Question for Chemists

Okay.  I know there's a difference between food calories and chemistry calories.  I think that's the short answer to my question.   But I'd like to understand more about that difference, at least a little bit. 

If a calorie is the unit of heat that it takes to heat a unit of water by a degree, wouldn't a glass of milk that you have heated to 100 degrees have more calories than the same volume of milk that is at 50 degrees? 

If not, why not?  Doesn't warm liquid have more energy than cold?  If so, could you explain the difference between food calories and chemistry calories in some way that I can get it, finally? 

(In the meantime, I'll be eating ice cream, savoring its negative calories....) 

Comments

[pjm]

If I remember my high school chemistry, there are two differences here. First, food calories are some order of magnitude difference - like a food calorie is 1,000 chemistry calories, or something like that. It's really just a rounding thing. Second, while you've got the idea right - calories are a measure of energy - the thermal energy of food doesn't have much effect on the chemical energy your body extracts from it.

It's true you can burn a few extra food calories (like one or two) by drinking ice water, which requires your body to use heat energy to raise it to body temp, but you'd probably drown (or go hypothermic) before you drank enough to burn any significant number of calories.

[Damien]

I think the main difference is that the heated milk would have a higher kinetic energy than the cold milk.

But both milks would have the same potential energy, because no matter what temperature milk is heated at, milk will always have the same potential energy.

[ap]

Yep, pjm is right. The whole 1000 cal : 1 kcal concept is critical. Let's take the example Ben and Jerry's Coffee Heath Bar Crunch (one of my favorite flavors). According to the website, 1/2 cup has 290 kcal. If you assume that ice cream has a specific heat similar to water, raising 1/2 cup (118 mL) by 40 degrees C takes a little less than 5 kcal.

[Scheherazade]

I know about the order of magnitude unit difference. But is the answer that we don't pay attention to temperature differences because they're too small to be meaningful (as PJM suggests) or that food calories and heat calories measure different things (kinetic vs. potential energy) as Damien suggests? This is what I want to know.

[J]

It's not how warm the food is that determines it's caloric content, it's how much the food can warm a given quantity of water when burned.

[Dreadmouse]

It's been a long time since I took organic chem, so this may not be 100% correct. That said, the difference has to do with how the body gets useful energy from food.

It is absolutely true that heat is energy, but raw heat won't fuel a body. If it did we could power up by lounging in front of a sunlamp. The problem isn't so much getting energy as it is transforming that energy into something we can use to get ourselves moving.

A general rule in organic chemistry is that energy is generated when molecular bonds are broken and energy is consumed when molecular bonds are formed.

The body generates useful energy by breaking chemical bonds in food molecules, usually starches and sugars (which are very similar in molecular structure.) You may remember "mitochondria" from your high school biology days? These organelles, present in every cell, are the energy factories of our body that process the nutrients passed along by the bloodstream.

Mitochondria break the chemical bonds in sugars and convert the energy released into a form that the cell can use to fulfill its function i.e. contraction for muscle cells, transmission of electrical impulses for nerve cells, etc.

I could go on ad nauseum if you wish, but that's the basic concept.

(Minor interesting sidenote -- different foods have different caloric values for different species. Grass, for instance, has a caloric value of about 0 for humans. We can't digest it so it's of no value to us. For a cow, however, grass has a much higher caloric value.)

[Marcin Tustin]

A calorie is a unit of energy not of heat. It is the energy required to have the heating effect that it is (was) defined from. That could have been expressed in an equivalent way, as the Joule is, as the energy to move a unit mass over a unit distance.

So, to go to the glass of milk, the answer rather depends on what you mean for it to "have more calories". Ignoring the effect of cooling on your body requiring more metabolism to re-heat itself, which is negligible, hot milk will allow you to do no more work than cold milk. However, if you were to convert all of the mass of the milk into a burst of radiation, the hotter milk would be a slightly more energetic flash, although not by much because the extra energy from heating the milk is negligible compared to the energy of the mass of the (cold) milk, because of the correspondance e = mc^2.

Is that clear?

[David]

I'm going to sum this all up.

You are not exactly right about the amount of heat because 50 deg is not half as hot as 100 degrees. That's because absolute zero is -273, so it has 373/323 as many calories. I'm talking celsius, but the idea is the same for fahrenheit.

If you could drink that milk, then you would have to find a way to 'expend' that heat until it got down to your body temperature. That would be easy because it would be the same mechanism as if you left it on the counter, it would radiate that heat. Think hot chocolate in the winter. It helps keep you warm.

OK, as for the calories 'in' the milk. Those don't refer to the temperature related heat in the milk, they refer to the calories that someone reckons would be released when the molecules are chemically broken down by your body until they can't be broken down any more.

As much as I like Marcin's answer, we don't really reckon THAT kind of energy in calories (although we do build big ol' calorimeters, see www-d0.fnal.gov). Last checked, we aren't breaking things down to quarks and gluons, yada yada yada.

[Monjo]

As a final note, 1 food Calorie is 1000 calories. Please note that when dealing with food you should use a capital C.
To confuse the issue some food manufacturers may refer to kilocalories (1000 calories = 1 kilocalorie = 1 Calorie)

Just to add some confusion to the subject, purely for my own enjoyment. I will assume by 50 & 100 degrees we're talking Fahrenheit. In this case milk at 100F *could* be argued to have more useful energy for a human than milk at 50F. The reason for that is the human body operates optimally at around 100F (just under), so our body doesnt need to consume energy to warm the milk up to body temperature.

[turboglacier]

It may help to know that "Calories" in food are actually determined by incinerating the food item in a highly insulated vessel (bomb calorimeter) and measuring the heat released-- that's how much chemical energy was stored in the food. In practice, what your body can extract is a bit less, so what's reported on food labels is reduced by a standard factor (about 10%, depending on the type of food).

If we could turn heat back into chemical energy, then hot milk would have much more useful energy than cold. Perhaps we wouldn't have to eat really hot weather! Unfortunately, we can't go that direction.

If your body is actually struggling to stay warm, of course, a direct shot of heat in any form will help save energy through direct transfer. But staying warm alone isn't enough to stay alive-- you need energy for a lot of different functions.

[ilona]

ok. You are all geeks and I am math challenged and so cannot deal with the equations, etc, you are setting forth.

However.

I believe you are going about the question the wrong way. The units of calories have more to do with the fact that your intake of food is used as fuel. You burn certain amounts in the action of living- but the calorie values have nothing to do with temperature, but rather with the fact that it takes certain amounts of action to burn the food components.

So even if milk is hot it still has the same amount of lactose,protein, and fat... and that is what your body uses to burn. So there wouldn't be a difference in the calorie levels of the hot or the cold milk.

At least this is how it appears to me.

[Heidi]

You're right that temperature differences in calorie count are too small to matter. Drinking hot liquids does mean, to some extent, that you need to expend less energy to heat up your body, and conversely, that you need to expend more energy to heat up cold foods. But this energy expenditure is really negligible: if you eat a 3 ounce scoop of ice cream, you'll spend around 2 to 3 food calories to raise it up to body temperature. Compared to the ~200 calories you'll get from the ice cream, you can neglect it.

There's a funny article somewhere that describes what would happen if you ate (could eat) steaks frozen to absolute zero. The answer? You'd use a little more energy heating up the steak, but that's about it.

one year late: ok, none of this really answered my question.

[peter jean eryan]

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