# Hot food = more energy for your body?

Gokul43201
Staff Emeritus
Gold Member
Other problems with this correlation have been discussed. I'll address this one little place where you do a calculation (and maybe say a wee bit more).

pivoxa15 said:
On food packets, they label the nutritional value of the food, the first one is usually energy. Obviously heated food have more energy than cold food. For example, consider heating a glass of milk (400ml) for a minute in a 1000W microwave oven. A total of 60kJ would have been transfered to the milk after it has been heated. The milk contains a total of 1100kJ before heating, now it contains 1160kJ. That is a small extra amount of energy but still energy which may be valuable in some circumstances. Imaging heating the milk for 10 minutes. That would be 600kJ of extra energy
Wrong. And here's why.

Milk boils at 100.2C.

It has a density of about 1030kg/m3 and a specific heat capacity of 0.93Cal/K-gm. The maximum amount of heat that 400ml of milk can absorb before it begins boiling is :

$$\Delta H = \rho V C \Delta T$$

That's about 150kJ. This is less than 15% of the calorific value of milk. And milk has a pretty low nutritional value per unit mass (since it's mostly water). If you were heating a typical meal to 100C, the heat content will remain below 2% of the nutritional value.

http://www.nationaldairycouncil.org/NationalDairyCouncil/Nutrition/Products/table14.pdf

Further, even if this extra heat content were usable (which it is not, in the manner described by you), all the work done in heating the food above body temperature would essentially be lost through thermalization in the esophagus.

A quick calculation for the thermal time constant for a swallowable piece of food gives me a value ranging between a few seconds to a few minutes. In three time constants the piece of food loses over 95% of its heat contant. Seeing as how the digestion process takes several hours, I would confidently hazard a guess that by the time any considerable digestion begins, virtually all the heat content gained by microwaving the food is lost.

**{this part of argument redacted for lack of rigor; but perhaps it might be worth noting that enzyme activity is highly temperature specific - the point of enzyme catalysis being to reduce the activation energy for dissociation down to thermal energies present at body temperature}

As for your analogy with fossil fuels and immense gravitational potentials, that's incorrect too. You can synthesize pretty much every fossil fuel drilled out of the earth in a chemistry lab that has no capability of generating enormous pressures. Even diamond has been synthesized at way lower temperatures and pressures than those reached by mined diamond. The high pressures and temperatures that facilitate the formation of fossil fuels and things like diamond are only relevant to overcoming an activation energy, and are not related to an enthalpy. There is no conservation law (or Hess' law) however, for activation energies. All the energy used to activate the reactant could be lost in making the product from the activated state.

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uart
Yes of course there is only a limited amount of thermal energy that you can sensibly add to food without making it too hot for ingestion. Water heats up at approx one degree C for each 4.2 Joules per mL added. But that does not change the fact that this thermal energy can be used by the body in substitution for energy that would otherwise need to be supplied from metabolic processes as outlined in my last reply.

Gokul43201
Staff Emeritus
Gold Member
uart said:
But that does not change the fact that this thermal energy can be used by the body in substitution for energy that would otherwise need to be supplied from metabolic processes as outlined in my last reply.
I agree that heating up to 98F is not wasted in this sense (but doesn't provide the nutrition in the sense that pivoxa describes). Besides, it no longer becomes a question of nutritional value (strictly speaking). For then, as described here, the nutritional value of a food goes up if you eat it in the summertime (just because the ambient temperature is higher, not necessarily even the food temperature). It's just a question of how you do the accounting.

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Reshma said:
Umm...I have read cooking certain food stuffs like carrots and tomatoes actually increases their vitamin content.

I think it just increases their absorbtion into the body. Because it doesn't matter what you injest, all that matters is what gets absorbed into your body. For example you take all the calcium you want, but without certain transport mechanisms that can be induced by vitamin D it is difficult to absorb.

DM
Reshma said:
Umm...I have read cooking certain food stuffs like carrots and tomatoes actually increases their vitamin content.
This is most certainly true.

@pivoxa15

Andrew Mason said:
Not a very scientific approach. Since humans don't work as well when drinking gasoline does that mean that gasoline contains less energy than milk?

Is it that there is more energy in fossil fuel or is it that the energy is more accessible?

AM

Interesting question.

Since fossil fuels are simple Hydrocarbons they are all carbon backbones with Hydrogen atoms attached. For example, the simplest fossil fuel is Methane (C1H4), which is one Carbon surrounded by four Hydrogens. The next simplest is Ethane (C2H6), then Propane (C3H8), Butane, Pentance, Hexane, Heptane, Octance, etc...

To give you more perspective on the word Fossil Fuels, it includes all of these:

- Natural gas (C1-C4)
- Straight-run gasoline (C5-C11)
- Kerosene (C11-C14)
- Gas oil (C14-C25)

With this in mind, to answer your question, live plants have their Hydrocarbon content locked into complex molecules that they use for biological function, so that the hydrocarbons are not found isolated in any part of the plant. For example, a live plant may have compounds in it that have the basic hydrocarbon backbone structure to them, but they are also attached to atoms like Phosphorus, Nitrogen, Oxygen, etc. Whereas, dead plants which have been buried under heavy pressure for millions of years, have two factors that contribute to their liberation of their hydrocarbons; namely Decay, and Heat/Pressure. Both of these factors combined, over sufficient time, help to break the atomic bonds of a plant's hydrocarbon compounds and break off any undesirable residues like the atoms mentioned above, so that a purer, more combustable hydrocarbon emerges.

As a side note, the best fuels have high octane numbers (100 = best, 0 = worst). Octane number is determined prinipally by how 'branced' a hydrocarbon is. For example, Heptane (C7-H16) is a straight line of carbons that looks like this C-C-C-C-C-C-C and has an awful octane number of 0. While, 2,2,4-Tirmethylpentane (C8-H18) has three branches and has a perfect octane number of 100. For this reason, unbranched hydrocarbons are 'catalytically cracked' into smaller pieces and then recombined into larger branched hydrocarbons that are more useful as fuel.

Hope this helps.