Question of the Week

Can Cooking Cut Calories?

Sun, 15th Jun 2008

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Allan Wensky, Berkley, California. asked:

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?


John Fry, Food Scientist and Chemist:

A homemade pepperoni pizza.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 thatís left after youíve 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.  Itís 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 doesnít taste that great.


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What's a calorie?
A calorie is the amount of energy required to raise 1 gram water by 1oC.  In terms of dieting, we usually talk about Calories or kilocalories (which is 1000 calories), and the energy required to raise 1 kg of water by 1oC.  If you take in more energy than you expend, your body stores it in a convenient form for later use: fat.

How do scientists calculate food calories?
Food calories are calculated by putting food into a "calorimeter" and burning it.  The calorimeter measures the energy released as the food burns completely.  According to wikipedia, the human body is on the average 85% efficient in converting a food's energy to energy the body can use, so the calorie labels on food are only 85% of the energy read by the calorimeter. 

So what if I burn the food ahead of eating it?
If you burn the food to begin with, you've already released some of of its available energy.  Therefore, the amount of energy you can obtain by burning it the rest of the way is lower, and the amount of energy you can obtain by digestion is also according lower.  If you burn 1/2 the available energy out of it, you'd cut the calories by half.

Any type of cooking that results in a "burning" or more technically a combustion reaction in the food, should lower the calories present.  Boiling or steaming probably won't do much, since they're often not hot enough to burn the food.  I have seen microwaves burn food before, so they probably can...

However, I do recall hearing about research showing that burned food was potentially carcinogenic, so this is probably not the best dieting idea around.


This is the textbook answer that I learned in talking about calorimetry in physics classes.  But it raises another question or two.  Is the human body really 85% efficient for everything that's "food?"  For example, I know cellulose is indigestible by humans, and there's a fair bit of cellulose in high-fiber vegetables and grains.  Since cellulose can burn, but humans can't digest it, is this accounted for in labeling food?  Or is it a flat 85% on everything nutritionists test (which is what wikipedia seems to suggest). jpetruccelli, Wed, 11th Jun 2008

Intuitively, one would guess that if you turn part of your pizza into smoke then it must lose mass and that means energy? chris, Wed, 11th Jun 2008

If the Pizza was not burned and cooked on a base with a covering of oil..would this then in fact increase the calories as the oil would be absorbed into the dough !

I too have heard that burned food can be carcinogenic....which is a shame because I used to love burned toast with lots of butter and marmite !! neilep, Thu, 12th Jun 2008

No but the charcoal from burnt pizza or toast is good for your digestion that is what my Mum told me.

Gastroesophageal reflux, a failure of the valve between the stomach and lower esophagus, allows stomach acid to back up and makes us reach for antacids to quell the subsequent heartburn.

Ulcers linked to infection with the Helicobacter pylori bacteria; use of NSAIDS (non-steroidal anti-inflammatory drugs) or other diseases can eat away the lining of the gastrointestinal tract. 

Gastroenteritis, distress caused by a flu infection or food poisoning. 
Ulcerative colitis inflammation of the colon. 

rosalind dna, Thu, 12th Jun 2008

definitely because the carbohydrates and other compounds which provides us energy are broken down into carbon particles which are not only harmful but also carcinogenic. The calories are cut down to a lower level but the free radical particles are more dangerous to us than the energy. qazibasit, Fri, 13th Jun 2008

Cooking usually involves some addition of oils and fats, doesn't it? So during the cooking process, before you get to the burning stage, you'd probably be adding some energy to food. I think that the heating of certain foods may also increase its digestibility, its availability for utilisation by the body, and consequently the amount of energy it is able to provide the body with. But then for other foods, the reverse may be true, so the net effect would depend on what you're cooking, how you cook it, for how long etc.

I reckon if you're going to eat something as terribly appealing as charcoal pizza, you may as well eat something a little more nutrious... cardboard. SquarishTriangle, Sat, 14th Jun 2008

If I take a steak and grill it a lot of the fat melts and runs off. If I don't eat that fat I have saved a fair few calories.
I think there's a risk from eatinf chared food. Some of the compounds produced are the same as those in soot and tobacco smoke. At the least they are possible carcinogens.
Simply eating less pizza would be a better (and nicer) option. Bored chemist, Sat, 14th Jun 2008

Why not just EAT LESS of the nice stuff? lyner, Sat, 14th Jun 2008

Pizza isn't nice anyway! SquarishTriangle, Sun, 15th Jun 2008

Especially if it's of the "pavement" variety... chris, Sun, 15th Jun 2008

BTW - love the fact that the expert answering this foody question was called "john FRY"

C chris, Fri, 20th Jun 2008


You pour off some of the caleries in the water down the drain , of course it might be better to drink and not waste

Alan Alan McDougall, Wed, 25th Jun 2008

Cooking Pizza is always very easy but the right cooking gear and the cooking temperature is very important while doing it.

Mod edit - spammy links removed Sandra1980, Fri, 23rd Jan 2009

You like cooking Sandra1980? Chemistry4me, Fri, 23rd Jan 2009

Looks like you're trying to advertise a website! I like getting my pizzas from PizzaHut  How do you 'cook' your own pizza? Chemistry4me, Fri, 23rd Jan 2009

Spam pizza anyone? dentstudent, Fri, 23rd Jan 2009

OK Hows this for an easy to understand answer. Bear in mind that there are a whole lot of different foods that gain different properties when cooked Foods that are mainly carbs Potatoes, cereals, wont increase their calorie content by cooking, but will increase their digestibility for humans or makes available more carbohydtrate than the raw product, but you must remember that doesnt mean you make it more nutritious, heat processing diminishes the nutritional aspect of food, vitamin, enzymes etc. The higher the temperature that the food is cooked the less nutritious it will be, coupled with the fact that you will introduce undesirable properties and other compounds within the food which in some cases might be and in others definately are carcinogenic compounds such as Acrylamides. Proteins such as meat and fish if cooked dry will lose calories as they will lose fat and oil content pulses such as beans, lentils dont increase their calorie content when heat processed in water, and most of those need to be heat processed because of enzyme inhibitors that need to be disrupted. there are other ways to process pulses that dont require heat to make their macronutrients available to humans. fats, gram for gram the most calorific food 1g of fat = 9 cals, protein and carbs are 4cals per gram. Assuming that you catch all you cook when it comes to fats there are no calorific gains or losses with respect to heat processing. Heat processing food decreases its water, oil and fat content, water being the greatest loss which of course is calorie free, therefore by virtue of weight by weight i.e 100grams of raw chicken will have less calories than 100g of cooked chicken. Other methods of cooking such as in oil will increase the calorie content of food, but that should be obvious because you are adding another food element to it. I hope the above helps Dickie, Wed, 10th Mar 2010

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