Calories

[Famous5]

How are the calorie values of foods calculated

[omg]

they aparently have a calories count machine

[Bangkok]

I recall the a calorie is the amount of energy required to raise one gram of water by one degree centigrade.  Therefore is provide a constant measure of energy.  Food provides energy, so it is measure in calories.  Quite how you set fire to a Mars Bar and decide how many grams of water it would heat is beyond me.

[booksworth]

But the ones you eat are really called killercalories, because Mars Bars are so bad for your health.

[New Forester]

Bangkok -- your definition is for a "true" or "small" calorie, with small initial letter.  The ones usually used for food are "large" Calories (with an initial cap) or kilocalories -- a thousand times as big.  Nowadays, of course, we are supposed to use joules instead -- "the work done when the point of application of 1 newton is displaced a distance of 1 meter in the direction of the force" (I don't understand that either), or "the energy expended in 1 second by a current of 1 amp flowing through a resistance of 1 ohm".  One (small) calorie = 4.184 J.

 

The energy released by respiration has been calculated for each type of food, such as sugars, polysaccharides, proteins, amino acids, fats and alcohol (theoretically it ought indeed to be the same as that released by burning the purified food).

 

You work out or measure how much of which types of food the food contains, then add them up.

 

Burning the whole thing wouldn't necessarily work with human food, because the roughage, sweeteners etc would contribute as well as the metabolisable constituents -- for example wood-shavings would count much the same as shredded wheat.  I doubt if a Mars bar has much roughage though...

[Gef]

Nonsense from newforester. The calorific values of foods (and Mars bars) are determined experimentally. The food is homogenised (blended) and burnt in a bomb calorimeter where the energy released is used to heat a known volume of water etc. etc.

[New Forester]

Nonsense...?  Oh dear.  But how does burning it take account of roughage then?

 

I don't happen to have a packet of Allbran handy, but does it really give the calories including the roughage?

[Gef]

Apologies New Forester I take your point. Perhaps the amount of roughage is relatively small though. By the way 1 joule is the amount of energy needed to raise the the temperature of 1 gram of water by 1 degree Celcius.