h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

All this talk about the lid increasing the pressure and thus the boiling temperature is true, but negligible for the average kitchen pot. Just think of the weight of a lid, surely less than a pound. Even a small pot will have an area over 10 square inches, so the pressure increase must be less than a 1/10 of a psi, or about 7 mbar. That's easily within the normal change in barometric pressure, and only corresponds to a change in boiling temperature of 0.32ºF / 0.18ºC. Significant?

lappydappydandy you are correct. And you are sadder than me for working it out. Ta!

Mu mentioned that the temperature would drop when it would actually increase (minutely of course). Pedantic but correct...which is what the SEX forum is all about!

Is it me or is this more complicated than I thought?

Pan of water and food with lid off boils at 100DegC. Much heat lost to the atmosphere due to evaporation from entire surface area of water-->loss of boiling liquid and risk of boiling dry. Also more energy input.

Pan of water and food with lid on boils at 100DegC. Less heat lost and less water lost because the lid keeps heat and steam in the pan. Therefore less risk of boiling dry and lower energy input. Therefore more energy efficient

Sealed pan of water and food boils at a higher temperature due to increase in pressure inside. A pressure cooker cooks food more quickly, uses very little water (it steams food) and preserves flavour and nutrients. It is the most energy efficient of the three systems.

turvy

Hmmmmm.


What about the temperature of the pan base?

If that becomes too hot may it not start decomposing some of those tasty organic molecules and taint the food?

(If anyone thinks the pan base is at the temperature of the liquid please have a quick scan of http://www.wlv.com/products/databook/ch5_1.pdf )

So perhaps simmering with the lid on (basically just a bit of insulation) will allow you to keep the liquid at boiling point at minimum heat input and hence minimum necessary pan temperature ?

Aha! That could be it - faster boiling (lid off) may ensure there is a layer of gas on the surface of the pan base, preventing the food from touching it and burning?

Or perhaps it is the extra mechanical jiggling caused by the extra bubbles?

It clearly isn't pressure - 0.18ºC is not significant, and would in any case depend on the type of lid (my pans have little holes to release the pressure).

<< faster boiling (lid off) may ensure there is a layer of gas on the surface of the pan base, preventing the food from touching it and burning? >>

Actually Dave, it's fast boiling that causes the problem. If you turn up the heat sufficiently to have film boiling (or even strong nucleate boiling) the low conductivity of the vapour film can allow the surface of the pan base to rise many tens of degrees so any 'delicate' foods touching the base are carbonised (particularly moong dal as I know from bitter experience).

On a low heat, keeping the lid on greatly reduces heat loss by bulk convection and prevents the liquid temperature dropping below boiling point which would prevent the food cooking properly.

OK, but the original question was why do so many recipes instruct me to boil stuff wth the lid *off*? Given that the temperature would be the same regardless but leaving the lid off requires more energy, it seems counterintuitive.

Discounting escaping enzymes, I don't reckon anyone's come up with a satisfactory explanation.

Could this thread be an example of science overturning received wisdom?

>>Discounting escaping enzymes<<

I hope you're discounting it through reasons of it being cobblers and not for it being a reasonable explanation.

All the 'open saucepan' cases I can think of off the top of my head are either to reduce a stock or heat gently with continuous stirring.

O yes, of course

http://everything2.com/index.pl?node_id=978200

Ah, blanching, yes. *That* I believe has rather hit the nail on the head

(Assuming people can link to that rather tediously slow to load link )

And you're not loading it from Nigeria, Orcus

That explains why you want a rolling boil before you put the food in, but not after. Surely the best strategy to ensure it returns to the boil quickly is to boil fast at first, put the food in and then put the lid on immediately.

Just to add abit fo confusion my dad (now deceased, so I can't ask for any clarification) was a vegetable cook in a restaurant in the 1930's. He always said cook with a lid if the veg was from under the ground and without a lid if it was above. To be honest, I use a lid for everything.

Sounds perfect scientific to me that one, Vestboy

Brussel sprouts and other Brassicaceae are above the ground but a lid is normally put when cooking those (at least for me).

The lid issue only applies to green vegetables. As the vegetables cook they give off acid. The acid is carried by the vapour and condenses on the lid, dropping back down into the water. There is a reaction with the pigment in the green vegetables that causes them to go drab. The effect is purely aestetic and doesn't affect the nutritional values significantly.

The reverse is true for vegetables that are higher in beta-carotene. They look better if the water is acidic.

If you want to cook green vegeables quickly with a lid it's a simple matter of altering the PH of the water with a bit of baking soda. I actually use this cheat quite frequently

Aha, that could be the answer! Thank you

Evidence people, evidence.

The only reason to keep the lid off a pan that I can see is if the intention is to reduce something like a stock or a gravy. This way the steam can escape and the water content of the liquid decreases.

As for Brussel Sprouts and cabbage, sprouts should be started in the first week of September at the latest to get them tender for 25 December. Cabbage should be Savoy (all others should be pickled or fed to rabbits!) and cooked in 5mm of boiling water for 2.5 minutes with the lid on and then drained and a knob of butter and a pinch of nutmeg added. Ditto spinach.

turvy

Graham Garden on I'm Sorry I haven't A Clue said, "Sprouts aren't cooked if you can still count them."