Is the heat used by cooking something actually used up?

Factual Questions
1

Forgive me, I don’t have the knowledge to probably express what I’m wondering about correctly.

Consider an old fashioned wood kitchen stove. There’s an area where you put the wood you burn (I think this might be called the firebox) and an at least partially separated empty box where you put food you want to bake/roast, the ‘oven.’ In addition the top generally has areas you can open to set pots on to heat up, and maybe a flat surface that would act with a griddle.

So you load it with a given weight of wood, call it X pounds, and set it afire. The burning of the wood makes the air hot and the metal of the stove gets hot, too. And eventually all the hot air gets out and mixes with the general air in the room, and the heat in the metal gradually cools off after the fire goes out, also shedding that heat to the room. When everything has settled down, you’ve added, let’s say, Y units (BTUs?) of heat to your room. Right?

Now, suppose you had built the exact same fire, same amount of wood, in the stove, but this time at some point you put a pan of cake batter into the oven. So the air and metal get heated, just as before, but some of that heat gets absorbed by the cake batter. In the cake batter, chemical type stuff happens – things break down or get changed somehow – as well as physical things like the materials actually getting hot and water turning to steam and so on.

Again, eventually the wood finishes burning, and the cake gets done and is taken out and set aside to cool down. and the metal of the stove and its hot air dissipates as before.

My question is, do you still end up adding exactly Y units of heat to the room? The heat from the cake and its pan goes into the room’s air, and the steam from the cake went into the air, too. But was some of the energy actually ‘permanently’ captured by the chemical changes in the cake? IOW, you end up with only Z units of heat added to the room, with X-Z units of the heat somehow ‘stored’ in the finished cake, and maybe only recovered when you actually digest that cake later on?

Which would mean, I guess, that the cake batter started out with a certain number of calories, and baking it caused it to contain more calories?

OTOH, if Y and Z are the same, wouldn’t that mean you magically created those extra calories worth of energy for ‘free’?

2

Some of the energy can, in fact, go into chemical energy. Of course, sometimes cooking will also release some of the chemical energy that was already in the food, and I can’t say offhand which effect would be larger for any given dish.

In any event, I’d expect the energy gained or lost in chemical energy would be negligible compared to the amount of heat.

3

Also, phase change (boiling) can use up a lot of energy.

4

This is ambiguous. In normal speech, the Calories contained in food are representative of the energy released when the food is processed in the body, nine Calories per gram for fats, four for proteins and carbohydrates. In chemical terms they are actually kilocalories, 1000 small calories. (Food Calories are sometimes written with a capital C, to distinguish them from small calories.)

Baking may produce changes in the digestibility or bio-availability of foods, but normally you can use the amounts in the ingredients to calculate the caloric output of a food. The Maillard reaction, e.g., is responsible for the browning of the top of cakes, created from amino acids and sugars.

These Calories are different from the amount of heat you would get by setting the food on fire, charring the cake batter until it was completely gone. That’s called pyrolysis. It’s a different chemical process, not involving amino acids.

I suppose the added heat from the chemical changes would result in an infinitesimal longering (is that a word?) of the time the cake would need to return to room temperature. But no Calories or calories would change.

5

First of all, I’m pretty sure I do understand your question and it’s a perfectly sensible one (no pun intended!)

The answer is yes, your intuition is correct. The oven will not heat the room to exactly the same extent if there is food being cooked as if it were empty. Some of the heat will be used to create or break chemical bonds in the food and this energy will remain bound up in the food even when the food cools. Some energy will be used to turn liquid to vapour which may exit the kitchen in vapour form through the ventilation fan or the flue, carrying its latent heat of vapourisation with it.

The above is assuming typical endothermic cooking reactions. As Chronos points out, if you leave the food in the oven long enough, exothermic reactions will begin to dominate as the food starts to char, at which point you will end up getting more heat into your kitchen than you would have if the oven were empty.

6

Aha! I thought there had to be something like that, otherwise it’s just too much like magic, as in, you are accomplishing something (work) without using energy. Like all those magic users who can cause boulders to levitate by contorting their faces and stiffening their fingers a bit.

I’ve seen articles (casual, like in women’s consumer magazines) boasting about getting ‘double the work’ out of energy by getting some benefit as a side effect. Like, oh, pouring the hot water from a pan you boiled the potatoes in into your dish pan, using that heat to also help wash your dishes instead of ‘fresh’ hot water from the tap. Or that vacuuming your carpet/running your fridge will also help warm the room. But in those cases, aren’t you just diverting/appreciating the heat that would otherwise go down the drain out of your house or simply not being noticed?

Similarly, turning on your car’s heater. You are not actually gaining additional heat from the gas you burn, you are just diverting some of it from where it would normally go (outside your car’s mechanism somehow?) by instead using some of the ‘waste’ heat to warm up the air that gets blown inside the passenger cabin. And thus turning on your heater doesn’t affect your mileage, but running the AC does, yes?

7

Worth saying that a lot of cooking doesn’t involve chemical reactions that change covalent bonds. Which is the sort of reactions we usually think about as chemistry.

A lot of cooking occurs when we heat stuff up enough that the proteins in food start to denature- and often start to coagulate and ravel themselves up. The best example is boiling an egg. But cooking meat (excluding any browning reactions) is much the same. These changes are probably very slightly exothermic. If only from an entropy argument.

Baking bread is mostly about getting the gluten to form long stable structures. So again mostly about proteins rearranging themselves. That and cooking off the unwanted water.

A boiled egg is amusing in that, in principle, it is possible to untangle the proteins, and in doing so, uncook it. Restoring order would tend to demand that net energy is input.

Browning reactions, aka Maillard reactions do create new chemicals. Which is how cooking injects flavour into many foods. I’ll bet most are exothermic.

8

In principle, no energy is ever created or destroyed. All energy comes from somewhere and goes somewhere and can be accounted for.

The energy ledger always balances out even if accounting for it all is difficult to impossible.

9

I doubt this is the case. I think the cooking process requires heat, not to fuel endothermic reactions, but to increase the temperature of the food so that the exothermic reactions can occur. The same way “cooking” a piece of wood in a fire doesn’t cause it to gain chemical energy, it gets hot enough to release energy.

Most everything used in food is created by life to store energy or use energy in a productive way, it’s unlikely that these structures are poised to gain energy by adding heat.

10

Sort of, but turning the heater on also turns the fan on, which takes energy from the battery which has to be replaced by an alternator that uses power from the engine.

The refrigerant has to be pumped around in the AC as it switches between liquid and gas. This also takes energy - and of course, the fan is running as well.

I have seen calculations that say lowering the windows is more economical up to some speed. Of course that doesn’t take the benefit of breathing filtered air rather than fumes.

11

I’ve certainly seen such calculations back in the day when they were talking about new cars built in the 1970s.

I’ve not seen any such calcs recently and I’d be willing to bet that the much greater attention paid to aerodynamic drag on modern (e.g. post-2010?) designs pretty well ensures the drag increment from open windows far exceeds the relatively small incremental load of providing electricity to the HVAC fan motor and the larger load of the HVAC compressor being clutched in and turning.

12

It seems AC on is the least fuel efficient. Windows down, AC off is more fuel efficient and, as you would expect, windows up and AC off is the most fuel efficient.

However, in general, studies have shown that having your windows down conserves more fuel than running the AC. Even the Discovery Channel’s MythBusters have tackled the debate. Their experiment revealed that, in their conditions, keeping the windows rolled down was also the more fuel-efficient choice.

A 2004 report from SAE and General Motors reached the same conclusions as the MythBusters. In their test conditions, they found that running AC was less fuel efficient than having the windows down. The test studied an SUV and a sedan at low, medium and high speeds. Not surprisingly, both cars were the most fuel-efficient with the AC off and windows rolled up. When the windows were rolled down, fuel economy dropped, especially for the sedan, due to the increase in drag mentioned above. For both cars, fuel efficiency was at its worse with the AC on, windows up. - SOURCE

13

The OP specified a cake in his example. So I had in mind endothermic processes such as the caramelisation of simple sugars, cooking egg proteins, browning (Maillard) reactions.

14

Waste heat is a real thing, but you may be able to use some of it, especially if it is hot enough.

15

Ultimately, all energy humans use ends up as waste heat. But if you’re clever, you can sometimes get more use out of it before that happens.

16

Just throw in a +C (for constant) or say it had to be entropy.

I really hope you have a stovepipe to guide the CO2 and other gasses out of the house. Not good things to breathe.

17

Can you explain further? This is beyond me.

My understanding is energy can neither be created nor destroyed. But, in any process, some energy is always lost to entropy. Put another way, you can never get more out than you put in and, in fact, will always lose some energy in the process. It is not gone, just can’t be used anymore.

And yeah…if you play this out you get to the heat death of the universe eventually (no worries though…many, many billions of years in the future…or more).

18

It wouldn’t work in a theoretical formula. But if you’re applying your best theory to a practical situation, and the values are still off, you can put a constant (C) into the formula and check to see if different iterations of the situation are off by the same amount or the same relative amount (Cx) or both (Cx + D).

This could include real world irregularities in materials or measuring equipment or whatever. If an actual C or D is found to work, they can only be applied to that exact situation.

19

If you’re just doing that blindly, to make things work after the fact, that’s what’s called a “fudge factor”, and is a really bad idea.

If you have some idea of the nature of the thing that you’re representing with the unknown constant, and that idea of its nature suggests that it probably won’t be different for different cases, then it can be OK.

For instance, heat loss to the environment is going to be constant… if the environment is always the same temperature, and your system is always the same temperature, and there’s always the same kind of wall between the two. If you have all that, then you can account for heat loss by just saying “it’s a constant”, without having to worry about how much it is. But if you don’t have all of that (say, in two different cases, your system has different temperatures), then you can’t.

20

There is a specifically endothermic chemical reaction in cooking that Adam Ragusea on YouTube did once. He made sodium citrate from baking soda and lime juice, which gets quite cold. Granted that’s not something you would do on the stove or in the oven, but I suspect such acid/base reactions would cause a slight reduction in the total temperature increase for certain baking situations since they’re using input energy to create gas (CO2 bubbles in this case).

Regarding vehicles and A/C, windows up and A/C off is most efficient obviously, but if you need cooling, then windows down and A/C off is more efficient up to a certain driving speed. After that point, aerodynamic disruption overtakes the additional mechanical load on the engine. My recollection is that the cutoff point is somewhere around 40-50 mph, so if you’re driving on the highway, windows up and A/C on is more efficient.