why is it important to introduce air when sieving flour?

[Mary]

i am making pasta shells in a ricotto sauce served with spinach for my home-ec exam. for a roux sauce i need to use sieved flour but need to know why its important the flour is sieved. i know its to introduce air into the mixture but need to elaborate on this. any help would be greatly appreciated

 

[Bilbo]

It is less to do with introducing air and more to do with stopping it going lumpy if it is for the roux - so it combines evenly with the fat. You'd be hard pressed to notice a light or heavy sauce as it is liquid.

If you are using it for baking or something that is supposed to rise then entraining air is rather more important.

 

[goingmad]

it not only thins out the flour but the introduction of air into the flour helps to make the pastry or pasta light and not stodgy and heavy, how it works i don't now but it does.