Question
Recently a friend of mine was telling me about a pizza that he burned to the point of becoming a charred husk ten times smaller than the original. At that point I realised that this pizza had now become a very low-calorie alternative to its former self. My question is whether all types of cooking result in loss of calories as well?
Answer
John Fry, Food Scientist and Chemist:
Burning the pizza will certainly reduce its energy content because some of the energy that you might otherwise have digested and turned into you goes up in flames and smoke. The black carbon that's left after you've burnt the pizza has got a lot less energy in it than the original.
Other cooking processes also cause loss of fat. Roasting a joint of meat is a good example. It's also common to rescue the fat and meat juices that drip from the joint for use in gravy or that great staple of my youth, bread and dripping.
Cooking can also directly increase the energy content of food by making it more digestible. Starch in particular is made more easy to digest by cooking it. Starch crops up as small tight granules in many cereals, vegetables and fruits. Humans have thought to find it easier to digest starch once these granules have been burst open and the starch released in a gelatinised form by cooking.
In short cooking can increase or reduce the energy content of a food depending what you do. If you want to eat pure calories then consuming less food is preferable to incinerating your pizza. Burnt food may have fewer calories but it also contains a lot of very toxic materials created by excessive heat and it doesn't taste that great.
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