Cornstarch, arrowroot, potato starch, and tapioca flour (another in this family of starches) exhibit the same characteristics. Like flour starch, they absorb water and swell in size, again like a ballon. But because they are 100% starch, they have more thickening power that does flour, which also contains protiens, vitamins, and other molecular compounds.
The pure starches have virtually no flavor and do not brown. In addition, you make a slurry (thin paste) from them by adding water, before adding them to the liquid you wish to thicken. When heated to a boil, they thicken very rapidly, and must be stirred into the sauce with medum vigor to avoid creating lumps. They turn transparent and give no color to the sauce. They only thicken it.
These starches have subtle differences and so are used for different things. You can, for instance, thicken a fruity pie filling such as apple or blueberry with cornstarch. But it is difficult to get just the right consistancy. I have had blueberry filling that came out like soup, and some that came out like school paste when using corn starch. However, when I used tapioca starch, the filling came out perfect.
A neat little trick, if you are thickening a filling that's to be used in a pie, is to cook the fruit on the stove-top, and thicken with a starch slurry to the desired consistancy before adding it to the pie-shell. Then you are gauranteed the proper consistancy.
Also, as was mentioned in a previous post, starch slurries make great puddings (I prefer cornstarch for this job). Lemon pie filling is an example. And because there is no protien in the starch, the pudding will not curdle when the acidic lemon juice is added.
For jams and jellies, and ice cream, gums such as arabic, xanthum, and pectin are used. They are very similar to fiber in structure, but have more thickening power. Added to the sticky-thick texture of cooked sugar (think the semi-liquid center of a caramellow bar) they create the texture we recognize in our jellies, jams, and fruit preserves.
And finally, there is that last thickening agent, gelatine. You know when you open a canned ham, and it has that jelly surounding it? That is the same substance from which Jello is made. It is derived from animals and in its pre-cooked state is called collagen. It is a cousin to protien and has nutritional qualities, unlike starches. If you crack the bones of a turkey carcass, and boil it for a couple hours (especially if you include root veggies such as celery root, rutabega, or turnup), you will extract (dissolv) the collagen from the bone marrow, connecting tissues, and cartillage. You won't particularly notice it when its hot as it will be liquid in nature. But it's there.
When you strain the broth and refrigerate, the broth will attain the texture of gellatin. When heated again, it will liquify. Soups and broths with a good amount of collagen exhibit a slightly thicker, almost slippery mouth feel. You can tell that there's more body, and it feels somehow more substantial, but its so very subtle.
If you serve that cold, jello-like broth, it's called an aspic. It can be flavored with pork, seafood, beef, any kind of poultry, veal, or whatever you desire.
A memorable dish I once had was a slam-juice flavored aspic that had pieces of cooked lobster meat throughout. It was molded into a squat cone and was used as something to spread onto Ritz or Townhouse crackers. It was a phenominal flavor, and very luxurious. And because it was mostly aspic, allowed the host to treat her guests to something spectacular for a modest cost.
I like to brown hamburger with the lid on and pour off the relultant juice into a bowl. I refrigerate that broth. The fat, being lighter than everything else, rises to the top and is easily removed in its hardened state. The broth has solidified into an aspic, and can be used ot make fat-free gravies and sauces. It has a wonderfully beefy flavor. And as for the ground beef, it has had much of the fat rendered out and discarded, making it healthier as well. After pouring off the juice, I finish browning the meat without the lid on.
You can also puchase this substance (gelatine) to add to foods. A common brand is Knox - Unflavored Gelatine. You just add the flavoring of choice (and with this, you can make home-made finger jello, or regular fruit flavored gellatine), and voila, instant aspic.
A word of caution when using collagen based products; If you try to add raw kiwi, pineapple, papaya, or a few other fruits that I can't quite remember right now, the gelatine will not set. This is because those fruits contain natural enzymes that denature protiens, and collagen. Those same enzymes are extracted from those fruits to make comercially available meat tenderizers (Adolph's uses papain from papaya, while McKormick's uses bromelain from pineapple). Heat destroys, or denatures the enzymes, and so those fruits, once cooked, can be added successfully to gelatines.
I hope this wasn't confusing and helps everyone understand a bit more about thickeners and their properties. And as always, the best way to learn those properties is by experimentation, and careful observation of the results. Enjoy the experiments. I know that I do, even when they fail, because I've learned something from them.
Oh, and for Frenchoinionsoup, in answer to your question, you can add virtually any liquid to a roux. This can be milk, with any percentage of fat that you like, including unsweetened condensed milk, or cream, or meat/vegetable broth. The roux will thicken all of them. The only difference is is what you call it when the sauce is complete. And this is for gravies as well.
Be aware though, that the different dairy products have differing flavors. Cream is not only more viscous, but has a less sweet flavor than does milk. And condensed milk has a cooked, intensified flavor. Each adds its own character to the sauce. And you can mimick cream by adding unsalted butter to milk. After all, butter is simply the butterfat fro cream with the whey (water, lactose, and milk protien) removed.
Seeeeeya; Goodweed of the North