It may depend on the food, of course. A couple examples to get you working:
[ul]
[li]Celery is not digestible, so cooking changes the calorie content from (negative?) to (positive?).[/li][li]The protein in an egg is denatured when cooked. Does that change the calorie content? Does the process of protein folding release or store energy?[/li][li]When toasting bread, some of the carbon is oxidized, releasing energy. [/li][/ul]
I am particularly interested in the toast example. Does lightly browning toast “burn off” a couple calories?
I don’t think it changes much, but I’ve got another to add to your list which may, literally, burn off more: flambé . Surely some of the calories in the alcohol burn off when (some) of the alcohol burns, right?
Any heating process where volatile (short chain) fatty acids are evaporated off will reduce the calories. Anything burnt will reduce calories. Anything that causes oxidation reduces calories.
Denaturing protein has no significant effect on its calories, as measured in vitro. In vivo it gets less clear, the body may not be able to digest the material in the time it’s in the gut.
So frying onions will reduce the calories of the onions, but by a lot less than the calories added with the oil they’re fried in.
Lightly browning toast will burn off a few calories, replaced by several orders of magnitude from the butter/peanut butter/jelly/honey/vegemite etc spread on when serving.
Cooking food can change the nutritional value by a huge amount, depending on the food.
The most obvious example is potato. A human will starve to death if they eat raw potato, simply because potato starch is indigestible. There are other starches in in potatoes, and the gut bacteria can liberate some nutritional value from potato starch, so there is some value in eating raw potatoes, but the human body can’t process it fast enough to obtain the calories needed to survive. A raw potato diet will result in death.
Boil the potatoes for just a few minutes and you denature the starch, making it perfectly digestible. At that point a human can get fat eating just a few potatoes a day.
The same is true to a lesser extent of almost all starches. Cooked flour has about 1/3 more calories than raw flour, cooked carrots about 1/4 more. Onions I don’t know, but I suspect that cooked onions also have about 1/4 more calories than raw onions. So dry frying onions probably increases the calories, rather than reducing it.
Plant foods have the added problem that the cell walls are indigestible to humans. Cooking ruptures the cell wall, freeing the cell contents to the gut enzymes. As a result cooking almost any plant food will increase the nutritional value by a measurable amount. Even fruits such as apples will be more nutritious when cooked.
The story with meat is more complex. Some meats, such as most red meat and liver form almost any animal, contains more calories when cooked. The muscle of many fish contains fewer calories when cooked.
I would like to add that even if the nutritionally relevant (i.e., calorie-bearing) ingredients of food were totally unaffected by cooking, the calorie-per-gram-of food content would still change: Heating food evaporates some of the water in the food. This means that even if the total number of calories remains unchanged, the cooked food will contain more calories per gram than before. I’m not a nutrition expert, but I guess this accounts for most of the effect mentioned by Pasta.
(I thought pasta was making a joke – ie hotter food has more energy stored as heat. Drinking a glass of cold water can be said to have negative one or two calories for the same reason).
Richard Wrangham’s book, Catching Fire goes into some of this in a fair amount of detail. Here I will simply say that it is quite difficult for a human to get enough nutrition from raw food to even survive. Probably fruit is the principal exception. This suggests that we’ve been cooking for a long time and have lost adaptations to living on raw.
Yes, I was referring to the heat itself, not water loss or anything. If you eat cold food, your body spends calories warming it up. If you eat hot food, your body saves calories not having to generate as much heat. A significant fraction of your daily caloric expenditure goes toward preventing your body from reaching thermal equilibrium with your surroundings. So, if you eat 12 calories of heat(*), that’s 12 calories of chemical energy (from whatever source) you’ll be storing instead of burning.
It wasn’t a joke at all (though I admit it was packaged like one).
(*) ETA: that is, 12 calories of heat that bring the food above room temperature, since that’s the temperature of the “heat bath” your body is working against.
OK, so I misinterpreted Pasta’s statement as not referring to the heat energy, but as implying that a cooked steak has more digestible (to your body) calories per ounce or gram than a raw one. I don’t know if that is true or not, but if it is I think it would at least partially be attributable to the loss of water.
The thing that comes immediately to mind (for me) is fatty meat. The more thoroughly you cook, say, bacon, the more fat renders out. Fat, containing 9 calories per gram, can account for a lot of the calories in a slice of bacon. But if you cook it thoroughly and then drain it, you’ve disposed of a lot of the fat instead of eating it.
Not that this is easily answerable, but for some reason (frivolous curiosity) I would be delighted, just delighted to know the calorie content of a slice of white bread as a function of its toasted hue. Assuming of course (spherical cows and all that) that the toasting is uniform throughout the volume of the bread.
For example (anyone have a color wheel?):
slice of wonder bread: 100.0 calories.
toasted to a shade of almond: 99.5 calories
toasted to atomic tangerine: 98.0 calories
toasted to cinnamon sprinkle: 95.4 calories
toasted to beaver brown: 83.9 calories
toasted to aged tamarind: 75.2 calories
toasted to black fig: 49.4 calories
toasted to blood sausage: 19.5 calories
toasted to jet black: 0 calories
I have no justification for the above list, other than that jet black, assuming it is ash, is probably zero calories.
Well, that’s true, but you’ve still reduced the caloric content of the bacon itself. and you can justify adding the calories to the eggs by saying “well, if I didn’t fry them in bacon fat, I’d have fried them in butter, which is just as high in calories, and this way, I’m doing my part to ‘reduce, reuse, recycle’!”
