Why do tomatoes get so hot?

Question

Ever noticed how certain foods seem to get a lot hotter than others? I almost never burn my tongue or mouth... except on tomatoes. Pizza sauce, tomatoes in panini sandwiches, spaghetti sauce. Tomatoes always seem to get hotter and retain their heat longer than almost any other food I've encountered. And they are nearly always the culprit when I succeed in burning my mouth. Why would that be? Is there something about their chemistry that causes them to have a higher heat capacity? Do they hold their heat longer? Or is it simply a figment of my imagination and bad luck with hot tomatoes?

Asked by Daniel Bingham

Answer

Another physics digression.

All cooked food gets hot, and everything in any given dish will have the same temperature {*}. The tomatoes don't get hotter than the other ingredients. But they do have a tendency to burn more than certain other substances, so the question is "Why?".

You get burned when a portion of your flesh reaches a high enough temperature{+}. The food warms your tongue, lips, etc. by heat conduction until either you move the food or your mouth parts and the food reach the same temperature (a condition known as thermal equilibrium). What that common temperature is depends on the amount of heat (i.e. thermal energy) in the system. Some of the factors that come into play are:

• How much (mass of) food there is.
• How much (mass of) your mouth is involved (see below).
• The initial temperature of the food.
• The "heat capacity" of both the food and your mouth parts, which is a property of each substance that appears as a coefficient in the thermal equilibrium equation. (Don't worry, I'm not going to make you read any math.) Water has a (very!) high heat capacity, so watery foods tend to drive high final temperature and thus to burn you more easily. There is an added complication for the extra heat needed to establish a phase change (i.e. melt solids or vaporize liquids) called the heat of fusion or heat of vaporization. Again water has a high value for both of these numbers.

How fast the common temperature is reached depends on

• The area of contact between the food and the mouth.
• Another coefficient called the thermal conductivity. This one is complicated, but liquids tend to have a high thermal conductivity and solids less so. This is where soups, sauces, and melted cheese really get you. Note that your mouth parts has a pretty low thermal conductivity, so you only get to count the surface layers in finding the equilibrium temperature. Sorry.

Some consequences of all this:

• This is why you can peel the aluminum foil off of a pan that has just come out of a 400 degree (F) oven without trouble, but if you get your hand stuck in the steam plume (which is only around 212 degrees F) you get scalded: Aluminum has a low heat capacity, and steam has a (very, very!) high one.
• Small bites help in two ways: less total heat means a lower common temperature, and may allow you to move the food around in your mouth, reducing the temperature of any one part.
• Some foods are just dangerous this way. You know what they are from experience: steam, hot soups and sauces, melted cheese, etc.

{*} Well, sort of. But take that as true for any particular region of any particular dish.
{+} What temperature is that? Good question. Maybe there is a medical professional around, 'cause I don't know. I'd guess around 140--150 degrees F (call it 60--65 degrees C), but don't quote me.

Answered by dmckee

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