What is the physical effect of sifting dry ingredients for a cake?

Originally, before flour was as processed as it is now, sifting helped remove things like twigs and other contaminants.

Sifting just helps remove clumped up dried ingredients (flour, powered sugar, etc, ) so that when you add in the wet ingredients you do not have to mix too hard to remove the clumps.

Edit : When mixing the mixture too hard or too long you risk of "creating" gluten and that will render the cake mix too "bread" like (too dense, too chewy)

You could just use the flour as is and whisk it in the bowl to try to remove the clumps.

see : https://www.thekitchn.com/is-sifting-flour-for-baked-goods-really-necessary-213894

Even though now a days most ingredients come quite sifted anyways and they don't usually have any other ingredient mixed by accident, it can still have lumps. It has sometimes happened to me that a cake (OR PANCAKES!) had like a bubble and inside some flour. Which is not the best flavour or texture when enjoying a nice cake or some pancakes with syrup!

And think that the smaller the pieces of each ingredient (every miligram of Flour), the better they can mix with other components and have the desired reaction with the heat of the oven and the rest of the ingredients.

So as an advice, Always shift the dry ingredients!

What I found while sifting and not sifting sugar:

Sometimes granulated sugar can be too big and depending on the cooking time and the thickness of the dough, it might not melt all of the granules. Sometimes sifting can also tell you if the sugar granules that you re using are going to be too big or decent size (even if not all is sifted).

Besides the other answers that hold true for sifting individual ingredients, it will also help make the ingredients more evenly distributed if you sift them together.

If I'm sifting, I'll typically place part of the flour in the sifter, then the smaller quantity items, then the rest of the flour, and sift them all together.

This especially helps for ingredients like baking powder, baking soda, and cocoa. Chemical leaveners are used in such small amounts that a small clump will really mess you up (and taste disgusting if you find it). Even if you don't sift your flour, it's still worth sifting any leaveners into the flour before you mix.

It's also worth mentioning that when you're dealing with volume measurements (as it typical in the US), sifting flour helps to aerate it ... that is, you're working air into flour that might have settled during shipping or just over time, so you'll get a more consistent measurement. But that requires sifting flour and then measuring it, which is rare to see in recipes. (it'll say something like "2 c. flour, sifted").

Powders are funny, chemically. The large surface area makes for some interesting effects. For one, adsorption of gases (oxygen, nitrogen) is rather fast and can be significant. Another is static electricity, which would be very annoying for the one tasked to mix the different powders. It also promotes (or disfavors) the formation of agglomerates, minuscule "clumps" if you will.

Now, can a reasonable general argument be made for all possible cases? Not by me at least. But that there can and will be differences in making batters of sifted powders as compared to straight-from-the-bag? Very understandable.

As a funfact/example; baking ingredients now are usually spiked with minute amounts of anti-caking agents and other surfactants. Flour is spiked with ascorbic acid (vitamin C) - and this reacts with oxygen and improves dough/batter gluten development. Any process that exposes the surface to more oxygen, sieving being certainly one example of such, would improve the action of the ascorbic acid.

Right before my mother taught me to how to sift, I found out in a frightening way, WHY I should sift.

I was young, and baking cookies, and when I got to the flour, I scooped it out of the container using the measuring cup. Then I packed it in pretty well with a spoon.

When the cookies were done, they looked wonderful. They were great big, and fluffy. I just had to try them. I got one and took a big bite, and then a big puff of dry powder exploded in my mouth. I started choking, and coughing. When I could finally take a breath, I took a big one, and I swear the powder seemed to go down into my lungs, and then I really couldn't breathe. Luckily my mom heard me, and she rushed in and pounded on my back, and I was struggling to breathe, and crying, but I was able to calm down enough to take a drink, and wet down the desert in my mouth, and I was okay. If she hadn't been there, who knows what would have happened.

She looked at the cookies, and knew that I had put in too much flour. She had me show her how I got the flour out, and then packed it, and she took the cup of flour and set it aside. Then she brought out this metal thing with wire mesh at the bottom of it, and a handle on the outside, that turned.

She dumped that cup of flour into this thing, and turned the handle while holding it over a plate. The flour came out looking like soft snow. Then she took a spoon, and placed the sifted flour very delicately back into the cup until it was full. When I looked, I saw that there was at least a whole other cup of flour still on the plate. I had used at least twice as much flour as the recipe called for!

I continued to accidentally ruin many desserts by adding too much flour, and it was just because I was lazy. It took me a while to learn that sifting the dry ingredients was worth the extra step. Incidentally, when my boys started baking, I had to show them how to sift, for the same reason, and with the same sifter.

I have found that recipes for baking often say to use sifted flour, and I've even read recipes that said to use a spoon "...to place it gently into the measuring cup." This is because it clumps together very easily. When too much flour goes in, you can't really tell, until it's done baking, and then the whole thing is ruined.

That was a true story up there. I hope people will keep it in mind, because you really don't want someone trying something you made, only to have the Sahara Desert suddenly appear in their mouth, and you have to call 911...