how do you work out calorie values?

[mollydolly]

I know most food has the calorie value printed on the packaging but I have only just realised that I dont have a clue how this is worked out!! answers simple or technical would be most welcome

[dougielogan]

It has something to do with how much energy needed to heat  1 litre water by 1 degree C. I think.

[Sammy Snake]

Yes, I see to remember in school setting fire to a peanut and using the flames from it to heat a beaker of water.

Something to do with how long it took to heat the water and to what temperature.

Not that this method is any use whatsoever in working out calories in food as you are wandering round a shop!

[5029]

We did something similar to sammy, with the peanut, but we measured how long it look before the flame went out of the peanut

[Sammy Snake]

5029 I think you might be right,  it is a looong time since I left school and the memory is a bit hazy!

[LoungeLizard]

Place an accurately weighed piece of the food in a special cylinder, fill the cylinder with compressed oxygen,  set fire to the food with an electric current, the cylinder has already been placed in a bath of water which rises in temperature by a carefully measured amount.  Heat loss is controlled by placing the bath of water in another larger bath of water which has a heater / thermostat that follows the temperature of the inner bath.  The calculation is: Heat rise x Bench Factor / Mass = Calorific Value

The above method is usually used to determine the calorific value of oils; and I have seen the method used to calculate the calorific value of foods.  The lady who did the food tests was an over-weight life-time dieter so personally very interested in the results.

As I recall, the calorific value of the complete oxidation of the food stuff in compressed oxygen gave a figure that seemed much greater than those quoted in tables in dieting books.  I'd be very interested to know whether this is true and why.  

[CT]

In my old physics text the calorie is defined as, 'the amount of heat energy needed to raise the temperature of one gram of water on degree Celsius'. The kilocalorie is the heat energy used to raise one kilogram, one degree C. The calorie value on food is the kilocalorie (kcal).

[gazzawazza]

Funnily enough CT, I recently looked up the definition of calorie in numerous dictionaries and they all gave two definitions as you describe, however they specified the difference in definitions as "small calorie" and "large calorie."

[Space]

Everybody gave you the physical definition, but not how it is worked out for food... So here it is:
4 kCalories per gram of carbohydrate 
4 kCalories per gram of protein
9 kCalories per gram of fat.
That's how it is worked out. No need to burn it or anything!!!