Why does it take longer to cook in lower pressures?


by indianaronald
Tags: phase change, saturation, thermodynamics
indianaronald
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#1
Jan20-13, 01:23 AM
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The textbook says "It takes longer to cook at lower temperatures because the saturation temperature is lower". But if the T(sat) is lower then it should take less time to cook, shouldn't it? Since the stove can reach that temperature sooner and can hence cook faster.
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jedishrfu
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#2
Jan20-13, 01:36 AM
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perhaps this will explain it:

http://en.wikipedia.org/wiki/High_altitude_cooking
indianaronald
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#3
Jan20-13, 01:54 AM
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Quote Quote by jedishrfu View Post
No, couldn't get it. It still says it takes longer to cook and the boiling temperature is lower. I don't understand why it would take longer.

tensor33
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#4
Jan20-13, 02:04 AM
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Why does it take longer to cook in lower pressures?


The way I understand it is, because the boiling temperature of water is lower, if you want to boil something without all the water evaporating into steam, you have to keep the water temperature lower than you would if you were at a lower altitude. Hence the food takes longer to cook at high altitudes because the water it's being boiled in is not as hot as boiling water at low altitudes.
Drakkith
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#5
Jan20-13, 02:06 AM
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I think it's the fact that when water evaporates it takes energy with it, keeping your food from heating past the boiling point of water. The water in food that you cook evaporates at a lower temperature at higher altitudes and prevents the food from being brought up to as high of a temperature as it would have if it were at a low altitude.
jbriggs444
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#6
Jan20-13, 07:14 AM
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When you are boiling food in a pot of water on the stove the important goal is not boiling the water. As you point out it's easier to boil water at high altitudes than at low. The goal is normally to increase the temperature of the food. This allows the desirable chemical, structural and sanitary changes you are after to take place.

For instance, if you are boiling an egg you want to get the insides to coagulate. If you are cooking spaghetti you want to get the liquid water to hydrate the starches. If you are frying chicken you want to get the internal temperature up to an appropriate temperature to kill bacteria.

There are some cases where boiling away water is the intent, but that is the exception rather than the rule. Thickening sauces is normally done with starches or eggs, not by desiccation.
CWatters
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#7
Jan20-13, 03:46 PM
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What jbriggs444 said.

Google suggests that at the top of mount everest water boils at 69C. So that's as hot as you can heat water in a saucepan up there. Food cooks slower at 69C than it does at 100C.

The answer is to use a pressure cooker not an ordinary saucepan. If you can increace the pressure in the saucepan it will raise the boiling point of the water, hopefully back to near normal.
Chestermiller
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#8
Jan20-13, 11:27 PM
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To properly cook food, the parameters involved are temperature and time. If the food is at a lower temperature, you need to cook it for a longer amount of time. If the food is at a higher temperature, you can cook it for a shorter amount of time. One of the key considerations is to kill the harmful bacteria, which follow this type of rule. The food processing industry has data on the times required as a function of temperature. If I recall correctly, during their canning operation, Campbell's Soup cooks their concentrates for about an hour at 258F, corresponding to 15 psi gage.
e.chaniotakis
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#9
Feb18-13, 07:18 PM
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I guess that the bacteria need the same amount of energy to be killed regardless the height (same goes for all chemical reactions taking place in cooking food).Right? If so, then, if the water boiling point drops,the cooking temperature drops and thus the energy transfered to them per unit time drops. So , the bacteria need more time to die and cooking lasts longer.
Not so sure if the argument is right..
skeptic2
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#10
Feb18-13, 08:27 PM
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A word of caution, we live at 4,700 ft. and use our pressure cooker a lot even though the instructions to the pressure cooker say "Do not use above 3000 ft. elevation." We've never had one explode nor have I heard of anyone else's exploding either. I've checked the instructions for a number of other pressure cookers and they all put the limit at 3000 ft.
Nugatory
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Feb18-13, 08:43 PM
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Quote Quote by e.chaniotakis View Post
I guess that the bacteria need the same amount of energy to be killed regardless the height (same goes for all chemical reactions taking place in cooking food).Right? If so, then, if the water boiling point drops,the cooking temperature drops and thus the energy transfered to them per unit time drops. So , the bacteria need more time to die and cooking lasts longer.
Not so sure if the argument is right..
Pretty much right... The only thing I'd add is that we're not just trying to transfer energy into bacteria to kill them... we're also trying to break down various molecules in the food itself to make it more tender, more digestible, or just more palatable. But in any case, the key is to transfer enough energy, and as you say, that takes longer at lower temperatures.


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