how are we able to calculate the calories in food?
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They literally burn the food and measure the output. Yes. They’ve set fire to so much food. Not everything everything NEEDS to be burned, some stuff you calculate by how much calories the ingredients have and how much you put in.
But yeah. They literally just set fire to food and that’s calories.
A calorimeter is used.
Take some base ingredient, like fat, and place it in a sealed chamber. Submerge it in water, and then burn the ingredient. We can than measure the temperature of the water and how much it changed while the ingredient was burning, and then using some math know how much energy was in that ingredient.
Do this to a variety of ingredients, and now we can determine for any given food the number of calories. Carbs and proteins provide 4 calories per gram, fat 9.
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Adding onto this, there’s a lot of debate over whether that is actually even useful or accurate information, from a nutrition standpoint. Calorie intake is likely more complex than a simple chemical reaction. Human digestive processes don’t work the same as bomb calorimeters.
Some articles:
https://www.livescience.com/62808-how-calories-are-calculated.html
https://www.scientificamerican.com/article/science-reveals-why-calorie-counts-are-all-wrong/
https://www.health.harvard.edu/staying-healthy/stop-counting-calories
And the podcast Maintenance Phase has a great episode on it as well.
It’s called a bomb calorimeter. They take a very well insulated bath of known mass of water at a known temperature – with very precise instruments.
Then they burn a known small quantity of food in a chamber completely submerged in the insulated water bath.
The heat has nowhere to go but out…through the water. So they measure the temperature rise of the water and calculate how much energy the food has released based on that.
For example one BTU (British Thermal Unit) will cause one pound of water to rise by one degree F. One calorie will cause one gram of water to rise by one degree C. (Food calories are called “calories”, but are actually kilo-calories.)
They do this with basic ingredients, flour, different fats, different proteins, etc. and then use this data to calculate the content of more complex foods – using the recipe….meaning: they don’t use a bomb calorimeter to test a bacon-lettuce-tomato sandwich. They already know the calorie content from the individual ingredients, so they add them in the correct proportion based on the recipe.
We dry it out, burn it, and measure the energy we collect from burning.
The device used to do this is called a calorimeter. A very simple way to make one is to simply burn the food under a known mass of water, and measure the temperature chance of the water.
1 calorie is the amount of energy it takes to change the temperature of 1 gram of water by 1°C
Calories used for nutrition are actually kilocalories, so keep that in mind in your calculations
[How they Actually Count the Calories](https://youtu.be/UuN5HXctmYk?si=NGcW-izuLgO9C0VP) — great Howtown video on it.
Despite the answers about bomb calorimeters… pretty much *no one is determining caloric content this way* in practice. That is the scientific way to see how much heat energy something can provide, but that’s not what we actually do with foods.
Studies establish a rough calorie-to-gram ratio for protein, fat and carbohydrates. This isn’t how much energy you can burn out of the food: it’s how much a human being can actually digest out of food. It’s not suuuuper accurate and we know that (it’s close, most of the time… it’s famously not-very-close with nuts, perhaps 20% off), but it’s easy. E.g. 9 calories per gram of fat. Are you using some new, exotic pressed oil? Is it pure oil? Ok: 9 grams of fat. They won’t check if it’s actually 8 or 10 for that exact oil. They’ll just call it 9 and move on.
So if you make a new food product, to determine the calories in it we just look at your ingredients. Every one of those ingredients have probably been analyzed before and is in the books already (the USDA maintains reference lists of this stuff). You’d just add them up in the proportion you’re using in your recipe. Even the nutrition info in those reference lists is usually calculated for that ingredient, not analyzed for it.
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