A physics question - conservation of energy

ArbitraryRandom

Apologies - I freely admit this is not a moneysaving topic therefore technically a violation of the rules... but I have been asked a question by an annoyingly curious child and I wonder if you can help me with the answer as it's making my brain hurt. 

Known facts: Food contains calories - which is a measure of energy. We measure those calories by taking a known volume of food and burning it in an oxygen rich environment - the amount of heat generated is the energy in the food.

The broad question is... when we are cooking food in the oven and it gets burnt/converted to carbon... what happens to the energy in the food (allowing that the conversion isn't complete/100% efficient)?

i.e. when the oven is at 200 degrees - is part of that heat coming from the food vs the gas or electricity?

and if a kg of bacon contains ~5000 calories (per google) - that's ~5kw - and it can be converted to carbon fairly completely in an hour (don't ask me how I know...) in a 2.5kw oven... so what's the energy input requirement for the output of burnt bacon given the bacon is itself an energy source? 

Edit: and my own question - does that mean the calorie calculations on packets are only 'accurate' in terms of raw rather than cooked food? 

QrizB

Correctly cooked, very little of your food is burned away. Most of the weight loss during cooking is (or should be) water.

Your bacon question will need a bit more thought, and some maths.

ArbitraryRandom

My unsatisfactory and aborted answer was stumped when we googled the amount of energy in a kilo of bacon - because I find it hard to believe that a total (or anywhere near) 7.5kw of energy was released over the course of an hour (the combined power of the stove and energy in the food) given I know how much heat my 4kw multi fuel stove can kick out in the same timeframe... but it was fairly well carbonated and even assuming the stove wasn't drawing the full 2.5kw, the energy released had to go somewhere. 

doodling

Hi,

OK, some maths, first thing to do is to convert units into sensible ones so we will measure energy in Joules.

1 calorie is 4184 Joules

5000 calories is therefore 20.92 Mega Joules.

(For reference, although we don't need it here, 1 kWh is 3600000J).

1W is the conversion of 1 Joule per second.

If we assume that the food is entirely converted to energy then if that conversion happened in 1 second then we would have e a power of 20.9MW, albeit for only a second.  If we are more realistic and consider that the conversion takes an hour (3600 seconds) then we would be looking at a power output of 5.8kW for that hour.

Unfortunately as others have noted most cooking doesn't release much of the energy in the food.  Cooking generally involves driving off water and some fairly minor changes to the chemical structure rather than full combustion.

In order to achieve full combustion you would need something more than a traditional oven.

Qyburn

It probably doesn't add anything much but one of my attempts to make ginger biscuits resulted in burning biscuits. Not just burned but actually burning with real flames Inside the oven. I didn't record energy consumption at the time though.

theoretica

So you put some bacon in the oven and first it warms up and the chemicals begin to change - cooking.  Then it begins to char - more chemical change.  But burnt bacon, or other food, is in large part carbon - and carbon is itself flammable (coal, charcoal etc) so the 'burnt' by cooking standards food is only slightly 'burnt' by physics standards.  Burning that carbon to form carbon dioxide gas will give out a lot of the total calories of burning the food.

Think about charcoal. Wood is partially burnt once in a very controlled way without much air in order to make charcoal - which is a great fuel when burnt the second time.  Burnt food seems similar - beginning to become charcoal.

debitcardmayhem

Not an answer per se, but the bacon contains 5000 calories, ergo let’s say you consume 125gm at one sitting what power do you set your stomach to to convert those 625 calories into fat, oh with tomato ketchup or brown sauce? Of course it will never be converted to carbon in the oven some of it will be given off in oxides, some will remain in sodium compounds etc. In my view cook the bacon properly and stick it in a thick sandwich with loads of butter and or sauce then you probably will end up with about 900 calories and a satisfying smile.

ArbitraryRandom

debitcardmayhem said:

Not an answer per se, but the bacon contains 5000 calories, ergo let’s say you consume 125gm at one sitting what power do you set your stomach to to convert those 625 calories into fat, oh with tomato ketchup or brown sauce? Of course it will never be converted to carbon in the oven some of it will be given off in oxides, some will remain in sodium compounds etc. In my view cook the bacon properly and stick it in a thick sandwich with loads of butter and or sauce then you probably will end up with about 900 calories and a satisfying smile.

We were actually trying to make candied bacon (thanks to an American tik tok!) so I reckon more than 900 all in if it had worked  

theoretica said:

Think about charcoal. Wood is partially burnt once in a very controlled way without much air in order to make charcoal - which is a great fuel when burnt the second time.  Burnt food seems similar - beginning to become charcoal.

But charcoal is the combustion of wood in a restricted oxygen environment - and it does generate huge amounts of heat (with charcoal having a significantly lower energy density than the original volume of wood)... so the question isn't IF energy is released as much as how much energy is released. 

Reed_Richards

ArbitraryRandom said:

 so the question isn't IF energy is released as much as how much energy is released. 

If you know the calorific value of your bacon, "cook" it until it becomes "charcoal", or the equivalent, then weigh it, you should be able look up roughly how much chemical energy remains by assuming what you have left is carbon or charcoal: https://www.engineeringtoolbox.com/fuels-higher-calorific-values-d_169.html

The rest of the energy will have been lost as extra heat during the cooking process,    

debitcardmayhem

ArbitraryRandom said:

We were actually trying to make candied bacon (thanks to an American tik tok!) so I reckon more than 900 all in if it had worked  

!!!!!! is that? Never mind, try cooking the bacon in the microwave until it’s almost crisp i.e. most of water etc is gone. Then melt butter and sugar/honey/golden syrup or god forbid maple syrup(too expensive) in a heavy pan chuck in your bacon for a while , cool and voila ….. nothing like what you want but just a guesstimate, based on candied walnuts…yum
Edit dont do this during a savings session… there it*s now a mse thread