Cake flour is a finely milled, low protein flour that is known for its ability to create cakes with a very even, soft and tender crumb. As the name suggests, it is excellent for making cakes and is commonly found in pound cakes and angel food cakes. Almost every recipe that calls for cake flour asks that you sift it before using, while recipes that call for all purpose flour (including mine) simply ask that you stir or whisk the flour before incorporating it into your batter, so I often get asked if it is necessary to sift cake flour before using it.
To put simply: yes, cake flour must be sifted before using it. Cake flour is so fine that it clumps together very easily. While large clumps can be broken up with a spoon or spatula, small clumps are tenacious and will show up as lumps of uncooked flour in your finished cake if you are not careful. All purpose flour and bread flour do not have the same fine texture of cake flour and often do not need to be sifted in a recipe to prevent lumps of flour from popping up unexpectedly.
Some recipes ask that you sift the cake flour before measuring it, while others will ask that you sift the flour after measuring it. Either way, the flour should be sifted at least once to remove those small lumps and help ensure that your cake turns out as perfectly as possible. In the event that you come across a recipe that calls for cake flour but does not ask you to sift it, I would recommend sifting it just to be on the safe side. There is no harm in sifting flour for your recipes and a lumpy cake is just one less thing you’ll need to worry about.