The Science of Gravy

What makes the perfect Thanksgiving turkey gravy? Depending on your preference, it might be the smooth texture, the rich flavor, or the glossy sheen. No matter what the end result, the magic of gravy lies in the science of starch.

So, what is a starch, exactly? A starch is a complex carbohydrate; in other words, a fancy chain of sugars. Simple sugars form neat little rings that dissolve readily in water, making a uniform solution. Starches, on the other hand, have bonds arranged differently, and form amorphous semi-crystalline layers. This arrangement allows the starch to absorb water, rather than dissolving in it. From the plant's point of view, the starch is a handy storage system for extra simple sugars.

When you are making a sauce, however, you need the starch to do more than just hold water. You need heat. When the starch and water combination is heated, the starch molecules expand and press up against one another. This creates the viscosity that gravy is known for. So, for the basic gravy, you need these three things: starch, water, and heat.

But which starch? Let's take a look at the differences between cornstarch and wheat flour. Cornstarch is basically pure starch. Wheat flour, on the other hand, contains starch, plus proteins (gluten) which behave quite differently than starches. These proteins are chains of amino acids, which essentially fold over and stick to themselves.

This stickiness is important when you're making bread or cookies, but not so much when making gravy. On the other hand, proteins have advantages starches don't, especially when considering the human palate-they can taste good. The combination of starch and protein in flour creates a thicker, more velvety texture in gravy than cornstarch.

There's only one problem: we don't typically like raw flour (cookie dough aside.) Adding flour to the mix, and heating until thickened isn't enough to break down the sticky glutens. So, made at the last minute, flour-based gravy just doesn't cut it. In that case, mixing cornstarch with cold water and adding it to the hot liquid is your best bet. But if you prefer the more complex flavor of flour-based gravy, you're going to need to bring some extra heat. Enter the roux.

A roux is the classic French cuisine answer to all sauces. Traditionally, it is a combination of butter and flour which is cooked on the stove before pouring in the liquid. This cooks out the raw flour taste, and gives a nice color to the sauce-anything from a light golden hue to a nutty brown. When it comes to gravy for the Thanksgiving turkey, a nutty brown roux can be just right. Of course, there is little need for butter... the drippings come with plenty of fat.

The roux method has a few drawbacks. The mixture burns easily, sticking to the pan, and needs to be stirred almost constantly. Since flour contains less starch than cornstarch, you need to use almost twice as much for the same thickening power. Heating the roux also reduces thickening, so the darker the mix, the more it will take to work. Don't let the complexity of the roux scare you off, though. With a little patience, it is well worth it.

Before making your Thanksgiving gravy, no matter which method you use, set the drippings to cool, and then remove the fat as it rises to the top. If you decide to make a roux, set the fat aside. If making a cornstarch slurry, toss it for a healthy gravy, or add it in at the end.

For rich, velvety, flour-based gravy:

In a large saucepan, heat about 1/2 cup of the fat from the drippings. (Alternatively, you can use a stick of butter.) Stir in 2/3 cup flour. Whisking constantly, cook for at least 1 minute. If you prefer, cook until the mixture turns a deep golden hue-just don't stop stirring! All at once, whisk 8 cups of stock into the pan. (Using hot stock will make a nice silky texture, but needs to be stirred quickly. Unheated, the stock gives a little more flexibility.) Bring the mixture to a simmer, and cook for another minute, stirring constantly. Meanwhile, add the remaining drippings back into the roasting pan, over the stove, along with about 1/2 cup of port, white wine, or water. Over medium heat, scrape and squish the browned bits. When the drippings are a fairly uniform mixture, pour them into the gravy in the saucepan. Cook for another 5 minutes, or until the flavors have blended together. Season with salt and pepper.

For a quick cornstarch-based gravy:

Heat the remaining drippings, adding enough chicken (or turkey) stock to make about 8 cups of liquid. This is easily done right in the roasting pan. Use a flat spatula to scrape and squish the browned bits from the bottom of the pan. In a separate cup, stir about 1/3 cup cornstarch into 1/2 cup water. While briskly whisking, stir the cornstarch mixture into the pan. Bring to a simmer. Keep stirring and heating, until thick and glossy, about 1 minute. Season with to taste with salt, pepper, and a dash of port wine.

For either recipe, you can use homemade turkey stock (made from the giblets and vegetables) or canned chicken stock. The recipes can be halved to make less, but gravy freezes nicely, and everyone wants seconds.... so make a large batch anyways!

Image notes: Cornstarch, magnified via Corn.org; starch chemical structure via Food Resources; gluten, magnified via NCP Poland; gluten, structure via Emerald; gravy in pan by sfegette via flickr; cornstarch gravy by la fluer via flickr.