Keeping a Lid on it
Today's question comes from the kitchen of DaveBlackeye, who has been poring over his recipe books...
When boiling stuff, I usually put a lid on the pan to keep the heat in, which allows me to turn the gas down. But many recipes specifically tell you to cook uncovered. Why would this be? What possible difference could it make to the food?
The first stab at an answer was supplied by sigsfried, who pointed out that boiling without a lid 'reduces the liquid, increasing the flavour'. Well, so far, so accurate, but Dave went on to clarify that he was 'talking about boiling or simmering stuff, not reducing sauces etc'. So, where do we go next?
One common suggestion was that removing the lid would reduce the pressure in the saucepan, allowing the water inside to boil at a lower temperature. This seemed plausible enough, until lappydappydandy did some calculations.
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 0.1 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.
Moving slightly away from science for a moment, Researchers chimed in with some of the more-spurious reasons they had come across in the past. Fanny confessed that her comment was 'in no way scientific', but observed that...
...on various cooking programmes I have heard it said that certain green vegetables, eg peas, should always be boiled with the lid off to preserve their colour
Returning to his cookery book library, Dave noted:
I referred to my Leith's Cookery School book last night which said that if you leave the lid off, certain enzymes that would otherwise discolour the vegetables, are allowed to escape
As is often the case, such advice is often contradictory. SqMike recalled that...
I was taught to put lid on the pan to 'keep the goodness in'. This is based on the principle that many water-soluble vitamins escape in the steam. By having a lid, the steam is condensed and returns to the cooking liquid with its disolved vitamins.
So, you have to leave the lid on to keep the vitamins in, unless you want the enzymes to escape..? Dave wasn't impressed:
I think this sounds like a made-up excuse to justify an age-old practice; I see no reason why these enzymes should evaporate with the water. I guess the same applies to 'keeping the vitamins from escaping' - even if true, you'd be chucking the vitamins out with the water anyway
Tops and Bottoms
Valiantly bringing us back to the world of physics, Seth of Rabi started off on a tack that seemed to be getting somewhere...
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 food1?
...before it became clear that he was actually providing a reason for boiling with the lid on:
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?
So, with the reasons suggested so far being of dubious merit, or contrary to the advice to boil without a lid, Dave was starting to wonder, 'Could this thread be an example of science overturning received wisdom?' But then Seth had a flash of inspiration...
On a Roll
What Seth had found was a handy link to a site discussing the concept of a 'rolling boil' (ie, continuous, vigorous boiling that keeps the surface of the water disturbed):
While all boiling water is at the same temperature (100 degrees celsius, or 373.15 degrees kelvin, assuming one atmosphere of pressure), water at a rolling boil has more kinetic energy (much of which is being lost as liberated steam), meaning that it will continue to boil, or resume boiling faster, when something below its temperature is immersed in it. This is important for procedures that require brief, high-temperature exposure, such as blanching vegetables
Finally, a valid reason for boiling with the lid off. Dave could go back into his kitchen knowing that at least some of the advice in his cookery books had a decent scientific grounding.