection oven does use convection to heat food since convection is the mechanism of the

oven it-self. I can also kinda see the other point of view, since convection as a mechanism for delivering heat

to be conducted into the food does represent a different mode of cooking at the macro level, and as such that

entire mode is generally referred to as "convection".

I think it boils down to point of view. Basically its the difference between asking how did the heat get to the

food and how did the heat get into the food. To illustrate what I mean by that let me use a small analogy.

Lets say like millions of Americans you commute to work in a car. If I ask you, How did you get to your

home? you would probably tell me you drove there. But if I ask, How did you get into your home? then you

would probably tell me you opened the front door and walked in. In the very same sense convection can be themain mechanism that transports heat to your food, while it is conduction that gets the heat into your food.

The author alludes several times to a low Heat Capacity being a limiting factor on thermal transfer, which I

think is partly in error. Heat Capacity is definitely part of the picture, but a low heat capacity doesn't equate to

slow heat transfer. In fact a pan with high heat capacity takes longer to heat up, but it will also stay hot longer

and maintain a more even temperature over time. High or low heat capacity may be desirable, depending upon

what you are cooking. Heat transfer depends upon the heat conductivity of both your heating medium and the

food as well as the difference in temperature. The quicker your heating medium heats up the sooner it will be

heating your food. In fact aluminum is used in a lot of cookware specifically for this purpose.

Keep in mind that the purpose of putting heat into the food is to raise the temperature of the food it-self. Thevarious chemical reactions we're trying to trigger happen at certain temperatures. The importance of controlling

the rate of transfer is about controlling the distribution of temperature in your food over time. Time of course

is the essential ingredient. Those chemical reactions take time. Increasing temperature generally increases the

speed of those reactions, but also causes additional reactions, and can change the food in other ways. Also

there are times when you want very uniform temperature gradients and times when you want very uneven ones,

as noted.

The heat alone won't cook your food; you need to raise its temperature. If you cool the food at the same rate

you heat it, simply pass ing the heat through the food without raising its temperature, you will achieve nothing.

I thought I 'd mention a couple of points not really covered in any of these articles.


11/15

The first is surface area. This is somewhat covered, but not really directly. Surface area defines the size of the

boundary we have to heat the food. Generally speaking the larger the surface area the faster the heating and

thus cooking can occur. When you cut up a piece of meat you are increasing its surface area, at least relative to

the air. This can greatly increase heat transfer, particularly from boiling, steaming, or baking. It also can

expose more area to infrared radiation. It may or may not increase surface area relative to a pan, however.

Here it depends partly on technique. If you cut up your meat and leave it sitting in the pan more is exposed to

the hot air ris ing off the pan, but depending on how you cut it, you may not have exposed any more to the

surface of the pan it-self. If you simply cut it but don't turn any of the chunks then the same surface area is in

contact with the pan. Similarly if you bake cookies you should already be very familiar with the fact that smaller

cookies will cook faster. As the authors mentioned, cooking with oil fills in the gaps normally filled with air

between your food and an otherwise dry pan. The oil acts like an extension of your pan, adhering to thecontours of your food and creating a larger surface area through which to transmit heat.

I also wanted to bring up induction. Its not really entirely within the scope of these articles. (I dont know of

any dishes that even could be cooked directly by induction) But I thought Id bring it up, mainly to refer to

induction ranges that heat pans directly by induction instead of using a flame or heat coil. These ranges induce

a current in the pan that converts directly to heat within the metal of the pan it-self instead of absorbing the

head by conduction. The process results in less waste heat since theres no burner or coil heating the air as

well as your pan. (In fact the stovetop it-self generally stays cool to the touch until you put a pan on it and even

then its only heated by conduction from the pan back to the stovetop.) Induction heating can also be very

fast.

And now my disclaimer All of this is just off the top of my head, so please dont be too critical. Ive done

zero research since Im merely posting this as a comment rather than writing an article. I believe it is

reasonably accurate, though Im sure that Ive glossed over several things, and may even have a flat out

error or few for all I know.

enjoy

-mannon

On April 27, 2009 at 12:06 AM, Jdusterb (guest) said...Subject: mass

It is true that gravity moves mass, but, more generally, it is force that moves mass. It can be the force of

gravity, but it can also be a number of other forces.

On May 18, 2011 at 03:45 AM, Hillman321 (guest) said...

Subject: "Continuation" Cooking

One thing I did not see was what the chefs/cooks refer to as "contintuation" cooking.

Once the medium used to introduce the heat transfer to the food is removed, cooking continues. The energy is

still in the food until such time as it has had time to move from the higher energy state to its final state.

Cooking continues until the temperature that no longer changes the material is reached.

Meats are allowed to "rest" before serving because the liquids that move out return inside as the temperature

cools.

Once food is cooked, it should be allowed to cool completely before being refrigerated or frozen. The rapid heat

transfer of the outer edge of hot food creates a cold "barrier" to cooling inside. Bacteria can even grow before

the temperature reaches the "magic" 40 F that impedes growth.

On October 03, 2011 at 12:20 AM, hischrispyness (guest) said...Subject: Just thanks!


12/15

I just wanted to say thanks for helping me understand better how heat transfer in food effects the result. I was

cooking a pork tenderloin roast

for my new wife and it came out perfect. I am a young air conditioning mechanic and know the basics of heat

transfer, but you helped me broaden my knowledge. Again thank you for taking the time to share with the

world!

Chris

On December 30, 2011 at 09:07 AM, harveybowden (guest) said...

Subject: Heat Transfer and Cooking

Hi, I cooked my first ever meal this Christmas and have a query. The turkey instructions said it required 3

hours 20 mins but was not fully cooked in that time. I put the Turkey inside a large pot with a proper steel lid onin accordance with my wife's instructions. My mother would have put it in a tray with aluminium foil over it.

Question: Did the steal pot slow down the heat transfer from the oven to the Turkey in comparison to

aluminium foil?

Also, why?

Thanks

On December 30, 2011 at 03:43 PM, Dilbert said...

it's a bad idea to rely solely on "time to cook" by a pound chart.

in addition to the pan, the quirks of your specific oven, covered, uncovered, stuffed, unstuffed . . . .

a big variable is the temp of the bird when your start. completely thawing a large frozen bird - like a turkey -

takes 4-5 days in the fridge.

even "fresh" birds often have ice crystals in their cavity - poultry and other meats do not freeze at 32'F/0'C

because of the mineral/other contents of the water entrained in cells - so they can be kept below "water

freezing" temperatures and still considered "fresh." any free water standing in the cavity will freeze however.

I've bought "fresh chickens" and had a devil of a time getting the giblets out - they're "frozen" inside the bird!

I'm not a fan of the pop-up "done" indicators - but if that's all you got . . .best to invest in a decent thermometer.

On January 01, 2012 at 09:37 AM, Harvey Bowden (guest) said...

Subject: Heat Transfer and Cooking

Hi Dilbert, Thanks for responding. The question I meant to ask was this:

All other things being equal, will a Turkey, or anything else for that matter, cook quicker under circumstances A

or B.

A. In a tray covered in aluminium foil.

B. In a steel pot covered with a lid.

Given the three hour time span would the heat transfer into the Turkey thourgh the aluminium or the steel pot

be the same?

If not, why not?

Many thanks.

On January 01, 2012 at 02:23 PM, Dilbert said...

ah.


13/15

most likely B "assuming" the usual sorts of roasting pans.

heat "transfers" convection, conduction and radiation.

heated air inside the oven makes up the convection part.

the pot/tray/whatever in contact with the oven rack does conduction.

exposed elements (if present) and the hot walls of the oven do the radiation part.

dull / dark objects absorb and emit radiant heat more readily than light colored / shiny objects.

the aluminum foil will reflect a lot of the radiant heat, so the typical dark color enameled roasting pan would be

faster.

now, if it is a brand spanking new shiny polished stainless steel roasting pan . . . results will be different.

On July 05, 2012 at 04:17 PM, Burr (original author) (guest) said...

Subject: Responses to earlier queries

A couple responses to some posts over the years...

Hillman mentions "continuation cooking", but I believe it is more commonly called "carryover", and which I would

consider to just be another example of conduction. The hot exterior of the food (because it was in contact with

the pan, grill grates, etc.) is hotter than the interior, continues to move heat into the cooler interior. Even

when removed from the cooking vessel, the exterior of the food is still hotter than the interior. For example,

the outer surface of the steak is very hot from the grill, and thus will warm the cooler interior (pull your steaks

off the grill when they're a little underdone!).

To Harvey and Dilbert, I agree with Dilbert's second answer. In an oven, the radiative mode is the most

important, so a light-colored/shiny surface will slow cooking (aluminum) by reflecting the radiation, whereas a

dark/dull black surface (like an enameled steel roasting pan) will absorb and re-emit heat more, and speed

cooking. Another factor to consider is keeping warmth around the bird -- a close-fitting lid, or even cooking in a

bag, will keep hot air / steam near the bird's surface, which will cook it faster than just the dry oven air. (Italso inhibits browning, but that's not part of the question) The *best* advice to ensure doneness is to ignore

time/weight guidelines and use a good probe thermometer. Stick it into the thick part of the thigh and pull the

bird about 150-155F (it will carry over to 165F, which is a great level of doneness for poultry).

On July 19, 2012 at 07:35 AM, Abayomi (guest) said...

Subject: ENQUIRIES

Could anyone give me the formula/relation used in calculating the cooking energy in joule or otherwise

expended by cooking a particular meal when using a particular fuel? For instance, if I cooked 500 grams of rice

using kerosene as my fuel (the source of heat) in say: 45 minutes, how would I calculate the cooking energy

expended in performing the cooking task?

I read through a final year project by a university student who worked on evaluation of cooking energy for

selected agricultural products and he stated somewhere that: ''Time (T) spent in cooking a food is directly

proportional to the cooking energy (E) expended.'' I agree with this but in his analysis, he wrote that he spent 45

minutes in cooking a particular food using charcoal as the fuel; he then stated that:

Let 2 minutes of cooking = 1 Joule of cooking energy

then, 45 minutes of cooking will use up 45/2 Joule = 22.5 Joule of cooking energy.

Please, is this basis true?

He later stated in another analysis (with the view that cooking energy can be deduced from d amount of fuel


14/15

consumed by a cooking process) that:

Let 1000 grams of fuelwood (solid biomass) = 1 Joule of cooking energy

then, 250 grams of fuelwood = 250/1000 Joule = 0.25 Joule of cooking energy

Is this argument right?

And lastly, he further stated that:

Let 1000 cubic centimetre of kerosene (liquid fossil fuel) = 1 Joule of -cooking energy

then, 50 cubic centimetre of kerosene = 50/1000 Joule = 0.05 Joule of -cooking energy

Is this argument equally valid?

These are my enquiries for the experts in the field of energy. I'll really appreciate it if someone could specify

the most accurate method/formula/relation used in calculating cooking energy expended while cooking for a

period of time using a solid or liquid fuel and also shed more light on the expressions stated above. Thank you

all.

Abayomi Adewuyi, AMIMechE

On July 19, 2012 at 05:57 PM, Dilbert said...

I'm a bit puzzled about the term "cooking energy" - for example to cook 500 grams of rice, in water, one could

calculate how much heat is needed to raise the water&rice&pot& lid to boiling from ambient. once it is "at

cooking temperature" the pot&lid radiates heat, the water evaporates, taking more heat out of "the system" -

so a continuing heat input is required to keep the pot at temperature over the cooking time.

how much fuel is burned determines how much heat is generated - each fuel contains "heat energy" which is

released when burned.

for example a kilogram of coal would produce roughly 3.6x10^7 Joules

you can look up the energy content of various fuels - the fuel density can be used to convert between fluid /

weigh for liquids.

the precise amount of heat released - especially for "natural" fuels - coal, wood, charcoal - will vary. petroleum

liquids will also vary depending on how refined they are.

burning a kilogram of charcoal will produce X amount of heat energy whether there is a pot of rice over the

charcoal or not. if there is forced air - the charcoal will burn faster - but excepting for the question of "complete

combustion" - will not produce more or less absolute heat release - a kilogram is a kilogram - whether ir burns

really fast or really slow.

same with kerosene or propane - a certain mass is burned and produces a specific amount of heat.

keep in mind that a large amount of heat released is not "absorbed" by the food being cooked - there is a _lot_

of "wasted" energy.

cooking appliances - like stoves - usually have knobs to regulate the amount of fuel being consumed. that

means the rate at which fuel is being consumed varies - so going strictly by time will be inaccurate. if the fuel

consumption rate varies, the heat energy produced in 2 minutes of cooking will not be the 45/2 ratio you

mentioned for 45 minutes

for fuels like wood and charcoal, they will continue to burn after the food is finished cooking - they would have

to be "extinguished" in order to "save" the remainder.


15/15

the example

"Let 1000 grams of fuelwood (solid biomass) = 1 Joule of cooking energy

then, 250 grams of fuelwood = 250/1000 Joule = 0.25 Joule of cooking energy"

is simple math - but true only if the wood is very consistent.

your first question:

"For instance, if I coked 500 grams of rice using kerosene as my fuel (the source of heat) in say: 45 minutes,

how would I calculate the cooking energy expended in performing the cooking task?"

easy - weigh how much fuel is consumed, kerosene contains about 46,300 Joules per gram.

On July 25, 2012 at 03:27 AM, GUEST (guest) said...

Subject: ELECTIC OVENS SUCK RAW EGGS

Microwaves are for people who don't cook too.

These are the important facts.

Remember food does NOT heat form the in side out in a micro. Yes some otherwise intelligent people still

believe this.

Reheating sketti sauce or other things that boil at lower temps than water means use 50-75% power depending

on oven wattage or there will be cleaning to do. Since microwaves heat by moving the molecules, give your food

a minute to stop dancing around before you eat it - just in case. We don't know how bodies actually react long

term to food that moves.

When cooking on an electric stove :( remember cooking directions are typically for gas cooking where stove top

heat gets turned off things cool rapidly. Electric coils keep the heat going longer and removing the pot/pan to

another burner is advisable when temperature changes are critical to the recipe.

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