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do i need to sift icingsugar when baking lemon squares
To"sift" and to "sieve" are the same thing, except that when using a sieve with flower people call it "sifting" flower. It means passing the flower through a fine sieve to make sure there are no lumps in it and holding it high when you do so so that as the flower falls though the sieve, air gets incorporated into it to make the baked products lite and fluffy.
Sift the all purpose flour before measuring it
Powdered sugar is made out of a sugar that has been finely processed - it is, basically, granulated sugar that has been finely ground. Cornstarch is added to commercial powdered sugar to keep it from clumping and being pesky to sift.
do i need to sift icingsugar when baking lemon squares
Yes, beautifully too. You can use a flour sifter, or a basket strainer with a handle, these are both smaller in size. Put your confectioner (powdered) sugar into this, hold over the cake and gently sift the sifter or shake the basket strainer around it. MAKE SURE YOUR CAKE IS COMPLETELY COOLED. Very important. Or it melts like snow flakes.
That would depend on the context. You can "sift through recipes" or "sort through recipies" but if a recipe says "sift flour with sugar" you'd better sift it.
if you did that then the sugar probably wouldn't disolve
Practicably you can't. However there is a way that will work somewhat. Using a fan with a steady air flow you can sift the mixture of sugar and flour through a wire strainer, so that it falls into the flow of air. The air will carry away most of the flour and the granules of sugar, which are heavier will fall to the floor, or tray you use to catch them. Some flour may fall with the sugar, and some sugar will get carried away, but they will be separated somewhat.
do i need to sift icingsugar when baking lemon squares
No, it is not necessary to sift whole wheat flour. If you made the flour by yourself, and the flour isn't like a smooth powdery texture, then you should sift it to remove any unblended grains
you sift it before using it to remove lumps