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.
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.
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.
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.
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!!!
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.
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?
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 email@example.com so we can get a more accurate article up on this subject.
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!
Actually, the brown in beer is from roasting the malt prior to mashing. The degree of roast determines the darkness of the beer.
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.
As promised, here is a follow-up article:
>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.
Lately I've become interested in "live food" especialy for my dogs since the recent pet food recall. Many advocates of raw or live food diets say "do not microwave". Can any of you smart engineers explain what happens in food breakdown during microwaving that results in less nutritious food?
man this is a very intresting topic this made no sence at all and i didnt even read it worse ever :lol: :lol: :lol: :lol: :lol: :lol: :lol: :lol: :lol:
The only bad thing that can happen in a microwave is that overcooking, as with any method, may destroy vitamins.
This thread reminds me of that old question:
"How many engineers does it take to change a light bulb?"
"Two. One to do it, the other to tell him he's doing it wrong!"
When you said that the temperature must reach above the boiling temperature of water (212 F/ 100 C) for the Maillard Reaction to occur, is this always true? I'm trying to understand why it is specifically at 212 F. Is it because all the water must be evaporated in order for the crust to form? If we lowered the pressure, is it still 212 F or is it just above the boiling temperature of water at that pressure? Or is 212 F the temperature where the special interaction between the carbohydrate and amino acid takes place?
Yes. If water is boiling and steam is being formed, the temperature won't get high enough to carmelize any sugar and do whatever else is necessary for the Maillard reaction.
Please explain how you will lower the pressure with common cooking apparatus. A reverse pressure cooker?
Maillard reaction occurs at temps much higher than 212° F. Might be fun to Google the temp at which sugar carmelizes, if you're really interested.
It was a great article on Heat Transfer and Browning Foods. It help to understand the working of microwave oven and how to cook food in microwave oven. Thanks for sharing.