Re: low power testing

[AJH <andrew.heggie@dtn.ntl.com>]

On Thu, 7 Mar 2002 17:46:33 -0800, "Dean Still" <dstill@epud.net>
wrote:

>
>In my limited experience, a pot when simmering, needs to be kept close to
>boiling temperatures for food to cook in normal time periods.

Tami hinted at something temperature related here. No one has really
defined cooking for me. For starters I see it as a means of:

1) sterilising pathogens, 70C for a number of minutes seems to achieve
this for common ones (salmonella, listeria and e.coli??). I have
previously posted differing temperatures for other less common
infectious agents.

2) hydrating the food such as starch

3) providing warmth

?4)? other more complex chemical reactions

> Hayboxes need
>to keep as close to boiling temperatures as possible since cooking almost
>ceases to occur below 170F.

Hmm 77C

With the thermal mass of water (being a major part of the pot's
contents) what insulation is required to slow the cooling from 100 to
77 in the time required for cooking?

> When the water is kept near to boiling, there is
>a lot of evaporation, not a little.

Pete Verhaart has explained this, it is why even a poorly fitting lid
is better than none. As the air temperature above the liquid increase
its RH goes down, it is able to carry away more water if there is air
circulation (i.e. no lid). This removes 2.3MJ+ from the fluid at
whatever temperature it is. Steam is a very good medium to transfer
heat, it will still do this via condensation to the lid, the lid to
air surface will limit this loss to what can be carried away by air
convection over this surface, which sits at 100C. Because of the
latent heat of steam being so much higher than the specific heat of
air no such limit exists for an air vapour mix rising from the pot.

An illustration of this is in the design of a condensing clothes
dryer, the ambient air fan is far larger than the circulation fan,
pointing to the fact that removing heat from the lid is more limiting
than producing a saturated hot air/steam under the lid.

> If the temperature dropped significantly
>so that there was little evaporation, cooking times greatly increase which
>requires a greater input of fuel.

Which of course is why a steam pressure cooker works fast, its
temperature is not held at 100C.
<snip>

> Wisps of steam start
>happening around 140F and build up to raging plumes at 212F. 

I think you are mistaken, if you can see visible water droplets what
you have is a sol. What you are seeing is an air/water phase being
created below 100C above the liquid surface, this subsequently rises
into the colder convection stream above the pot. This cools the air
water vapour below its dewpoint, and hence produces the visible sol,
and must be a significant means of transferring heat (which has been
thermally costly to get into the pot as you have many times pointed
out) water having a latent heat  some 500 times its specific heat.

I believe a lid should be used in most cases.
-- 
AJH

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