On 6/13/2012 10:18 AM, Dave Smith wrote:
> On 13/06/2012 3:55 PM, dsi1 wrote:
>> On 6/12/2012 9:56 AM, spamtrap1888 wrote:
>>>
>>> I know what you mean. I cooked a beef roast at 325F the other day. Did
>>> you know it took well over an hour for the roast's internal
>>> temperature to reach 140F? What gives?
>>>
>>> I think the answer in both cases is that pressure cookers transfer
>>> heat mostly via steam, while ovens transfer heat mostly via air.
>>>
>>
>> I think you're right about this. Steaming is an interesting process.
>> When the steam hits the cooler food, it condenses on the surface and the
>> latent heat of the steam is released. It's the opposite of sweating
>> where the sweat changing into a gas absorbs heat. The energy it absorbs
>> will again be released when it either turns back into a gas or when it
>> freezes.
>>
>>
>
> I think you maybe wrong about that. You need a physics lesson. Water
> changes to a gas vapour at 212F at sea level and at a lower temperature
> at higher altitudes, so when you are cooking at sea level you water is
> only going to heat up to 212F, though adding salt will raise the boiling
> point a bit. Cooking under pressure allows the temperature to get up to
> about 250 degrees, and the steam pressure is forced through the food.

What part did I get wrong? Everything you said is pretty much correct
but my point was that water condensing on food is an efficient way to
cook. This was my little intro to simple phase change thermodynamics. If
you think you're better qualified, feel free. Pressure cooking is
probably mostly boiling but I have to believe that the steam condensing
on the food surface at 255 degrees has got to release a lot of heat.