Why do we prefer cooking/boiling food in pressure cooker, I know there is a reason but don't know what is it?
I can't show my work here as it is a conceptual question.
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Cooking things in water is mostly a chemical reaction called hydrolysis. Specifically it hydrolyses the connective tissue in the meat. Connective tissue is what makes meat tough and it is largely composed from a chemical called collagen. Hydrolysing this converts it to gelatin and makes the meat more tender.
This reaction is quite slow at $100$ºC and typically takes one to two hours to hydrolyse enough of the collagen to make the meat tender. However like most reactions the rate of the reaction increases as you increase the temperature.
And this is where the pressure cooker comes in. If you cook meat in water at atmospheric pressure then the temperature is the boiling point of water, which is $100$ºC. But the boiling point of water increases with pressure. A pressure cooker increases the pressure of its contents by about $1$ atmosphere, and this inceases the boiling point of the water to around $120$ºC.
So you are now hydrolysing the collagen at $120$ºC instead of $100$ºC and as a result the reaction goes much faster, so your meat becomes tender faster. And that's why pressure cookers cook food faster.
Consider that when cooking in an open pot much of the heat, steam is driven off into the air above, and along with it the energy that was transferred from the burner below.
But in the pressure cooker the steam, energy imparted to the fluids in the pot is trapped, and that trapped energy has no choice but to permeate and re-permeate the food within thus accelerating the rate at which it is cooked.
From another perspective the increase of pressure that a pressure cooker causes increases the diffusion of vaporized water particles that permeate the food within.
But as far as preference many people prefer normal, slower cooking methods over a pressure cooker. A pressure cooker tends to make all the food within taste the same. Carrots taste like beans, taste like potatoes.