If I microwave a piece of bread until all the water is evaporated and then weigh what is left, is the caloric content estimated by finding the calories in the same weight of flour?
Conventionally, drying is only the first step. The second is burning it and seeing how much energy is given off.
But this isn't always the best way to determine the calories that your body gets from the food, as it doesn't deal with bio-availability - basically, can your body extract that energy from the food?
Diet foods often cellulose or other fiber added to them -- which can burn and have heat, but your body can't absorb. So for nutritional reasons, they're considered 0 Calorie.
For the case of unenriched bread, we basically have only a few ingredients ... water, flour, yeast, and maybe salt. Once we remove the water, the yeast and salt are lower percentages, so we can estimate (stress estimate), but we also need to know what type of flour was used.
- whole wheat flour : ~339 kCal / 100 grams
- white flour : ~364 kCal / 100 grams
(but this is likely for American whole wheat, which is white flour with bran mixed back in, not ground up whole wheat berries)
Of course, it's also worth mentioning that calorie counts on menus and food packaging in the US are only estimates. There are tables of calories per item, and they just add them up in the amounts used to get a number. (so all wheat bread is considered to have the same kCalories/gram, no matter how it was made) Some of those values might just be estimates based on the ratio of carbohydrates, fat, and protein in the ingredient.
But how something is cooked, and the particular person (their gut biome, how well they chew, etc.) can affect how much energy they can get from the food, so it's always going to be a really rough estimate
- If your ingredients are just flour, salt, yeast and water, you’ll be reasonably close, but not really exact.
- If you are dealing with a more complex recipe, added milk, eggs, sugar, fats, seeds... the values will be way less precise.
But: There’s always some deviation, even between different batches of flour, and all values you will find in books, tables, the Internet, will be a kind of average. You may assume that the differences even out over time and counting down to the last single calorie is except for very few special cases (where you would need a lab setup and scientific methods) less crucial than most people may assume.
Bread is not just flour, as already stated. It is flour that has undergone a variety of physical and chemical changes (even if we don't consider the other ingredient for simplicity's sake).
Those changes not only change the caloric content of the product (some may be exothermic, some are endothermic, meaning some lower and some raise the caloric content of the product).
It would be an interesting experiment to see the difference between the caloric content of a pound of flour as compared to a pound of bread created using that same flour.