Why does hot cheese burn your tongue more than pizza crust?

[sciontc03]

Consider a hot pizza from the oven. Why do you tend to burn your tongue on the cheese, but not the crust?

 

[berkeman]

Different heat conductances:

http://en.wikipedia.org/wiki/Heat_conduction

.

 

[sciontc03]

So does the cheese conduct more heat than the dough so it transfers more heat to your tongue. Or is it because the cheese was originally a solid and it melted so it absorbed more energy than the bread, so the cheese transfers that energy to your tongue?

 

[mgb_phys]

It doesn't matter that the cheese melted (it's not the same as a solid-liquid transition)
Cheese does have a higher heat capacity than crust, because of the water content, and so does transfer more energy for a given temperature change.
But the main difference is as berkemen said, the lower conductivity of crust.

 

[russ_watters]

It is both the heat capacity and the thermal conductivity, but probably more about heat capacity - remember, the crust is mostly air.

 

[marcusl]

Specific heat (c) is important. In particular, c of water dominates over that of the dry ingredients (wheat, milk fat), so the relative water content of the bread vs. cheese (approximately 20% vs. 85%) gives you an immediate clue to which will burn you. Also important, and not mentioned above, is the latent heat of fusion of the cheese.

We can look up the following values for specific heat:
c(bread, 36% water content) = 2.9 kJ/(kg-C)
This bread (the only value I could find) is very moist, 36% is a soggy dough. For a crispy pizza crust, let's assume 20% water content so c drops to 2.6.

c(whole milk) = 4.1 kJ/(kg-C)
Cheese has more fat than whole milk, but fat has a low density compared to that for water so the whole milk value will be quite close.

Latent heat of fusion of milk fat = 84 kJ/kg


Now we can calculate the heat energy Q given up to your mouth in cooling the pizza from, say, 90C to skin temperature 37C. Assume that all ingredients were evenly heated, the cheese transitions from molten to solid somewhere around 45C(?), and that a 50 g bite of pizza is divided evenly into 25g bread and cheese.

Q(bread) = 0.025*(90-37)*2.6 = 3.4 kJ
Q(cheese) = 0.025*[(90-37)*4.1 + 84] = 7.5 kJ

The cheese gives up twice the heat of the bread. Ouch!

 

[sciontc03]

Wow! What an explanation! That makes sense. Thank you.

 

[russ_watters]

Heh - I had no idea cheese even had a latent heat of fusion! I didn't think it was really a solid.

 

[marcusl]

Well, you can definitely chew a piece of cheese that is cooled below the melting point. That's the way we usually eat it!

 

[russ_watters]

Well, you can definitely chew a piece of cheese that is cooled below the melting point. That's the way we usually eat it!

The ability to chew something does not necessarily imply that it is actually frozen. In fact, this is an important characteristic in a lot of food: butter and other fats, for example, have no melting point and thus a small/difficult to define, if not nonexistent, latent heat of fusion.

I had assumed cheese was in the same category.

 

[marcusl]

Fats can be liquid or solid, and substances like butter have a mixture of both; furthermore, butter contains a range of solid fats that have different melting points. Thus butter begins to melt around room temperature but isn't fully liquified until around 40C. Butter (and cheeses) have a definite latent heat in going from solid to fully molten state.

Values can be found in
Fox and McSweeney, Dairy Chemistry and Biochemistry, 1998