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Chefsteps: "all frying is convection"


gfweb
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What a load of crap. Convection is not the process of heating food through a medium

 

We’ve been air frying up a storm in the test kitchen lately to bring you our new Air Frying Hub. This cooking method is a great way to satisfy your cravings for crispy, crunchy foods, while avoiding the calorie bomb that comes with deep frying.

Air frying has become the standard industry term for super convection. Some people object to the term because they point out that convection isn’t the same as frying.

Here’s the funny thing: all frying is convection.

Convection is the process of heating food through a medium. In sous vide, the medium is water. In an oven, the medium is air. In deep frying, the medium is oil.

The key difference between oil frying and air frying is that in oil frying, as the water evaporates from your delicious breaded chicken, it is replaced by the limitless supply of cooking oil surrounding it. In air frying, the water in the food evaporates but can only be replaced by any thin coat of oil you might have dressed it with before placing it in the fryer basket. The result from the latter is crispy food that is much less caloric than that achieved through oil frying.


 

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actually deep frying (as in when the oil/liquid is hot enough) it is the gas surrounding the food that does the cooking. The bubbles formed when we heat anything with water in it (most food) the water turns to steam and that steam as it boils away from the surface of the food does the cooking This steam becomes hotter, heated by the oil to well above waters boiling point and expands (the laws of physics says so) but it rises to the surface of the oil and dissipates as steam. The oil is bubbling & boiling! (well no it isn't the water in the food is boiling - take the food out the oil stops bubbling)

So round the food is essentially a layer of super heated steam which does the cooking

Once the food runs out of water or there is sufficient crust formed then the oil may well contact the food and the food will absorb some of the oil, though if the remaining steam can't get out past the crust. oil will certainly not go into the food.

Once the water is fully used up the food is likely inedible anyway.

The heat transfer is conduction (even though the air/steam is moving the heat is being transferred from the gas to the food and the gas is moved on by gravity (it bubbles to the surface) and is replaced by new hotter steam. That is not convection

Convection occurs when the medium circulates around a body because of differences in the temperature of the medium itself, which is not what the gas/oil does.

There IS convection happening while the oil is being heated but that is within the oil itself.

If the oil is not hot enough to boil the liquid in the food then the oil may well be absorbed. (confid?) The heat transfer is conduction.

In an air fryer the air itself takes the place of the oil but the food will still have a layer of super heated steam at its surface.

In boiling foods the water/liquid and the internal water of the food are at close to the same temperature and there is no super heating of any steam so the heat is transferred directly from the water to the food. The method of heat transfer is mostly conduction.

Below boiling the heat is transferred by conduction (in SV the bag is just a moisture barrier not a heat barrier)

But it really doesn't matter as long as the chips cook😀

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35 minutes ago, Bernie said:

The heat transfer is conduction (even though the air/steam is moving the heat is being transferred from the gas to the food and the gas is moved on by gravity (it bubbles to the surface) and is replaced by new hotter steam. That is not convection

Convection occurs when the medium circulates around a body because of differences in the temperature of the medium itself, which is not what the gas/oil does.

There IS convection happening while the oil is being heated but that is within the oil itself.


Maybe there are a few terms used too loosely here:

 

Convection is heat transfer by a liquid or gaseous medium. It consists of mainly two different underlying processes:

 

Conduction is heat transfer by thermal diffusion, thus two media in contact with each other try to equilibrate their temperature.

 

Advection is the heat transfer by movement of bulk medium (this is what you refer to here as convection in the last paragraph).

 

Convection encompasses the two processes above and is the correct term to describe both deep frying and air frying.

Edited by Duvel (log)
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2 hours ago, liuzhou said:

 

 

So, pretty much all cooking.


Pretty much, except for the cooking techniques that rely on thermal radiation, e.g. grilling, some types of baking, …

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8 hours ago, mgaretz said:

 

What if they had said "convection cooking is the process of heating food through a gaseous or liquid medium"?

Different.

But convection happens in a heated chamber. The cooking is still thermal cooking it isn't because convection occurs.

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I think they are using semantics to sell an idea of lacklustre results on my wedges in an oven compared to the deep fried ones I made last week. 
 

I’ve had an “air frier” I’ll remain with traditional cooking like sous vide, gels, muddling with ph, fire and spices. 
 

 

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And while we are at it, convection ovens work not because of thermal convection, but because of a fan moving air around

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3 hours ago, gfweb said:

And while we are at it, convection ovens work not because of thermal convection, but because of a fan moving air around

 

Which is heat transfer by a moving medium (air), so nothing wrong with the term convection oven. I doubt anyone would claim that the movement is solely due to density changes in the heating environment ...

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49 minutes ago, Duvel said:

 

Which is heat transfer by a moving medium (air), so nothing wrong with the term convection oven. I doubt anyone would claim that the movement is solely due to density changes in the heating environment ...

 

Well that is the point.  Convection doesn't involve a fan and did not (originally anyway) mean any air that is moving for any reason. 

 

I suspect oven marketers thought "convection oven" sounded sexier than a "fan oven".

 

So how many angels fit on the head of a pin?  I'd say none, but am open to discuss it.  😉

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18 minutes ago, gfweb said:

Well that is the point.  Convection doesn't involve a fan and did not (originally anyway) mean any air that is moving for any reason. 

 

I suspect oven marketers thought "convection oven" sounded sexier than a "fan oven".


I think the term convection oven was coined to distinguish it from a conventional oven, where most of the heat transfer is done by radiation. I also think the term fan-assisted oven is used interchangeably, so I can not detect any ill will here from the marketing department ☺️

 

I don’t think it matters much how the convection (again, just the term to describe the heat transfer process) is caused. 

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So as it turns out it was for product sales, joule oven which appears to be a rebadged breville smart fan oven that doesn’t sous vide or do combi.

 

kinda dislike chef steps being a product placement “thingy” as I like all their recipes and work they put into teaching.

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On 3/12/2022 at 2:42 AM, Duvel said:


Pretty much, except for the cooking techniques that rely on thermal radiation, e.g. grilling, some types of baking, …

 

And cooking processes that rely on conduction with something stationary, like sautéing. 

 

I think you could argue that most cooking methods use a mix of of thermal processes, often with one of them dominating. But deep frying? That's pretty pure convection.

 

I'm not sold on this "air frying" business. Just because two cooking methods use a roughly similar thermal transfer processes doesn't mean they're similar in ways that matter.

Notes from the underbelly

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