Re: [Phys-L] physics and potatoes

[John Denker <jsd@av8n.com>]

On 01/12/2015 06:18 PM, Anthony Lapinski wrote:
> I saw a question in Hewitt about why potatoes are wrapped in foil for
> baking.

Rule #1:  Whenever Hewitt or anybody else asks "why",
assume it is the wrong question.

In this case, as is very often the case, start by asking
"whether".  I haven't seen a foil-wrapped potato in so
long that I literally cannot recall where or when I saw
one.  If I wanted to find one in the wild I don't know 
where I would look.

It is difficult to think of any scenario where it makes 
sense.

It would "almost" make sense if the goal were to keep
the skin from drying out ... but there are far better
ways to accomplish that goal:
  a) Start with smaller potatoes.  This guarantees that
   they will cook before they dry out.  At some point
   if you want a crispy skin you will need to put them
   under the broiler for a short time.

  b) If for some reason you want to bake a humongous
   potato, turn the oven on to preheat and stick the
   spud in the microwave.  When it is about 75% done,
   move it into the oven.  Adjust the percentage to
   achieve the desired amount of crispiness.  This 
   approach makes it cook faster ... in contrast to 
   foil which would make it cook slower and would 
   generally be a waste of effort and resources.

  c) A little bit of oil on the skin helps produce
   a nice crispy-but-not-dry skin.

If the potatoes come out early and you want to keep
them from cooling off and/or drying out, put them in
a covered dish.  Or just cover them with a towel.
This solves any problem the foil could possibly have 
solved, with less effort and less waste.  Potatoes
are proverbially known for /not/ cooling off very 
quickly.

The only exception is if you are cooking in a campfire.
Wrap the potatoes in foil, maybe two layers, and bury
them in the coals.  The foil will make them cook more
slowly and more evenly.  Without the foil, the skin is
likely to scorch, but the rest of the potato will be OK.

=================

> 1. The foil will trap the steam inside, making the potato cook faster.

Nonsense.  Foil provides thermal insulation.  It makes 
them cook slower.  I haven't done the experiment, but 
I'd be willing to bet money on the outcome.

> 2. The foil is a good conductor, making the potato cook faster.

Nonsense.

> 3. The foil should be wrapped tightly around the potato to minimize air
> inside, which is a poor conductor.

Twisted logic.  *IF* you were going to wrap a potato,
I guess tighter would be better than looser ... but this
is sloppy reasoning insofar as it fails to consider simpler
and better alternatives, such as no foil at all.

> 4. Put the shinier side of the foil toward the potato. This will reflect
> more heat back to the potato, making it cook faster.

Nonsense.  Does anybody really think IR radiation cares
whether the foil is shiny or not?  We're not trying to
form an image.  Liouville's theorem doesn't care either
way.  There is a difference between black and white, but
not a difference between white and shiny.  The non-shiny
side of the foil is not black, not anywhere close to black.

> 5. The foil does nothing during baking, only to create less mess.

Huh?  Potatoes are one of the least messy things you
could possibly put in the oven.  And there are simpler
and better alternatives.

> 6. The foil does nothing during baking. But after baking, wrapping a hot
> potato in foil will keep it warmer because the foil traps the heat (less
> energy lost via radiation).

Not entirely crazy, just mostly crazy.  Sloppy reasoning 
insofar as it fails to consider simpler and better alternatives.