## How is the caloric content of a food item measured?

I'm curious how calories are measured in foods.

I assume it is basically the amount of energy that could be extracted from that food item via complete combustion. Is this right?

Does it take into account the "activation" energy that would be needed to access the stored chemical energy?

 PhysOrg.com medical sciences news on PhysOrg.com >> Obese British man in court fight for surgery>> 2008 crisis spurred rise in suicides in Europe>> New food labels dished up to keep Europe healthy

Mentor
Blog Entries: 4
 Quote by pa5tabear I'm curious how calories are measured in foods. I assume it is basically the amount of energy that could be extracted from that food item via complete combustion. Is this right? Does it take into account the "activation" energy that would be needed to access the stored chemical energy?
Here is the current formula for food calories.

 The large calorie, kilogram calorie, dietary calorie, nutritionist's calorie or food calorie (symbol: Cal)[2] approximates the energy needed to increase the temperature of 1 kilogram of water by 1 °C
http://en.wikipedia.org/wiki/Calorie

http://en.wikipedia.org/wiki/Food_energy

 Recognitions: Gold Member Science Advisor Food calroies for are usually 'Gibbs Free Energy' measurements in the lab. This is the total number of calories available. Not the net calories you get after the cost of "processing your food". Start here to learn the basics: http://fnic.nal.usda.gov/food-composition The thermic effect of food is the amount of energy expended above basal respiration to digest and otherwise utilize food. http://en.wikipedia.org/wiki/Thermic_effect_of_food

Mentor
Blog Entries: 4

## How is the caloric content of a food item measured?

 Quote by jim mcnamara Food calroies for are usually 'Gibbs Free Energy' measurements in the lab. This is the total number of calories available. Not the net calories you get after the cost of "processing your food". Start here to learn the basics: http://fnic.nal.usda.gov/food-composition The thermic effect of food is the amount of energy expended above basal respiration to digest and otherwise utilize food. http://en.wikipedia.org/wiki/Thermic_effect_of_food
Oh, much better links than mine!!

Recognitions:
 Quote by jim mcnamara Food calroies for are usually 'Gibbs Free Energy' measurements in the lab. This is the total number of calories available. Not the net calories you get after the cost of "processing your food".
According to wikipedia (http://en.wikipedia.org/wiki/Food_energy), food calorie measurements are based on heats of combustion (a measure of enthalpy), not Gibbs free energy.

 Quote by pa5tabear I'm curious how calories are measured in foods.
Ever heard of a bomb calorimeter? Yeah, you burn it and measure the heat given off by accurately measuring the change in temperature of a well-insulated water bath surrounding the calorimeter.

 It's also possible that many foods are not actually measured, but are determined based on knowing the caloric content and proportions of the various ingredients - this would probably be even more likely for processed foods.

 Quote by mp3car It's also possible that many foods are not actually measured, but are determined based on knowing the caloric content and proportions of the various ingredients - this would probably be even more likely for processed foods.
I would imagine that processed foods are more likely to be measured using a calorimeter, to insure consistency and for quality control purposes.
Basically, all that is required is for the sample to be dried and pulverized, before being measured and placed in the calorimeter bomb.
If I recall correctly (it's been nearly 20 years since I ran a coal lab), the test length is approximately 11 minutes for an isoperibolic O2 calorimeter, so any major processed food company would find this to be reliable for in house quality control and cost effective for insuring a consistent product for consumer interests.

edited test time from 7 to 11 min.

 How does a bomb calorimeter take into account the digestibility of the food? For example cellulose in the food will burn just fine and produce energy in the bomb calorimeter, but our bodies cannot digest cellulose. Is a bomb calorimeter really used to measure food calories?

 Quote by phyzguy How does a bomb calorimeter take into account the digestibility of the food? For example cellulose in the food will burn just fine and produce energy in the bomb calorimeter, but our bodies cannot digest cellulose. Is a bomb calorimeter really used to measure food calories?
As to the question of cellulose adding to the potential energy values, I regretfully have no answer, as dietary analysis was not my forte, although assuming the cellulose content is minimal, so would be any energy value attributed to cellulose. It's also possible that the cellulose values are subtracted from the final analysis during calculations.
To answer your second question, yes, bomb calorimeters are most assuredly used to measure calorific values in foods.

http://www.livestrong.com/article/48...orimeter-work/

and here is a pdf of sucrose analysis:

http://chemweb.calpoly.edu/jhagen/Bomb_Calorimetry.pdf

 Quote by pa5tabear I'm curious how calories are measured in foods. I assume it is basically the amount of energy that could be extracted from that food item via complete combustion. Is this right?
Correct

 Does it take into account the "activation" energy that would be needed to access the stored chemical energy?
Yes, the electrical energy activating the bomb, the amount of fuse burned to ignite the sample, and titration of the residual liquid is done to detect any acids produced - are all measured and corrected, either by the calorimeter's own control panel computer for the more sophisticated units, or by the operator/technician manually calculating when using a "bare bones" bomb calorimeter.