Your time is limited. Besides, you are concerned about tenderness and nutrition. So you are in the market for a pressure cooker. But you are just a bit curious about how and why they work. And you are not certain which one to buy.
Meats and Vegetables
Water ordinarily boils at 212°Fahrenheit (100°Celsius). So food cooked in an open saucepan¹, whether by boiling or steaming, cooks at approximately that temperature.
Using a Pressure Cooker
In an open saucepan, water boils at atmospheric pressure, something over 14 pounds per square inch. However, when water boils in a pressure cooker, steam pressure is considerably higher.
Now steam is essentially gaseous water. This brings to mind our high-school days and the dreaded Ideal Gas law.
A Very Little Math
The Ideal Gas Law is,
PV = nRT
P is pressure, V is volume, T is temperature, n is the quantity in moles, and R is the Ideal Gas Constant.
Since we are comparing two cooking techniques, we set up two equations, Equation 1 for the saucepan, Equation 2 for the pressure cooker. So,
P₁V₁ = nRT₁
P₂V₂ = nRT₂
For comparison purposes, we use identical quantities of water. That way, both n and R are the same in both cases. We rearrange the equations and equate them,
P₁V₁/T₁ = P₂V₂/T₂
Now since the quantity of water is the same, the volume also is the same. We cancel the volumes to get,
P₁/T₁ = P₂/T₂
In order for both sides of this equation to remain equal, if the pressure increases, the temperature also has to increase. Since the pressure is higher in the pressure cooker, its temperature of operation must also be higher.
Pressure Cookers Advantages / Disadvantages
Not only does pressure cooking take up less time, the food loses less moisture under pressure. Finally decreased cooking time decreases loss of nutrition.²
Pressure cookers require a separate utensil and require either time to cool down naturally before opening, or more rapid cooling under the tap. If insufficient moisture is provided initially, there is the risk of burning in the bottom of the pot. However, the same might be said for cooking in an ordinary saucepan.
Want some idea of what to look for when purchasing a pressure cooker? Check out this America’s Test Kitchen video. It is short, and to the point. And it may help you avoid pitfalls when making your purchase.
¹ The food could be cooked, of course, in a skillet or some other utensil, but if it is fried or cooked in some other manner, it is hard to compare with the same food cooked in a pressure cooker. It would be like comparing apples and oranges.
² Certain vitamins degrade with heat over time.