Questions have come up in recent weeks about how to appropriately measure out a quantity of flour in gluten-free baking. It seems like a trivial question, but the answer turns out to be a little more involved than you might suspect. The answer is important, though, because especially in baking, recipes are a lot like formulas... precise measurements matter.
At the professional level, flour quantities are almost always referred to by weight - either in ounces or grams. That's because weight is the most reliable and consistent measure. Why do we care about reliability and consistency? Simple. Because we want to be sure that when a recipe calls for a given quantity of flour, we're using exactly the amount intended (and not more or less). We also want to be sure that we're using the same quantity of flour from one time to the next for a given recipe. (As reliable as weight is, it can have some variability due to changes in humidity...from day to day, season to season, or region to region...necessitating subtle recipe modifications - usually in the quantities/ratios of flour and liquid ingredients.)
Compared to professional cookbooks, consumer cookbooks (like ours) typically use cup measures. Why? From our perspective, every kitchen will have a set of cup measures, but not every kitchen will have a scale accurate to the ounce or gram. But when measuring flour using cups, there's a lot more room for variability (and unintended error or deviation from the recipe). Allow me to demonstrate by example:
I compared four cup measures of our Artisan Gluten-Free Flour Blend.
For the first cup measure, I weighed one cup of sifted flour. (To properly measure a sifted cup of flour, place your measuring cup over a plate or other surface that will allow you to recapture sifted flour that doesn't fall into the cup. Using a sifter, sift the flour over the cup. Use a knife or other straight edge to level the cup. Voila! One cup of sifted flour.) Sifted flour is nice for measuring because the act of sifting evenly aerates the flour every time. My result: 1 cup sifted flour = 4 oz.
Next, I measured one cup of spooned flour. Here, I used a spoon to transfer flour from our master batch into a measuring cup. I also did this same method, but tapped the side of the measuring cup as I went along to settle the flour and remove any large air spaces. The result: 1 cup spooned flour = 4.5-4.75 oz. The one cup of flour is slightly heavier (meaning, more flour is packed into that cup) because it didn't have the even aeration of sifting.
Next, I measured one cup of scooped flour. Here, I used the measuring cup itself to scoop through the master flour batch and then level off a cup measure. The act of scooping through the flour serves to further compress the flour into the measuring cup. Predictably, the result is even heavier: 1 cup scooped flour = 5-5.25 oz.
Lastly, I measured one cup of densely packed flour. Here, I used the back of a spoon to tightly pack flour into a one cup measure. Of course, this returned the heaviest result: 1 cup packed flour = 5.75 oz.
That's an incredible amount of variability from sifted flour (4 ounces) to densely packed flour (5.75 ounces), all of which account for one cup of measured flour. In fact, the densely packed flour represents a 44% increase in weight! I'd call that a statistically significant difference, and one that would greatly impact whatever you're baking. A dough or batter would be drier. Yield would decrease. Breads would become denser and rise less. These are not good things.
In our cookbook, we advocate using the spooned flour method of measuring cups. It's generally reliable, and is both easy to do and expedient. However, it's not without its own pitfalls. For one, the density of the spooned flour will generally be influenced by the density of your master batch of flour. For another, if - like us - you mix up a large master batch of flour because you do a lot of baking, the flour near the bottom of the batch is going to be denser/heavier because it's been compressed by the weight of the flour sitting on top of it. Hence, as you work your way down through your flour, the same spooned cup measure may become gradually heavier...and that will impact your baking. You can account for this by using slightly less flour, or slightly more liquid. (You'll likely do this anyway with the coming of winter, when the air is drier and recipes need slightly more moisture content to achieve the same result that you would during more humid months.)
Or, you could sift your flour. It's one extra step to do when baking, but the small "hassle" may well be worth it for the reliability of the method. You'll have consistently measured cups of flour that hit the low end of the weight spectrum every time. As a result, your recipes will turn out as you'd expect them to, and that's more than worth it in my book.