Flashcards in CHAPTER 2: HEAT TRANSFER Deck (81):
Three main ways heat is transferred.
Radiation, convection, conduction.
A fourth type of heat transfer.
Induction, which takes place on a special stovetop surfaces.
What is radiation?
Rapid transfer of heat through space from a warmer object to the surface of a cooler one.
What is frictional heat?
The heat generated once molecules begin to vibrate rapidly on the surface of an object.
Why is radiation described as indirect heat?
At no time does the radiation body come into direct contact with the object.
Appliances that use radiation.
Toasters, broilers, in fared heat lamps and conventional ovens.
Do dark or light surfaces radiate more heat? Why?
Dark surfaces do because they absorb more heat energy to begin with.
Do dull or shiny surfaces radiate more heat?
Do dull black sheet pans bake food faster than bright shiny ones?
What is emissivity?
The ratio of power emitted by a body to the power emitted if it was a black body.
What is happening if something close to an oven wall is baking up darker than everything else?
"Hot spots," created by radiant energy.
Is radiation a means of transferring microwave energy?
What tube generated microwave energy?
Does microwave energy penetrate the surface of food more easily than radiant energy?
Why do microwaves heat food unevenly?
Partly because different substances absorb microwave energy differently, but also because some substances require less energy to heat up.
Why is heating with a microwave relatively fast compared to radiant energy?
Because the radiant microwaves penetrate further into the food--typically 1 to 2 inches--than radiant heat energy, which heats surfaces only.
When does conduction occur?
When heat passes from a hot area of an object to a cooler area; heat is passed molecule by molecule.
Is conduction a form of direct heat?
What happens to heat transfer when old aluminum sheet pans are pocked with bits of blackened food?
They will radiate heat unevenly.
Does conduction continue when a pan is removed from the heat source?
Yes, until the pan and food reach the she temperature.
Does water have low or high heat conductivity?
Does air have a higher or low heat conductivity than water?
Materials that conduct heat quickly are referred to as having what?
High thermal or heat conductivity.
Why do solids have higher heat conductivity than gases and liquids?
The molecules are much closer together in solids than liquids or gases; this makes it easier to pass from one molecule to the next.
Good team reference for conduction and radiation.
Two teams of ten stand next to each other. One (radiation) passes the ball from first to last while the other (conduction) can only hand it off one at a time.
Explain the conductive properties of heavy-gauge compared to this-gauge material.
Heavy-gauge material is thicker and conducts heat to food more slowly than thin-gauge.
Are heavy-gauge materials preferred over thin-gauge? Why?
Yes, because they transfer heat more evenly.
Why does marble feel cooler than wood to the touch, even in a warm bakeshop?
Marble has a greater heat conductivity than wood, so heat transfers faster from the body to marble than t does wood. Because the hand touching the marble cools more quickly, the marble seems cooler to the touch (when, in actuality the marble is now slightly warmer, because heat was transferred to it from the hand).
Put copper, silver, stainless steel and aluminum in order concerning heat conductivity.
Silver, copper, aluminum, stainless steel.
Put teflon, marble, wood, water and air in order concerning heat conductivity.
Marble, water, teflon, wood, air.
Why is marble often used in bakeshops?
Its good heat conductivity helps cool hot confectionary products quickly.
Why not use stainless steel instead of marble to cool confectionary products?
Generally, it's the price but they are also very thin and heat up too quickly.
Why is copper good to use for cooking sugar?
It helps reach high temperatures quickly.
What metals are typically applied in a thin layer to prevent copper from reacting with food?
Tin or aluminum.
How well does aluminum conduct heat compared to copper?
Half as well.
Pros and cons of aluminum cookware.
It's cheap and conducts heat very well but also reacts with foods, especially when acidic, and discolors fruit products while turning milk and egg products gray; this limits the use over stovetop cookware. And since it's a soft metal, aluminum is easily scratched an pitted
Where is aluminum commonly used? Why?
In bakeware, where discoloration is less of an issue.
How do you avoid burning baked goods on aluminum pans? What if you're baking delicate items?
Use heavy-gauge pans and parchment paper. Use silicon silicon baking pads or double sheet tray if food is delicate.
How does a double layer of sheets pans help with baking delicate items?
The double layer slows heat conduction to a manageable level.
What is dark hard-anodized aluminum?
Aluminum that has undergone an electrochemical treatment that changes the surface of the aluminum so that it is hard and durable.
Is anodized aluminum reactive with foods?
Does anodized aluminum conduct heat as fast as regular aluminum?
Why is some heat transferred by radiation with anodized aluminum?
Because it's dark in color.
Typical gauge of anodized aluminum? Expensive?
Typically comes in a heavy gauge so it cooks evenly, but it's more expensive than regular.
What is stainless steel?
A type of low carbon steel (iron alloy) that contains a mix of metals including chromium and often nickel.
Is stainless steel a good conductor of heat? Advantages (cleaning, pricing, durability)?
No but it is easy to clean, moderately priced and durable.
What is convenient about stainless steel's light-reflective surfaces?
Makes it easy to view food as you cook it.
How have manufacturers tried to "improve" stainless steel's heat conductivity?
Lower-quality cookware is manufactured to a thin gauge.
Why does thin gauge cookware have hot spots where food is likely to burn?
It is difficult to roll any metal to a thin gauge evenly. Because of its unevenness, thin gauge cookware has hot spots where food is likely to burn.
What does stainless steel cookware with an aluminum core provide for consumers?
Stainless steel provides nonreactive, light colored surface that makes it easy to view food and clean; aluminum provides improved heat conduction.
The best aluminum/stainless steel cookware has how much aluminum? Why?
Aluminum extending up the sides of the pan.
Best cookware for stovetop cooking of fruit mixtures, vanilla custard sauce and pastry cream?
Aluminum-core stainless steel cookware.
Is cast iron reactive? Is it pragmatic for bakeshops?
Yes, it adds a metallic taste and discolors food; this is why it's rarely used in bakeshops.
How do you season a cast iron skillet?
Coat with a thin layer of vegetable oil or shortening, then heat in an oven at about 350ºF (175ºC) for an hour.
Is tinware used in traditional French bakeware?
Benefits of tin bakeware.
A good conductor of heat, lightweight and inexpensive.
Problems with tin bakeware.
Rusts easily and darkens with acidic foods.
What must be done immediately after tin bakeware is washed?
It must be dried thoroughly so it doesn't rust.
Pros and cons of ceramic, glass, porcelain and stoneware.
They all conduct heat poorly but retain heat well once hot. This makes them useful for slow cooking. Ceramic ramekins, for example, are great for custards.
Do nonstick surfaces vary in durability?
Yes, some crack and peel, and most scratch after repeated use.
Why is it more difficult to brown food on nonstick surfaces?
The surface acts as an insulator between the source of heat and any food placed in the pan. This means cooking with nonstick is also slower as well.
Where are nonstick pans acceptable?
Wherever fast heating is not needed.
Pros and cons of silicon bakeware, molds and sheets.
Silicone is not a good conductor of heat. For this reason, items bake more slowly and brown more evenly.
Silicone baking mats (Silpat pads) are able to withstand what temperatures?
Ovens up to 580ºF/300ºC to freezer.
Why do liquids and gases become less dense when heat?
Molecules not only vibrate faster when heated, they move apart. This expansion lowers the density of hot liquids and gases as they rise and move away from the source of heat.
What happens to cold air and liquid as the hot air and liquid rises? How do convection currents play a role?
They are denser and, therefore, fall, moving closer to the source of heat. Convection currents set in, distributing heat more quickly and more uniformly throughout the product.
Where do convection currents occur? Only the air?
No, in the air, in thin batters baking in the oven, within thin liquids in a saucepan, and within fat in the fryer.
What is convection? How is an an invisible helping hand?
Aids heat transfer through liquids and gases, which otherwise conduct heat slowly. Convection involves the constant movement of cold currents of air or liquid toward warmer currents. The warmer, less dense liquid/gas and rise while colder ones sink; like a hand stirring the pot.
Rule of thumb when switching from conventional to convection oven.
Reduce oven temp about 25ºF (15ºC) and reduce baking time about 25%.
How do you maximize convection currents in any oven?
Make sure baking pans are placed so air movement is unobstructed; do not overload the oven.
How do you maintain oven temperatures during baking?
Minimize the number of times you open the oven. Even a few seconds carries a significant amount of the ovens warmth into the bakeshop.
When would convection currents need assistance?
They work without it, but the movement of liquid in a pot can be increased if it is stirred. This is especially important with thick liquids, where fewer convection currents set in.
Why do convection oven work faster than conventional ovens?
Whether air is being blown by a fan or food is moving in the oven (reel and rotating ovens), the hot air is moving more rapidly toward cooler surfaces.
Why do convection, reel and rotating ovens require a lower temperature and shorter baking times than conventional?
Because hot air is being forced into cooler spots faster. This is also why they work more evenly with fewer hot spots.
When are convection ovens appropriate and inappropriate? Explain.
They are best for products made from heavy doughs, such as cookies and pâte à choux. Cakes and muffins, for example, bake up asymmetrically if convection currents are too strong or oven temperatures too high. Sponge cakes and soufflés can lose volume and custards and cheesecakes over bake.
What is induction?
A new form of heat transfer where cooking takes place on special smooth-top ceramic surfaces, below which are coils that generate a strong magnetic field. The magnetic field causes molecules in the pan to rapidly flip, generating frictional heat within the pan.
How fast do induction burners work?
What characteristic must be present for an induction burner to work?
Flat surface and magnetic material; woks won't work.
What types of metal work on induction burners?
Cast iron and some stainless steel pans.
What types of metal don't work on induction burners?
Copper and aluminum.