Defrosting a chicken breast and the laws of thermodynamics?

Question

I was wondering how we could apply the laws of thermodynamics to defrosting some chicken breasts.

I noticed that when I put a bag of frozen chicken breasts in lukewarm water the defrosting process is sped when compared with leaving on the counter or in the fridge. This makes sense due to the higher temperature. Does the temperature of the chicken breast tend to the temperature of the water? And in turn what does the temperature of the water tend to?

In summary, how would you use the laws of thermodynamics to determine the most efficient way to defrost a chicken breast?

Update: Thanks to the answer and comments from @JohnRennie below, I'm updating the question to be slightly more specific.

The situation is slightly different now just so I can clear the situation in my mind and get a better grasp on the concept of temperature equilibrium:

I have the following set - up:

I have two sinks filled with water. One is filled with lukewarm (LW) water and the other with cold (CW) water each sink with their own frozen chicken breast. Can we use thermodynamics to decide which chicken breast would tend to room temperature the quickest?

The reason for the change in question is that (correct me if I'm wrong) that the two sink systems will eventually tend to temperature of the "kitchen" system but how do we figure out which one gets there first?

Answer 1

The laws of thermodynamics are the basic principles that form the foundation of the subject, but that doesn't mean they are the best starting point for analysing systems.

In this case frozen chicken breasts defrost faster in water than in air of the same temperature because:

1. water has a higher specific heat than air

2. water has a higher thermal conductivity than air

The higher specific heat means that for every 1 degree drop in temperature water delivers more heat to the chicken than air does, and the higher thermal conductivity means water delivers that heat faster than air does.

The laws of thermodynamics are involved. For example the first law tells that that because energy is conserved the energy that heats up the chicken must come from the energy released by cooling down the water/air. The second law tells us that the cold chicken/warm water system will equilibrate to a uniform temperature. but neither are useful in determining the kinetics of the process.

Answer 2

If you put two bodies with different temperature next to each other, then thermal energy flows to the colder one (a.k.a. the Second Law of Thermodynamics, though in a more serious context you would want to be aware that "it can be formulated in a variety of interesting and important ways." -- Wikipedia).

Depending on the materials involved, the speed of flow will be different. This is obvious from daily life: you can touch a very hot piece of wood with little pain because the heat stored in it takes a long time to transfer to your skin; you have plenty of time to let go of it. But touch a piece of metal with the same temperature and your skin will instantly blister because everyday metals transfer heat very quickly.

Same goes for air (bad heat transfer - good insulator) vs. water (relatively good heat transfer, relatively bad insulator). Example: heat your oven to 250°C, open the door and hold your hand right in the middle. Yes, you will feel the heat, but it will take quite some time before you feel any pain or damage occurs. In comparison, put your hand in a pot of 100°C boiling water, and you will have pain and real damage very, very quickly.

The rules for heat transfer are similar to Ohm's Law in electrics - thermal energy transfers faster if the difference in temperature between the two bodies is higher; and lower if the "heat resistance" is higher.

So, let's look at your system:

• Chicken at -10°C, air at 23°C, CW at 10°C, LW at 30°C
• Chicken-in-air: Difference of 33°C but very high resistance
• Chicken-in-CW: Difference of 20°C, low resistance
• Chicken-in-LW: Difference of 40°C, low resistance

So, assuming your room will always be constant at 23°C, this is what will happen:

• C-i-a will heat up very slowly and eventually end up at 23°C. Similar to when charging an electric capacitor, the rate of change will also slow down as the chicken temperature gets close to air temperature.
• C-i-CW will heat up much quicker but only close to 10°C (similar slowdown occurs). Better than nothing - from there, the whole mess of chicken+water will have to sloooowly heat up to 23°C.
• C-i-LW will heat up even quicker and easily pass the 10°C and even 23°C to get somewhat close to 30°C, assuming it is placed in a lot of warm water in a suitable container, so that the water contains enough thermal energy to give to the chicken before wasting it to the air. Eventually, if you leave it sitting, it will be at the same temperature as the water, and together they will cool down to 23°C.

Answer 3

The thing that comes to mind is more related to Heat Transfer because it has to do with heat rates, rather than plain Thermodynamics, which is usually used for determining quantities. But this is just terminology. Heat transfer goes hand in hand with the Thermodynamics laws.

I would analyze your system by applying the so-called "Lumped analysis." Such assumption can be used for legal studies to determine how long a person is dead, by measuring the temperature. Some criteria apply for this assumption, but it is not really relevant right now.

So, in heat transfer for a lumped system, the following relation applies to the chicken:

$$\frac{T_{final}-T_{water}}{T_{chicken}-T_{water}}=e^{-bt} $$

where

• $T_{final}$ is the final temperature of the chicken or the "desired" temperature.
• $T_{water}$ is the temperature of the water in the sink
• $T_{chicken}$ is the initial temperature of the frozen chicken

$$ b = \frac{hA_s}{\rho C_p V}$$

where:

• $h$ is the heat transfer coefficient which depends on the medium and skin temperature, which in your case should have close values if the temperature of the water is not that high.
• A_s is the surface area of the chicken, which is constant in both cases
• $\rho$ is the density of the chicken, which can also be assumed constant, although it would change a bit when it loses water.
• $C_p$ is the specific heat of the chicken, which can also be assumed constant.
• $V$ is the volume of the chicken, which can also be considered constant.

So putting the things together, one would end up with this relation for defrosting time:

$$t = \frac{1}{b}ln\left[\frac{T_{final}-T_{water}}{T_{chicken}-T_{water}}\right]$$

This formula allows you to compare different times of defrosting and therefore compare different scenarios.

Of course, this is only for answering your questions about relating Thermodynamics (Heat Transfer) to time.

In reality, things slightly more complicated and to apply the lumped equations, and your system has to follow certain conditions. If you think this is too complicated, you can also take the simple thermodynamic analysis of energy balance inside the chicken - the traditional $mC_p\frac{dT}{dt}$ - to see that a lower temperature difference gives a smaller flux, therefore a slower time. This last equations lays at the base of the whole theory.

If you are interested to find more about the derivation of these equations, assumptions and other particular information about such "lumped systems" I recommend that you read this.

Answer 4

This falls under the study of heat transfer, which is a subset of thermodynamics.

Conceptually, heat "flows" from hot areas to cool areas. From a classical, microscopic view, the hot item is comprised of rapidly vibrating atoms/molecules and these vibrations hit neighboring molecules and transfer this energy to their neighbors. Thus some energy is lost from the hot item, and some energy is gained in the cool item; a system always seeks equilibrium. In this case, the water or air imparts heat into the frozen chicken. The water/air cools, while the chicken warms.

For the analytical component, the critical concept you have to consider is, "how is heat flowing into and out of your system through an interface?" In this case we want to look at the chicken-fluid (air is also a fluid) interface. The bag is of negligible thickness, and I doubt it's surface qualities affect the rate of heat transfer appreciably, so let's ignore it from our analysis.

The chicken-air interface (leaving on the counter or in the fridge) will predominately transfer heat via natural convection. Air has a low heat capacity and a low thermal conductivity. In many day-to-day cases air can be considered an insulator because of these properties. This makes this a fairly inefficient process.

The chicken-water interface (chicken in sink full of water) will predominately transfer heat via conduction. Water is an excellent conductor of heat, and has a high specific heat, meaning it can absorb or release large quantities of heat relative to the temperature change.

As @Physther stated in their answer, heat transfer allows us to select the most appropriate model to predict the temperature of the chicken or fluid with respect to time and/or position. As they stated, lumped analysis would be correct for the chicken-water model, and natural convection would be appropriate for the chicken-air model.