Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

  1. Jun 27, 2012 #1
    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?
     
  2. jcsd
  3. Jun 27, 2012 #2

    Evo

    User Avatar

    Staff: Mentor

    Here is the current formula for food calories.

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

    And more information about energy use.

    http://en.wikipedia.org/wiki/Food_energy
     
  4. Jun 28, 2012 #3

    jim mcnamara

    User Avatar

    Staff: Mentor

    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
     
  5. Jun 28, 2012 #4

    Evo

    User Avatar

    Staff: Mentor

    Oh, much better links than mine!!
     
  6. Jun 28, 2012 #5

    Ygggdrasil

    User Avatar
    Science Advisor

    According to wikipedia (http://en.wikipedia.org/wiki/Food_energy), [Broken] food calorie measurements are based on heats of combustion (a measure of enthalpy), not Gibbs free energy.
     
    Last edited by a moderator: May 6, 2017
  7. Jun 29, 2012 #6
    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.
     
  8. Jul 2, 2012 #7
    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.
     
  9. Jul 6, 2012 #8
    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.
     
    Last edited: Jul 6, 2012
  10. Jul 6, 2012 #9

    phyzguy

    User Avatar
    Science Advisor

    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?
     
  11. Jul 6, 2012 #10
    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.

    You may find more information here:

    http://www.livestrong.com/article/481246-how-does-a-food-calorimeter-work/

    and here is a pdf of sucrose analysis:

    http://chemweb.calpoly.edu/jhagen/Bomb_Calorimetry.pdf
     
  12. Jul 6, 2012 #11
    Correct :approve:

    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.
     
    Last edited: Jul 6, 2012
  13. Jul 12, 2012 #12
    • The fact that food is not 100% digestible is a good point.
    • The fact that amino acids are also not completely digested adds to the complications (normally, amino acids in a bomb calorimeter are completely oxidized but in the body, they are not, rather they are oxidized incompletely to urea & creatinine, which still have energy).
    • The fact that each of the 20 amino acids has slightly different variables only makes matters worse (by a small factor, but hey, scientists sometimes want to know these factors).

    The values for carbohydrate (4), protein (4), and fat (9) are based on Wilbur Atwater's early studies that built on Max Rubner's bomb calorimeter experiments in the late 1800s. Rubner's early research only counted the energetic value of fully oxidized (combusted) hydrocarbons (of which fat, protein, and carbohydrates are more or less a subset) so that his values were larger than 4,4,9. Though Rubner figured out that protein in the gut is incompletely oxidized (as compared to complete oxidation in a bomb calorimeter), he never went as far as account for fecal losses. Atwater, on the other hand realized that fecal losses were an important variable and must vary according to the food, so he did what a true scientist would do...
    he stuck the caca in a bomb calorimeter to determine its caloric content :yuck:​
    He did this for all foods consumed so he could subtract fecal losses from food intakes to provide better estimates of what he called "digestibility".

    Thanks to Atwater, we finally had an idea of how much energy we can get from certain foods. He showed that digestion is amazingly efficient, but not perfect. He showed that high fiber foods definitely reduce the availability of calories. He also refined the use of bomb calorimeters for nutrition studies (which up to then had been used for non-digestive studies and in some ways, not as accurate for digestive studies). His original values were 4, 4, 8.9, but in 1910, the USDA rounded them off to 4, 4, 9.

    He figured out much more than that, but unless you are a metabolism nerd, you probably don't want a long, soporific outline of how he influenced dietary thought. Suffice it to say that most questions you might have about calories, he probably addressed in a very deep way.... go read his works to get an idea.


    There are many more ways in which energy efficiency is lost, but most of this is of academic importance only... not of much use in daily life as the losses are small in comparison to the truckloads of food we consume. Nevertheless, if you are curious, I made a chart a while back that outlines some of the many ways in which it happens. (click here)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook