It's time for another round of The Food Lab. Got a suggestion for an upcoming topic? Email Kenji here, and he'll do his best to answer your queries in a future post. Become a fan of The Food Lab on Facebook or follow it on Twitter for play-by-plays on future kitchen tests and recipe experiments.
I wasn't quite sure what I was getting into when I started this column last week, but man are there are a lot of great, thoughtful questions out there. I only wish that I had a few more hands or perhaps a flux capacitor to give myself the time to answer all of them as thoroughly as they deserve to be. As it is, unfortunately, I'll have to stick to a few great questions per week.
"Why the heck won't my cocoa dissolve?"
Why is it that when you stir cocoa powder into cold milk, when it resurfaces it's NOT EVEN WET?! How is that possible? When I submerge plastic or glass or steel cookware in water, it gets wet. My water-proof skin gets wet under running water. But cold cocoa powder does not get wet. Honestly, I'm stumped and my whole family is lawyers so they are no help at all.
—Sent by smsingram
Ah, those frustrating little dry cocoa pockets. Everybody knows 'em. They're like the Sideshow Bob of the powdered drink world. You can knock 'em down as many times as you'd like, but the keep coming back.
There are actually two different things going on here.
The first has to do with the nature of starch—a complex carbohydrate found in all sorts of plant matter, including dried cocoa—and water. Remember those little dinosaur-shaped sponges you'd get as a kid, which you'd drop into water, then wait for them to grow? That's exactly what starch molecules are like. When dry, they start out tiny and shriveled. They can flow freely past each other. But expose them to water, and they start growing, getting bigger and bigger, until they eventually rub up against each other and bind, creating a water resistant barrier. Now are you starting to see the picture?
When a clump of cocoa powder lands on the surface of a cup of water or milk, the first parts to get wet are the starches on the outside, which rapidly expand, forming a waterproof seal. As you stir and submerge these clumps, that seal ends up forming around the entire clump, protecting the interior from getting wet. Once it resurfaces, the shell falls off due to gravity and the surface tension of water, exposing the dry powder inside.
This is something you'll notice not just with cocoa powder, but with other starchy powders as well—anyone who's ever tried dropping raw flour or cornstarch into a hot gravy to thicken it will know that you'll end up with impossible-to-destroy little floating clumps of flour with a dry core.
There's a second thing going on with cocoa, and it has to do with fat. Fat molecules are naturally averse to water—they shy away from it like my dogs from a bath, and tend to stick together when confronted with it. Dry cocoa is about 12 percent fat by weight, so when you drop it into water, those fat molecules stick together (much like the starch), forming an impenetrable shield.
So how do you solve this problem? Two ways.
First, use less liquid to start. By starting with a smaller amount of liquid, the mechanical stirring action of your spoon, fork, or whisk is much more effective. Smaller amounts of liquid also get viscous more easily, making it simpler to bash up those pockets of dry powder. I use an equal volume of powder to liquid to start and stir it up until homogenous before adding the remaining liquid.
Second, use some fat. Starch does not swell in fat, so by combining flour or cornstarch first with a fat like butter or oil and mixing it until homogenous, you end up coating the individual starch granules, preventing them from swelling and sticking together when you first add them to the liquid. After you add it, eventually the fat melts away, the starch is exposed, and can be safely incorporate neatly and smoothly.
You don't want to add butter or oil to your hot chocolate, so with cocoa powder destined for the cup, the best fat to use is dairy—combine your powder with a small amount of full-fat milk, half-and-half, or—better yet—cream. It should dissolve much more easily. Once smooth, add the remaining hot water or milk.
Finally, if you've already added it to a liquid and can't get it to dissolve, your best bet is to expose it to as much turbulence as possible. Throw it in a blender, or use a hand-held immersion blender to get rid of the clumps. Even a small milk frother will do the trick.
"Why is whole wheat bread always denser than white bread?"
I don't do a lot of baking but recently have been trying to make some things 100% whole grain. I've actually had some luck with recipes that list weight instead of volume measurements. What exactly is going on that makes it more difficult for the dough to do what I want, and why the heck can't I even come close to perfecting a 100% whole grain croissant?
—Sent by aliglia
If you've tried replacing all or part of the white flour in a recipe with a whole grain flour, you've probably noticed that your bread has come out denser and dryer than it normally does. Heck, you've probably even noticed that whole wheat bread from the supermarket tends to be denser than white—part of the reason why many little kids (or little kid-like adults) have an aversion to it. To understand why that happens, you have to understand both the structure of a wheat grain, and of bread.
A grain of wheat has three basic parts: the endosperm (that's where most of the starch and protein resides), the germ (which contains fats and vitamins), and the bran (which is mostly indigestible dietary fiber). White flour is milled from only the endosperm and produces a very fine powder that is a mixture of mostly starch, along with anywhere between 8 and 15% protein, depending on the desired end product (bread flours tend to be higher in protein, cake flours lower, all-purpose in the middle).
Whole wheat flour, on the other hand, is milled with both the bran and the germ intact. This makes the final product healthier both in terms of additional fats and vitamins, as well as essential dietary fiber. It does, however, have a negative impact on texture. The reason?
Well, consider that bread is essentially a solid foam*—that is, a matrix of bubbles that have been hardened after being inflated. In order to create these bubbles, you must first create gluten by mixing flour with water. There are two proteins in the flour—glutenin and gliadin—that intertwine with each other as they're mixed together in the presence of water. As they continue to stick together, eventually you get a strong, net-like network. This network is called gluten, and it acts much like the rubber in a balloon.
* More on that concept here).
As yeast digests carbohydrates in your dough, they release both alcohol and carbon dioxide gas as bi-products. It's this carbon dioxide that initially inflates those little gluten-walled balloons, causing your dough to rise. Once placed in the oven, heat causes water to turn to steam, further inflating those bubbles and causing your loaf to rapidly rise. Eventually, the heat will cause the proteins in the gluten network to harden, setting your bread into its final shape.
So what happens when you try and use whole wheat? As we know, whole wheat contains ground up shards of germ and bran, neither of which can be ground as finely as the starchy endosperm. Look at a pile of whole wheat flour under a microscope and you'll see tiny fragments of germ and bran, sitting in there like little scissors*, ready to cut and cleave the gluten network as it forms. It's like throwing a truckload of razor blades into a fishing net factory. A few small pieces of net might still get formed, but you're still dealing with a seriously compromised product.
* This reminds me of a joke: What do Mexicans use to cut their pizza? Little Caesars! ha.
When that chopped up dough finally hits the oven, sure, it expands a little bit, but there's only so far a balloon with holes in it can be inflated. That's why whole wheat bread will never have the same light, airy, stretchy bubbles that white bread will have. It's an unfortunate truth.
Making whole wheat anything is a trade-off between flavor/nutrition and texture. Personally, when I make a whole wheat loaf, I try to limit the amount of whole wheat flour I use to about 1/3 of the total mix, using a high protein bread flour for the remainder. This gives me the optimal balance between flavor and texture. For you, that balance might swing a little bit one way or the other. Just know that the more whole wheat flour you use, the denser your bread will be.
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About the author: J. Kenji Lopez-Alt is the Chief Creative Officer of Serious Eats where he likes to explore the science of home cooking in his weekly column The Food Lab. You can follow him at @thefoodlab on Twitter, or at The Food Lab on Facebook.