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Kitchen Notes: Heat Transfer and Browning Foods
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Cooking For Engineers



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PostPosted: Fri Feb 09, 2007 9:01 am    Post subject: Kitchen Notes: Heat Transfer and Browning Foods Reply with quote


Article Digest:
Yesterday, in one of my lazy moods, I succumbed to the microwave pizza dinner. Opening the box, I noticed the instructions mentioned placing the frozen pizza on a shiny, metallic disk but only if I used the microwave and not the oven. This got me thinking about the effect of the disk on the pizza and so I did a little research on the browning of foods.

But first, a look at cooking methods and an examination of how each method affects browning.

Cooking, at its simplest definition, is preparing food through the use of heat. The different methods of cooking or heat transfer are broken down into conduction, convection, and radiation. Every method of cooking involves one or more of these heat transfer methods.

Conduction
This is the exchange of thermal energy through direct contact between a heating element and the food. Different materials result in different heating times and temperatures. Please refer to the article Equipment and Gear: Common Materials of Cookware for a complete breakdown of materials that directly affect conduction.

Pan-frying or sautéing are common forms of conduction. The pan heats up and, through direct contact with the food, cooks the food. Fat or oil used in the frying provides uniform contact with heat, lubrication to prevent sticking, and some flavor of its own. Oddly enough, cooking in oil is considered a dry technique because the oil acts more like a cooking material than anything else. The moisture in the food will still be contained because it will not mix with the oil surrounding it.

Convection
Whereas in conduction heat is transferred through direct contact, in convection, heat is transferred by the movement of molecules in either gas or liquid. The fast moving molecules of the convection medium collide with the slower molecules in the food and heat them up. Baking and roasting are common forms of convection cooking. The heating elements within the oven heat the air and that comes in contact with the food. Boiling and steaming are also forms of convection with water or steam acting as the convection fluid. In deep-frying, the oil envelops the food, like a fluid pan that completely encases the food and heats the surface evenly.

Convection relies much on the density of the fluid. Liquid convection, either through boiling, steaming, or deep frying, is a much more effective transfer of heat than gas convection. This is why boiling a potato is much faster than baking. The denser the fluid, the more often the molecules collide with the food and the fast the food heats up. Therefore in convection methods involving air such as baking, the temperatures must be much higher than in liquid convection. This is why you can stick your hand into a 500F oven without burning yourself but you cannot stick your hand into a pot of boiling water at only 212F.

Radiation
While conduction and convection are heating methods through molecule to molecule contact, radiation is the transfer of heat through waves of pure energy. Most of the heating energy comes from the infrared radiation below visible light. When you hold your hand near glowing coals or a stovetop burner, the heat you feel is infrared. Technically, everything emits thermal radiation including you and me, and so every cooking method has an element of radiation.

Grilling and broiling, the former with heat below the food, and the latter with heat above, are two methods of radiation cooking. Of course there is convection from the air in between the heat source and the food and conduction from the grate, but the heat is primarily radiated.

Microwaves are below infrared waves on the spectrum and so carry much less energy. Infrared waves have enough energy to heat up almost all types of molecules, but microwaves tend to only heat up polar molecules such as water, sugar, and fats. Foods containing water are heated by these microwaves which penetrate about an inch into the food's surface. The interior of the food is still heating by conduction of the heat from the surface into the interior.

Cooking MethodHeating MethodWet/DryBrowning?
Grilling/broilingPrimarily radiation from heat source, secondarily conduction from grate and convection of air between food and heatDryYes
Baking/RoastingPrimarily convection of air, secondarily radiation from oven walls and conduction from baking panDryYes
BoilingConvectionWetNo
SteamingConvection of steam and condensation of vaporWetNo
Pan-frying/SautéingConduction of pan and oilDryYes
Deep FryingConvection of oilDryYes
MicrowaveRadiationDryNo


The Browning Reactions: Caramelization and the Maillard Reaction

Heating foods intensifies flavors already latent within the foods; however, browning creates new flavors that are intrinsic to the cooking process. This is why a poached salmon and a grilled salmon both tastes identifiably like salmon, but you can also easily distinguish one food as poached and the other as grilled. There is flavor within the cooking method itself created by caramelization and the Maillard Reaction.

Caramelization

We have all had caramel candies before, but how many of us realize that those sugary delights are not much more than sugar itself. The caramelization of sugar is the simplest browning reaction happening at around 330F/165C. Plain table sugar melts into a thick syrup, then gradually darkens into a light yellow and eventually a dark brown. The flavor begins sweet and clean, but develops acidity, bitterness, and a rich aroma. The chemical process itself is complicated, but the reaction products include organic acids, sweet and bitter derivatives, fragrant molecules, and brown polymers.

The Maillard Reaction

Named for Louis Camille Maillard, the French physician who documented these complex reactions around 1910, Mailliard Reactions are responsible for bread crusts, chocolate, coffee, dark beers, and roasted meats. The sequence begins at about 220F/115C when a carbohydrate molecule and an amino acid bind together in an unstable structure, producing flavorful by-products. The involvement of amino acids brings nitrogen and sulfur creating meaty and earthy flavors. These reactions create that crust on seared foods and the brown coloring of a good roast as well as multitudes of other browned foods.

Both caramelization and the Maillard Reaction require relatively high temperatures beginning above the boiling point of water 212F/100C. As a result, wet processes such as boiling and steaming will never be able to brown foods because the temperature of the food will only get as high as the 212F with slight adjustment due to elevation and atmospheric conditions. Dry methods are able to reach much higher temperatures allowing the browning reactions to occur. This is why braised foods are usually seared first to create those flavors and colors that otherwise won't occur in a wet, low temperature setting.

There are notable exceptions to browning above the boiling point. Basic solutions, concentrated mixtures of carbohydrates and amino acids, and long cooking times can create the same reaction. Examples include reductions of stock to create demiglace and brewing beer.

Back to my microwave pizza. Metal placed in microwaves usually creates dangerous sparking through the buildup of electric fields. Very small amounts of metal however can be heated without creating a danger and when this metal is heated, it reaches temperatures far beyond the boiling point of water. This is the function of the metallic disk with my pizza. The disk is placed underneath the crust and so when it is heated by the microwaves, it subsequently heats the crust through conduction at temperatures high enough for Maillard Reactions to occur. This is how microwave pizza makers brown the crusts. The effect can also be seen in Hot Pocket brand stuffed sandwiches which utilize a microwave sleeve slipped around the Hot Pocket with similar metallic coating. The microwave sleeve heats up hot enough to brown the crust of the Hot Pocket.

Aaron Chan believes that since everyone needs to eat to live, food should be everyone's top priority.



by Aaron Chan
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Michael Chu



Joined: 10 May 2005
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Location: Austin, TX (USA)

PostPosted: Fri Feb 09, 2007 9:02 am    Post subject: Reply with quote

Aaron did a great job explaining the different methods of cooking in simple terms. A few notes that I would add are that dry cooking techniques just mean that the technique does not dominantly use water to transmit the heat. That's why pan frying and deep frying are not considered "wet". In addition, microwave oven cooking can be considered both dry or wet depending on what was cooked. The frequency of radiation used in a microwave oven (2.45 GHz - technies should realize that this frequency and thus the ramifications of using many popular phones and wireless networking devices while the microwave oven is running nearby) is used to excite both water molecules as well as fat molecules (the idea that the frequency is tuned to the electromagnetic resonance frequency of water is an urban legend - if it were true, we wouldnt' be able to heat dried foods effectively. Even so, if the food doesn't contain water or fat it doesn't heat well - and what's the point of eating it anyway?). So, if you are cooking a particularly fatty substance it can be considered dry cooking, but, in my book, most microwave cooking is wet cooking since we're heating up water molecules (maybe even to boiling/steaming).

One interesting thing to note is that because microwave ovens deliver energy efficiently to fats, you brown food in a microwave through the application of fat. In fact, you can cook a steak in a microwave and have it brown effectively by brushing oil or melted butter on the surface. You'll need to devise some way to catch the splatter and release teh steam or you'll end up with a soft soggy browned meat because of trapped steam or a really dirty microwave oven.

One last note - although the Maillard reaction occurs most apparently at higher temperatures, it also takes place at a much slower rate at temperatures below the boiling point of water (< 100°C). In most cases, the browning occurs so slowly as to be competely ignored when cooking (unless you're a chemist), but some proteins (such as milk solids) can be browned through Maillard reaction at these lower temperatures effectively.
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richard



Joined: 09 Feb 2007
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PostPosted: Fri Feb 09, 2007 11:36 pm    Post subject: Silver lining Reply with quote

Ive never really thought those silver disc work very well. Who could forget the old infomercial for Microcrisp? You could buy a roll of that silver paper stuff, and wrap anything in it place it on the included stand and it would allegedly turn your nuclearwave into an oven. My dad bought some, and sadly, his steak was mushy, and not too good. I put a hamburger in it, and burned it.
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gngbfly
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PostPosted: Sat Feb 10, 2007 9:37 pm    Post subject: microwave heater panels Reply with quote

I am amazed that "engineers" would refer to the microwave heater panel as a "metal disc". That disc is a die-cut piece of polyester film (Mylar) that has ben metallized with aluminum. The film is then laminated to paperboard to give it some dimensional stability. The thickness of the aluminum deposition determines the amount of heat produced.
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from an engineer
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PostPosted: Sun Feb 11, 2007 7:13 pm    Post subject: erroneous explanation of heat transfer in cooking Reply with quote

This article leaves a lot to be desired in terms of technical accuracy. Not only does the author erroneously refer to the browning disk simply as "metal", but the author is fundamentally incorrect in the entire treatment of heat transfer as it relates to cooking.

The explanation of various forms of heat transfer is fundamentally deficient. The author faithfully (if oversimplified) repeats the definitions of various means of heat transfer. But, as they relate to cooking, the author confuses generic concepts of 'heat transfer' with how heat actually transfers into food.

The author's description of convection is by far the most problematic. Heat transfer, and browning in particular, is about the temperature difference between the surface of the food and its surroundings and the thermal conductivity of the surroundings. Because material is not moving into and out of the food, convection does not play a substantial heat transfer role into the food -- it does move heat from heating elements to the food's surroundings (e.g. air, water, or oil), but not into the food. Convection only helps maintain a higher temperature at the food surface by allowing heated portions of the surroundings to move toward the food and cooled portions away. The author errantly states, "The fast moving molecules of the convection medium collide with the slower molecules in the food and heat them up." That description is more or less how conduction works, not convection. Likewise, it is heat capacity, not density, that helps heat conduction -- they are not the same thing! "The denser the fluid, the more often the molecules collide with the food and the fast the food heats up." Again, this is simply incorrect. A correct rephrasing would be, "The higher the heat capacity and heat conductivity, the higher the heat flux into food, and the faster the food heats up." For example, lava rock can be as dense as peanut oil, yet people walk can easily walk on 400F lava rock, but oil at the same temperature would burn you quite rapidly.

The author's treatment of conduction is also mistake-filled. The author misses the most basic function of oil in conductive cooking -- as a heat transfer medium. Oil's high heat conductivity and capacity make it a superior heat transfer medium for high temperature cooking. It's not just about 'uniform contact with heat' as the author states. It's about the temperature and heat conductivity. With respect to browning, water provides uniform contact with heat, but water boils before browning can occur. Air gives perfectly uniform contact as well, and with no temperature limit, but has poor capacity and conductivity. Cooking in a dry pan really means that small portions of the food are in contact with the pan, but mostly separated by air, which is a poor heat conductor. Oil, with its higher heat capacity and thermal conductivity transfers more heat from pan to food, thus getting more even browning and, by transferring more heat, faster cooking.

In the portion about radiative heating contains a very simple error, but this is indicative of the fundamental misunderstanding of heat transfer that runs throughout the article: referring to fats as polar molecules. Almost axiomatically, fats are non-polar. For those who avoid chemistry, try microwaving identical bowls of water and oil and compare the temperature difference. Water will be much, much hotter. In fact, it's usually the heating of the bowl that heats the oil, not the oil itself being heated by the radiation. The way the author describes infrared as having 'enough energy to heat up anything' is also not correct. Every material has a characteristic absorbance across the radiation spectrum; absorbance is not about 'how much energy' the radiation has, but the nature of the material and the nature of the radiation. It just so happens that foods absorb infrared radiation more uniformly than they do microwave radiation, and microwaves penetrate more deeply than do infrared waves.

Finally, and comparatively trivially, the 'metal disk'. As noted by a previous poster, microwave heating disks usually are a mixture of metal and polymer. Adding the polymer helps insulate the heated metal (usually particles, not film) to help apply the heat to the food more than to other surroundings. A metal film (consider for example a thin film of aluminum) alone has too low heat capacity to effectively transfer heat -- why else would you cover baking dishes in foil?. The polymer mixture helps insulate the disk, and is also cheaper and easier to manufacture than pure metal. Even with no a priori knowledge of the disk construction, just basic knowledge transport phenomena tells you a pure metal film is not the best material for the job.

In summary, this article contains numerous, significant errors that, in combination, completely misrepresent the nature of heat transfer in cooking. A better article would consider conduction and heat capacity more directly.

I enjoy 'cooking for engineers'. I hope this posting is one way I can help contribute to making this site better. I would love to hear a response from the author or editors regarding this posting. I will check back.

Best Regards.
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stonewallinn
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PostPosted: Sun Feb 11, 2007 11:04 pm    Post subject: Microwave Reply with quote

Great article. The only thing I thought of to add is this: with the amount of time you wrote the article, you could have made a fresh pizza instead of one of those disgusting microwave pizzas!!!
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scott123



Joined: 23 Dec 2006
Posts: 15
Location: Morristown, NJ

PostPosted: Sun Feb 11, 2007 11:56 pm    Post subject: Reply with quote

Michael Chu wrote:
One last note - although the Maillard reaction occurs most apparently at higher temperatures, it also takes place at a much slower rate at temperatures below the boiling point of water (< 100C). In most cases, the browning occurs so slowly as to be competely ignored when cooking (unless you're a chemist)....


This is not entirely true. Maillard browning plays a huge rule in stockmaking. When you get into simmering durations of 8 hours or more, sub boiling simmering temps will produce a substantial quantity of maillard compounds.

In heavily reduced stocks/glace/demi, maillard browning plays an even larger role. Demi and glace are 'wet' applications that involve a tremendous amount of maillard browning. Because of the concentration of dissolved solids, though, the boiling temps are slightly above 100C.
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mycoman



Joined: 12 Feb 2007
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PostPosted: Mon Feb 12, 2007 3:16 pm    Post subject: Reply with quote

The term metallic disc in the instructions is a bit confusing. I hadn't heard of these composite discs before, but I've never tried to make a microwave pizza. Initially, I thought they meant a metal pizza pan like a cookie sheet. If a solid metal sheet were used, wouldn't it just reflect the microwaves?
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Greg
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PostPosted: Wed Feb 14, 2007 3:55 pm    Post subject: Re-work article? Reply with quote

I found both the article and the comments very interesting. My wish is for "engineer" who pointed out some of the deficiencies of the article to help re-write it so that a lay-person can fully understand the correct science.

How about posting a re-write?

Thanks!
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Michael Chu



Joined: 10 May 2005
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Location: Austin, TX (USA)

PostPosted: Wed Feb 14, 2007 5:15 pm    Post subject: Re: Re-work article? Reply with quote

Greg wrote:
I found both the article and the comments very interesting. My wish is for "engineer" who pointed out some of the deficiencies of the article to help re-write it so that a lay-person can fully understand the correct science.

How about posting a re-write?

Greg, I was thinking the same thing! Unforunately, I don't have the contact info for "from an engineer", so we'll have to hope he/she reads this and e-mails me at submissions@cookingforengineers.com so we can get a more accurate article up on this subject.
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not quite anonymous
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PostPosted: Wed Feb 14, 2007 6:16 pm    Post subject: re-write Reply with quote

All,

Thanks for the feedback. At the same time as my first posting, I sent Michael Chu an email. I offered to discuss my comments with Mr. Chan, the author, but I never received a response. I had hoped to connect with Mr. Chan to work with him, not separately, to make any changes. I sent my note to 'cooking@...', which I found on this web site, instead of 'submissions@...', so I guess I just used the wrong address.

I have just now sent a second note to Mr. Chu at the 'submissions@...' address he provided in this forum. Mr. Chan is the original author, and I would prefer to work collaboratively with him instead of just doing a separate re-write. He wrote an article from scratch, which is a lot harder to do than poking holes (what I have done), so any changes that might be made should be at his discretion, and credit should remain his.

Hope this helps -- looking forward to hearing from you, Mr. Chu!
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jmcharry
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PostPosted: Thu Feb 15, 2007 10:27 pm    Post subject: Beer Reply with quote

Actually, the brown in beer is from roasting the malt prior to mashing. The degree of roast determines the darkness of the beer.
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Another engineer
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PostPosted: Fri May 25, 2007 7:58 pm    Post subject: Comments regarding Engineer's treatment of heat transfer Reply with quote

Good points, but I would like to suggest the following clarifications:

Engineer is correct in stating that the explanation of oven cooking is erroneous. However, to say that something is incapable of receiving heat transfer via convection simply because the convecting fluid does not enter the system's boundary is misleading. Anytime heat transfer involves a moving fluid could be said to involve convection according to how most engineers use the term (see http://www.efunda.com/formulae/heat_transfer/convection/overview_conv.cfm)

Otherwise, it would be inappropriate to describe a convection coefficient changing on a windy day to make us cooler, would it not? This is more semantic than anything else, but common engineers' usage of the term would imply that, for example, a convection oven DOES transfer heat via convection, whereas Engineer's definition would imply that the previous statement is inaccurate.

Back to ovens, I believe Engineer does not recognize that the effect of foil on something in the oven is not so much related to the reduction of the thermal conductivity as much as a change in the net radiation transfer to the food (tenting foil over food will create "shade" for the food, much like a canopy, keeping that area cooler. Tightly wrapping foil around an object will act like a "radiation blanket", keeping heat loss due to radiation very low and possibly making the food hotter). The foil further reduces heat transfer by actually reducing the local convection coefficient by making the air close the food at even lower velocity, or even at a static condition. Aluminum can actually be quite a good conductor of heat (I've got some camping pots/pans made out of the stuff that work just fine!): aluminum foil is typically not that great because the non-uniform contact surface that typically results will increase the thermal resistance at the boundary (mainly due to trapped air). Try coating a frying pan in foil and stir-frying: it still works if you can push enough air out between the foil and the pan. If you review Fourrier's law, you'll see that a very thin, metallic substance can actually be one of the best conductors: indeed, the thinner the substance, the better it will be.

Engineer is correct in pointing out that generally convection is not the appropriate mechanism, but this is mainly due to the low fluid velocities (occuring by advection only, except in the case of convection ovens where convection effects typically far surpass radiation effects).

An oven is actually quite complex when it comes to heat transfer: depending on how much fluid is inside, arrangement of coils, size, etc. conduction, convection and radiation could all play a very significant roll in cooking your food.

http://www.madsci.org/posts/archives/nov2001/1004895879.Ph.r.html
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Burr Zimmerman
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PostPosted: Sat Jun 09, 2007 10:36 pm    Post subject: re-write Reply with quote

As promised, here is a follow-up article:

http://www.cookingforengineers.com/article/224/Heat-Transfer-and-Cooking
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Burr Zimmerman (engineer)
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PostPosted: Mon Jun 11, 2007 5:47 am    Post subject: response to 'another engineer' Reply with quote

>However, to say that something is incapable of receiving heat transfer via
>convection simply because the convecting fluid does not enter the system's
>boundary is misleading.

No, it isn't. It's accurate. Convection increases conduction by reducing the width of the boundary layer, and by increasing the temperature of the bulk fluid at the boundary layer. Nonetheless, the heat that the food receives is only by conduction, not by convection.

>Anytime heat transfer involves a moving fluid could be said to involve
>convection according to how most engineers use the term.

Convection *does* play a role, but it's only conduction, not convection that is transferring heat into the food.

>Otherwise, it would be inappropriate to describe a convection coefficient
>changing on a windy day to make us cooler, would it not?

No, it wouldn't. The reason we feel colder in wind is because the boundary layer around us is reduced by the wind -- body hair serves to create a large boundary layer, and thus reduce heat transfer by conduction. Convective air flow reduces the size of the boundary, and thereby increases conductive heat transfer. Net result: we feel colder.

>This is more semantic than anything else, but common engineers' usage
>of the term would imply that, for example, a convection oven DOES
>transfer heat via convection, whereas Engineer's definition would imply
>that the previous statement is inaccurate.

A convection oven does transfer heat via convection, but not to the food. Convection only occurs up to the boundary layer around the food. The convection reduces the size of the boundary layer and thereby increases (conductive) heat transfer.

>Back to ovens, I believe Engineer does not recognize that the effect of
>foil on something in the oven is not so much related to the reduction of
>the thermal conductivity as much as a change in the net radiation
>transfer to the food (tenting foil over food will create "shade" for the
>food, much like a canopy, keeping that area cooler.

Yes, reflection radiation is another role that aluminum plays. I was not trying to explain everything that aluminum foil does. Thanks for this additional insight.

>Aluminum can actually be quite a good conductor of heat (I've got some
>camping pots/pans made out of the stuff that work just fine!):

Aluminum is a terrific conductor, but it has very low heat capacity. Hence, foil may not be a good medium for heat transfer.

>If you review Fourrier's law, you'll see that a very thin, metallic
>substance can actually be one of the best conductors: indeed, the thinner
>the substance, the better it will be.

LOL. Like pulling out a famous name changes physics. Um, no, it's not quite accurate to say thinner is better. Yes, in terms of simplified systems seen in introductory physics or heat transfer classes, a thinner layer provides less resistance to heat transfer. However, in cooking, you have to consider heat capacity as well. Please read the article I wrote (link posted above).

I'm really pleased that you have posted this comment. I'm glad to have a pair of conscientious eyes looking over this discussion.

Burr
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