calorie measurements

[RichD]

How do they measure the calorie content of food items?


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Rich

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Post by RichD
How do they measure the calorie content of food items?

The standard way of measuring the calorie content of food is to use bomb calorimetry. In this technique, the food is completely burned and you measure how much heat is produced. This is slightly deceiving because you can't necessarily use all of the calories that you would get by literally burning the food. If you ever study physiology, you will probably run into the concept of catabolizable energy at some point, which is the amount of energy in the food that your body really can use. If you're interested in these topics, you can check out section 5.11 of my physical chemistry textbook (http://www.cambridge.org/roussel).

Depending on the country of origin and legal standards, it's often permissible to calculate (rather than measure) the calorie content of foods from the composition and calorie counts of the ingredients, or sometimes just from the rough breakdown into carbohydrates, proteins and fats.

Marc Roussel

[Martin Brown]

Post by RichD
How do they measure the calorie content of food items?

Bomb calorimetry. Burn it in oxygen at constant volume which makes the
reaction go very fast. Then measure the temperature rise it causes in a
known volume of water (or other calibrated heatsink). eg

https://www.scimed.co.uk/product-category/bomb-calorimeters/

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Post by Martin Brown

Post by RichD
How do they measure the calorie content of food items?

Bomb calorimetry. Burn it in oxygen at constant volume which makes the
reaction go very fast.

A bomb calorimeter uses a high pressure of oxygen, not to make the reaction go fast, but to make sure the sample burns completely. This is extremely important because you can't calculate the heat of reaction very accurately otherwise. For analytes that don't burn well, top-notch calorimetry labs will collect the ash and subject it to further analysis so that they can correct for the presence of poorly (or non-)combustible material in the sample. For example, there is a necessary correction for samples that are high in calcium carbonate because the heat causes the carbonate to decompose into carbon dioxide and calcium oxide, which is an endothermic process. This process is clearly not relevant to someone eating a high-calcium food, so it has to be corrected for. (Along with obvious high-calcium foods like dairy products, this will be an issue for mechanically separated meat, which contains a lot of tiny bits of bone. Enjoy your cheap hot dogs and chicken nuggets.) Ash from such samples would therefore be analyzed for calcium content so that the appropriate correction could be made to the measured heat. Having said that, I'm not sure how many food labs would be that careful.

Marc Roussel