What Is a Calorie? The Science Every Food Business Owner Needs to Understand
How Calories Are Measured
A kilocalorie (what we call a "calorie" in everyday speech) is the energy required to raise 1kg of water by 1°C. Food calories were originally measured using bomb calorimetry: a sample is burned in a sealed chamber surrounded by water, and the temperature rise determines energy content.
Modern nutrition databases use the Atwater system — which is simpler but slightly less accurate for individual foods. It assigns fixed energy factors:
- Protein: 4 kcal/g
- Carbohydrates: 4 kcal/g
- Fat: 9 kcal/g
- Alcohol: 7 kcal/g
- Fiber: ~2 kcal/g (fermentation by gut bacteria)
Why Cooking Changes Caloric Availability
Raw eggs have approximately 10% lower digestible energy than cooked eggs, because heat denatures the proteins (particularly avidin, which blocks biotin) and makes them easier to digest. Raw potatoes are nearly indigestible in their starch form — cooking gelatinizes the starch and makes 90%+ of the energy bioavailable.
This is why calorie counts on nutrition labels are based on cooked or processed food, not raw ingredients where applicable.
The 10% Variance Rule
The FDA permits a ±20% margin of error for calorie declarations on nutrition labels (and ±10% for most nutrients). This tolerates natural variation in food composition. It does not mean food companies can deliberately underreport.
Calories in the Context of Labels
For nutrition labeling purposes, calculate calories using the Atwater system from your macro totals: (protein × 4) + (net carbs × 4) + (fat × 9). RecipeCalc does this automatically — and rounds to the nearest 5 calories as required by FDA guidelines for most calorie ranges.
