One of the most frequent criticisms of gluten-free foods is a texture thing. Even home-baked GF recipes tend to be dry and grainy. Mixes can be especially so. Unfortunately, GF flours just don't break down like wheat flour, and the "chewy" texture is an effect of gluten.
We've been playing with breakfast foods, using the Gluten Free Bisquick now on the market. The GF Bisquick pancakes were/are decent, and better after JD added chopped apples, dried fruit and honey to increase the moisture content. While we were experimenting, we were on a hunt for gluten-free buttermilk to add more flavor. The only type of powder sold in our local grocery stores was not gluten-free.
We finally found a GF buttermilk powder with no preservatives or additives at a specialty store (NOW foods brand). JD made the first batch of buttermilk pancakes by substituting reconstituted buttermilk (at about double concentration) for the milk called for in the recipe. The results were, as I say, "diabolically good". They were super-moist, chewy and perfectly flavored. I honestly don't know if I could tell the difference between them and the regular homemade pancakes.
The only thing that could explain the change was the buttermilk, and I went about testing it. Betty Crocker has a GF chocolate chip cookie mix that we've used. The cookies taste perfect but there's the texture issue again. After they cool they become super crumbly and grainy, even if kept in the fridge. I added 1/4 cup buttermilk powder and 2 tablespoons milk (always balance wet/dry ingredients) to the mix and otherwise prepared per the box. The result were super-chewy cookies, even the next day. There's still a slight after-effect of the rice flour grittiness, but they are significantly chewier and more moist than the mix alone. The flavor of the buttermilk adds a very tasty tang to the cookie as well.
I did a little digging, and the theory that makes the most sense to me is that the acidity of the buttermilk helps break down the dry ingredients. Some of the GF flours don't absorb moisture very quickly, but adding an acid helps it break down.
On the other hand, too much acid can affect the levening action of other ingredients. Pancakes and cookies work beautifully with buttermilk because they don't require that much rising. My buttermilk cookies were flatter than usual, but more than made up for it in improved flavor and texture. If you're baking a cake or bread, however, you need to take the Ph balance into account.
My theory, which awaits testing in a few weeks when I'm free of my despotic psychology professor, is that simply adding more baking soda or powder to counteract the increased Ph of the buttermilk is somewhat counterproductive; if the acid is neutralized it cannot break down the particles of the flour, but if it is too acidic you lose the levening action of the alkaline/acid reaction. I believe the solution is in the order of mixing ingredients.
I propose that you mix the GF flour and buttermilk first, then let it sit. If you're proofing yeast for bread you might use the same time for soaking the flour. Otherwise I'd give it about 5-10 minutes. Mix the remaining dry and wet ingredients in separate batches, then add it to the soaked flour. This gives the acid time to work before putting together the levening ingredients.
When you mix the remaining dry ingredients, you'll need to add more base to allow rising. The general consensus seems to be that for each cup of buttermilk you use in a recipe, you need to subtract two teaspoons of baking powder and add one teaspoon of baking soda. That would be tough to do in a pre-made mix (you may just need to add the baking soda) but simple for homemade baked goods.
I'll work on playing with the idea, but in the meantime would love to hear from anyone with first-hand experience on how to perfect the buttermilk swap!