Cooking For Engineers

[Hillman321]

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.

[hischrispyness]

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

[harveybowden]

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 on in 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

[Dilbert]

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.

[Harvey Bowden]

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.

[Dilbert]

ah.

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.

[Burr (original author)]

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. (It also 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).

[Abayomi]

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 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

[Dilbert]

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.

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.

[GUEST]

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.

[nh]

What method of heat transfer does "induction" fall under? Conduction, convection, radiation?

[Michael Chu]

[bennylynch]

It was suggested to me that oven cooking at 150C my beef curry would result in the curry reaching 150C. But I maintain it cannot exceed 100C unless it loses all the water I added as per the recipe for curries. Am I correct?

[Michael Chu]

[bennylynch]