Temperature and airflow: The complex dynamics

[A Patric]

Hello all,

I'm wondering if there are any cookbooks, food books, or other books or publications out there that do the topic of temperature and airflow justice. By this I mean that I would like to see a very clear explanation of what happens within an oven, for example, when one increases the airflow while keeping the temperature the same in terms of length of cooking time and any other interrelated changes. I understand that often cooking times decrease given the same temp but increased airflow, but I would like to know what is happening that causes the cooking time to decrease, etc.

I wasn't sure which place would be the best to ask this question, but I figured with all the bakers here, it would be relevant in this section.

Also, if you don't know of a book, but you are very knowledgeable about the subject, then I would greatly appreciate all the information that you have the time to share.

Many thanks in advance.

[jsolomon]

Speaking as someone who has taken thermodynamics courses, I dont' think you really want to think about this rigorously. Thermodynamics is a very unfriendly course of study.

But, what you are doing when you increase the airflow is increasing the thermal mass of the cooking medium. How it works is at a zero airflow, no convection or anything, the air and the food come into temperature equilibrium fairly quickly, as air doesn't have much heat capacity. When you increase the airflow a little, now, you have a constant influx of air that, asit travels over the food, comes into temperature equilibrium, again. As you increase the airspeed, you have the air traveling farther and farther across the food before it comes to thermal equilibrium. Then, you get to an airspeed where the air doesn't even come to thermal equilibrium as it travels all the way across the food. Having warmer air at the far end of the food is good because this warmer air has more heat to transfer which cooks that end of the food faster.

What this actually works more like, is having a denser cooking medium. Granted, you have to get a lot of airflow to equal the density of boiling oil, but you could get there, if you tried hard.

If that is unclear, please ask questions.

[A Patric]

Okay, I believe that I understand. Here is a new related question though:

With convection ovens often one is told to decrease the temperature slightly. This makes sense to me given your explanation as it seems if the recipe was written with a normal non-convection oven in mind, then the outside of the bread/meat/etc. could become very overdone before the inside is finished in a convection oven. So, there are two ways of doing it so far:

relatively stationary air and higher temperature

convection (moving) air and lower temperature

Am I to understand that we could come up with many more possibilities by increasing the airflow and further decreasing temperature? If so, given the two extremes of 1) maximal airflow and minimal temperature and 2) maximal temp and minimal airflow, if they are both within my capability to attain, then why would I choose one over the other? If I understand correctly, the option with maximal airflow and minimal temperature would probably take less time to cook (is this correct?), is there anything else that should cause me to choose one over the other?

Thanks again.

[dejaq]

much of what JSolomon says rings true, many of the dynamics that apply to heating a product can also come into play, regarding cooling it down super fast. With freezing though, esspecially cryo blast freezing, you are concerned with "breaking the microbial air barrier" around the product, to have cold air penetrate the product. One note I would like to add is the addition of air vapor, in the heating process, Combi ovens execute this quite nicely, and not only keep a product moist, but cooks it in a fraction of the time, barring at an added expense.

this is a Phenomenal resource, don't be afraid of giving them a call or writing them an email, these guys are at the top of their heap, and are way cool:

AIB

Michael

[jsolomon]

Okay, I believe that I understand.  Here is a new related question though:

With convection ovens often one is told to decrease the temperature slightly.  This makes sense to me given your explanation as it seems if the recipe was written with a normal non-convection oven in mind, then the outside of the bread/meat/etc. could become very overdone before the inside is finished in a convection oven.  So, there are two ways of doing it so far:

relatively stationary air and higher temperature

convection (moving) air and lower temperature

Am I to understand that we could come up with many more possibilities by increasing the airflow and further decreasing temperature?  If so, given the two extremes of 1) maximal airflow and minimal temperature and 2) maximal temp and minimal airflow, if they are both within my capability to attain, then why would I choose one over the other?  If I understand correctly, the option with maximal airflow and minimal temperature would probably take less time to cook (is this correct?), is there anything else that should cause me to choose one over the other? 

Thanks again.

←

You choose one over the other because the speed of heat transfer, as well as depending on thermal density of the material (heat capacity) and thermal conductivity, also depends on the difference in temperature. Actually, Newton himself wrote on this and formulated Newton's Law of Cooling.

MATH WARNING!!!!!!!

Newton's Law of Cooling simply states that the rate of change of temperature of an item is proportional to the difference between its temperature and its surroundings. So, dT/dt = K[O(t) - F(t)]

Where dT/dt is the rate of change of temperature of the food as a function of time

O(t) is the temperature of the oven

F(t) is the temperature of the food

If you integrate this, you get an exponential function.

T(t) = F` + (O` -F`)exp-K(t-t`)

(primes signifiy values at time = 0)

Now, I don't have any great treatments for how you would deal with changing air velocities, but given that air's heat capacity is fairly 1000J/kg*K, and a kg of air is a lot of air, you've got to move a lot of air to make the difference 50 degrees C makes. I'm sure there is a relatively straightforward method to calculating it, but I don't have the heart to dig further into my differential equations or physical chemistry textbooks tonight.

[nathanm]

Oven dynamics are quite complicated. There is no good cooking oriented book on the topic.

JSolomon's math is only the beginning because when you get to forced convection the amount of turbulence in the air and other factors come into play. The ambient humidity does too - which is why combi-ovens control the humidity.

An oven works by transferring heat to the object. Heat transfer depends on heat gradient (what JSolomon wrote) but it also depends on the heat capacity of the air, how fast it is moving, how much water is in the air, how much water evaporates from the food....it is quite complicated.

I have been through all of this. In the end I think that the scientific treatment does give some insights for a chef. However, nobody has spelled it out in a way that is digestible by chefs. I have been thinking about writing something up, but so far I haven't gotten around to it.

The key factors are:

- The actual temperature of the food surface only somewhat depends on the oven temperature. The key factor is on how fast water in the food evaporates from the surface. The food surface is at the wet bulb temperature.

- The moist part of food never goes above water boiling temperature - no matter how how the oven. It can't exceed boiling point, and usually is much lower (wet bulb temperature).

- When food browns or burns in a very hot oven, the parts that brown have to first dry out by having the water evaporate from them - only then can they start to raise above wet bulb temperature.

- This in turn depends on how fast you are carrying heat to the food, and how fast you carry water vapor away, which depends on convection speed.

- Speeding up the flow rate speeds up heat flow, and water evaporation rate. The reason you decrease temperature is that with faster flow you can transfer the same amount of heat as lower temperature with still air.

- However even with ultra fast flow there are limits - once the flow has "full developed turbulence" heat flow will level off.

- A far better way to increase heat flow is to increase the moisture content of the air (humidity) this dramatically increases heat flow. Moist air cooks faster than dry air - and again you have to reduce the temperature. Steam cooks FAR faster than dry air.

- As you increase heat flow, you can lower the temperature. However there is a limit to how you can put the temperature because you also want to brown things (carmelize sugars and/or malliard reaction), and this requires specific temperature thresholds.

- Heat conduction inside the food is totally different story - it is quite slow. In an oven with too much heat flow too fast into the food, the outside can be burned before heat conducts in so the interior is undercooked.

[bibbotson]

Oven dynamics are quite complicated.  There is no good cooking oriented book on the topic. 

Thank you, nathanm, your summary is beautiful!

As another eG'er who suffered through Thermodynamics courses en route to a career in the kitchen, I can't stress enough that the key factors nathanm provided above is as detailed an understanding you need. They give you a sense of direction of how changing the setup (like moving to an oven with steam injection) should lead you to experiment with the temperature and time of your food.

Any further specificity isn't worth it because it depends too much on the individual characteristics of the cut of food in the pan, the pan, the oven, the air humidity, etc. Its only worth running down that difficult-to-figure road in industrial setups where consistency is maintained among all those items, every time, hundreds of thousands of times.

Great thread.

[jsolomon]

I hate to be the pedant here, but with regard to a steam-injection oven, you get performance gains based on the food condensing the steam. What this does is transfers the energy it took to vaporise the steam into the food. This really is a tremendous amount of energy.

So, with a good steam injection oven, you can cook at a lower temperature for the same performance because you are using a gas condensing to a liquid--actually you are creating a very high-temperature air-conditioner for steam when you do this.

Ain't science great?

[A Patric]

Thank you to everyone who has added input. It is a quite interesting subject. It's too bad that someone hasn't written more about it formally. Maybe that will change someday soon.

[jsolomon]

There are more scientific treatments. You just have to look hard. I'd find more, but my scifinder login is acting funny.