Not particularly intuitive as a unit for food, right?
Food's calorific content used to be worked out by doing just this - burning it and seeing by how many degrees the temp of some water would rise.
Instead food labs and manufacturers use the Atwater system, developed by the father of American nutrition, Wilbur Olin Atwater
3.6g fat at 9kcal/g = 32.4kcal
0.8g protein at 4kcal/g = 3.2kcal
+ misc = 72kcal/biccie
There are around 72,000 calories in 1 of my biscuits. Not 72.
(1kcal can heat 1L of water by 1C, therefore each biscuit can heat 1L of water by 72C)