Oven – Do wire oven racks and solid plate oven racks have different functions

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In South Africa, domestic ovens have wire racks and solid plate racks. Does each type have a specific function or can anything be cooked on either type?

Best Answer

This sounds like you might be talking about a european style oven, which generally has 2 types of inserts, wire shelves and solid trays. Wire shelves are used when you want to cook something that is self-contained, like a casserole, baking dish, cookie sheet, etc. The space between the wires allows the free circulation of air, which is a good thing.

Trays usually come with a removable wire rack and are used when you are baking or grilling (in the US it's known as broiling) something loose like chicken pieces, burgers or sausages. The tray catches any drips and holds liquids that would otherwise fall to the bottom of your oven and make a very sticky and hard to clean mess. The tray slots into the same grooves as the wire shelves so you can put it on any level.

In the US many ovens have a separate drawer for broiling which comes with a slide-in tray, it's the same concept except you usually can't use the tray in the oven as the sizes don't match.

Here's a picture of the type of oven I mean, on the bottom you see the tray, on the top the wire shelf.

Bottom line

When you use both racks at once, your cooking pattern will become somewhat uneven.

Normally, you will only see this as a serious effect in thin, rapidly cooking foods like cookies. Some cookies on the tray may be browned and done, while others are still pale and underdone. This is why cookie recipes almost always specify swapping the trays between the racks, and rotating them front to back, half way through. The idea is to balance out the influence of these various affects.

For thicker and longer cooking foods like a casserole or a braise, internal conduction or convection will tend to even out the influence of the local hot and cool spots over time, making use of both racks fairly irrelevent to the outcome.

Background

Ovens transfer heat into food in two modes:

Infrared radiation (much like a heat lamp puts out) warms the surface of the foods (or their containers) directly
The hot air in contact with the food (or their container) conducts heat into the surface

Furtheremore, the conduction of heat into the food from the air is strongly influenced by the convection (circulation of the air due to temperature differentials). If the air were pefectly still (which it never is), then the part that is in contact with the food would soon come into thermal equilibrium with the food. As air is a very poor thermal conductor, it would then take some time for it to heat up from the adjectent air, and the food would cook very slowly.

In the real world, air also changes density when the temperature changes, becoming denser and sinking as it cools, and becoming less dense and rising when it is heated. This causes the air in the oven to circulate, even if there are no fans, and is called convection. It increases the ability of the air to transfer heat energy into the food.

There are also ovens which have fans to force air circulation, technincally called forced air convection ovens, although often simply called "convection ovens" for short.

Understanding these modes of heat transfer allow you to understand what happens when you use two racks within the oven:

Shadowing

As radiant heat is a major mode of heating, and infrared is light (just of a wavelength too long for people to see), it can be blocked and cast a shadow.

When you use only a single rack, the food (or its container) is bathed in the radiant heat from all of the walls (and floor and ceiling).

As soon as you use two racks, the food on one rack absorbs and blocks the radiant heat from at least one direction from getting to the food on the other rack.

This will cause the baking to slow down slightly, and change the pattern by which heat reaches the food.

Convection patterns

When you use more racks, the pattern of natural convection within the oven will change. Its like being in a city where the wind patterns are affected by the shape of the buildings. This can create hotter (more air going by) and colder (less air going by) spots within the oven.

If you have forced air convection (a fan), this affect is reduced but not eliminated, because the air still must circulate past the foods to actually transfer heat energy into them.

A note on on oven design

All ovens have radiant heat as a major cooking modality. The hot air from burner or heating element heats the walls of the oven, which then begin to emit infrared radiation back to the foods.

Some ovens also directly expose their heating element within the main oven cavity. In this case, infrared can be an even stronger mode, and become very directional.

This is particularly true of electric ovens with directly exposed coils. The shadowing affect becomes stronger in this type of oven.

Oven – the difference between Microwave, Microwave Oven, and Oven

First off, the terminology issue.

So, can someone please clear up this fog? What's the difference between Microwave and Oven and Microwave Oven?

"Microwave" is just short for "microwave oven". Both terms mean the same thing: an appliance that uses microwave radiation to heat food. Cooking food in this way is called "microwaving". An oven, on the other hand, has a heating element which heats the air inside, which then heats the food. Cooking food in this way is generally called "baking", though there are a lot of other things (e.g. roasting) you can also reasonably do in an oven.

So if all you have is a microwave, then all you can do is microwave (not bake). And if you have an oven, then you can bake (but of course you can't microwave).

There also exist combinations, which are capable of both microwaving and baking, i.e. they have a microwave emitter and a heating element. However, remember that in the US - and thus on much of the internet - something talking about a "microwave oven" likely does not mean this combination, but simply microwave heating. Names like "convection microwave oven with grill", "combination microwave/oven" do refer to these combinations, though. I've also seen "speed oven". You may sometimes have seen people referring to these combination microwave/ovens as a "microwave oven" (I think this may be common in Indian English), but that's really just imprecise language.

Some sites say that a "microwave" is only for heating or re-heating cooked food. Whereas, in "microwave ovens", you can microwave and bake. Is that true?

It sounds like in this statement, "microwave oven" is careless language, referring to a combination microwave and oven. (It doesn't make any sense at all otherwise; the two things are the same thing, so of course you can do the same things in them.)

So the question is really, what can you do in a microwave vs an oven, or a combination microwave/oven?

It's true that microwaves are mainly used for reheating cooked food, and it's true that they can't actually bake. However, they can do a lot more than reheat food; there are a lot of kinds of cooking besides baking. For example, microwaves are great at simmering/boiling and steaming, and it doesn't matter if the food was cooked already or not. The recipes you're talking about are probably mostly in those kinds of categories, but if you're interested, How do I know if a food or recipe can be made in a microwave oven? discusses in a bit more detail what works in microwaves.

The big things that you can do with ovens but not microwaves are the things that actually need the steady dry heat of baking. You can't bake bread or cookies or roast a chicken in a microwave. Microwaves hold in a lot of steam and don't get terribly hot, so you can't generally get things to brown or crisp up. They also don't really hold a temperature like you need for baking, they just pump more and more heat into the food.