STARCH THICKENERS: These silky powders are used to thicken sauces, gravies, pie fillings, and puddings. They're popular because they thicken without adding fat or much flavor, but some tolerate heat better than others.  

Starch thickeners often lump if not added to the liquids properly. To avoid lumps, mix the starch with an equal amount of cold liquid until it forms a paste, then whisk it into the hot liquid you're trying to thicken. Once the thickener is added, cook it briefly to remove the starchy flavor. Don't overcook--liquids thickened with some starches, such as flour and arrowroot, will thin again if cooked too long or at too high a temperature.  

If you get lumps in your sauce from a thickener, blend the sauce in a blender or food processor until it's smooth or strain it.

ARROWROOT:  A starch obtained from the rhizome of a West Indian plant. Sold as a dried and milled white powder. Does not mask or alter natural flavors. Produces sauces and pastes of remarkable clarity. Use as a thickening agent in place of flour or cornstarch for fruit sauces, puddings, salad dressings, dessert sauces, vegetable sauces, and meat glazes. Do not use to make gravy. Arrowroot reaches maximum thickening at lower temperatures than other thickeners, thus it is ideal for use with heat sensitive foods. Because it begins to thicken long before the boiling point of fruit fillings in pies, arrowroot is not a desirable choice. 

CLEARJEL® = ClearJel® starch = Clear-jel: This modified cornstarch is the secret ingredient that many commercial bakers use in their fruit pie fillings. Unlike ordinary cornstarch, ClearJel® works well with acidic ingredients, tolerates high temperatures, and doesn't cause pie fillings to "weep" during storage. ClearJel® is an especially good choice if you're canning homemade pie fillings, since it doesn't begin thickening until the liquid begins to cool. This allows the heat the be more evenly distributed within the jar during processing. 

ClearJel comes in a powdered form, just like cornstarch, and you basically mix liquid and sugar with it, heat over medium heat and stir with a heavy spoon until thick, add a little lemon juice and boil 1 minute. Then stir in fruit and cool and use, or you can cool it and then stir in fruit. It freezes well, and it's perfect for processing canned pie fillings.  ClearJel® is available from the supermarket or online

CORNSTARCH: Cornstarch, made from corn, is a fine, white powdery starch ingredient that is used to thicken pie fillings, sauces, gravies, and puddings, as well but it doesn't thicken well when mixed with acidic liquids. Cornstarch is called cornflour or maize cornflour in Britain, Australia, and New Zealand. Don't confuse cornstarch with the finely ground cornmeal that Americans call corn flour. 

Besides thickening, cornstarch has still two other contributions to make: cornstarch helps to prevent eggs from curdling—certainly a helpful contribution to make to a custard and second, it causes the heat to be transmitted more evenly throughout the custard; this helps to take care of the overcooked outer ring when making a custard pie. 

Cornstarch has twice the thickening power of flour, but like flour, it imparts a slightly starchy taste, especially true in juicy summer fruit pies; that's why I recommend cooking part of the fruit and cornstarch before baking a pie. It yields the smoothest texture and does not thin when reheating a slice of pie. But, it must be used properly.

Cornstarch doesn't stand up to freezing or prolonged cooking, and it doesn't thicken well when mixed with acidic liquids. See also "How to thicken with cornstarch". It can be kept indefinitely if stored in a cool, dry place.

FLOUR (See also, Roux): Flour is a good thickener for pie fillings, gravies, gumbos, and stews, since it gives them a smooth, velvety texture. It's best to mix it with fat first, either by making a roux or beurre manié, or by flouring fruit for a pie first before filling a pie shell. If making stovetop sauces, cornstarch and flour mixtures start to thicken at 144 to 162 degrees F. These starches complete the final thickening process at 205 degrees F. Under cooking does not allow starches to reach their maximum thickening capability. Sauces thickened with flour become opaque, and they may become become thin again if they're cooked too long or if they're frozen and then thawed. NOTE: High amounts of acid in food may prevent starches from setting.

FLOUR - INSTANT BLENDING: = instantized flour = quick-mixing flour.  You can sprinkle this finely milled all-purpose flour into liquids without getting many lumps, so it's perfect for making gravies and batters. It's also good for breading fish. Wondra flour and Shake & Blend are popular brands.   

POTATO STARCH (POTATO FLOUR): This gluten-free starch  is used to thicken soups and gravies. Its main advantage over other starch thickeners is that it's a permitted ingredient for Passover, unlike cornstarch and other grain-based foods. Liquids thickened with potato starch should never be boiled because it will loose it's power to thicken. Supermarkets often stock it among the Kosher products. 

ROUX: This is a thickener that's made from equal weights of flour and a fat, like butter or meat drippings, whisked together in a saucepan over heat in order to ameliorate the flavor of the flour and to remove lumps. It is then added to sauces and gravies to thicken while cooking together. Or, additional ingredient are added such as cream, cheese, tomatoes, herbs, egg yolks, white wine, lemon, onions, peppers, etc. or combinations thereof can be added creating different sauces.

The most basic white sauce is based on a roux, which is equal volumes of butter and flour. The thickness of the sauce is easily varied by changing the proportion of roux to liquid. For a thinner sauce, use 1 tablespoon each of butter and flour to 1 cup of liquid; for a medium sauce, 2 tablespoons each; for a thick sauce, 3 tablespoons each. These two basic sauces, a béchamel and a velouté, are the basis for classic French white sauces.

To make Roux, heat the fat in a pan, then gradually whisk in the flour. Cook the mixture, stirring constantly, for at least several minutes, then gradually whisk in the hot liquid you're trying to thicken. It must be cooked for at least 30 minutes in order to rid itself of the flour's starchy flavor, to thicken the liquid and to get rid of the flour's white color.  

TAPIOCA:  Instant or quick-cooking tapioca are little white "beads" made from the cassava root. It is nearly flavorless and gives a transparent gloss to fruits. It is most desirable in puddings and in juicy summer fruit pie fillings in a two-crust pie.  It is not recommended for a one crust or lattice crust fruit pie because the tapioca beads, which soften and rehydrate through baking, can be seen in the filling and as a result, are often called "fish eyes".  Also, the grains on the surface become hard and dry. See "How to use tapioca". 

Tapioca starch is a fine powder made from tapioca. It creates a perfectly smooth filling and imparts a high gloss for a tasty-looking result. It’s the perfect product to use with high-acid fruits or a lattice-type pie. If you find only pearl tapioca, just place it in a spice grinder, blender, or food processor and grind away. Now you have "instant" tapioca.

OTHER THICKENERS: Each thickener has a different attribute and is used based upon the recipe being made.

AGAR AGAR: Agar agar is a gelling agent made from a combination of algaes from the species gelidium. Other names include dai choy goh, Japanese isinglass, or kanten, in reference to the dish in which it is commonly used. The name, agar agar, is Malaysian in origin, and the harvest of the long red and purple fronds goes back hundreds of years. The fronds are freeze dried and dehydrated naturally, producing colorless sheets which are shaped into bars. Agar is available in the traditional bars, flakes, and powder, all of which can be used interchangeably for gelling purposes. Long strands of agar are one of the ingredients in the seaweed salad served at sushi restaurants.

The mechanics of cooking with agar differ slightly from those of gelatin. Generally, powdered agar can be substituted in equal measure for powdered plain gelatin. In substituting agar for gelatin, remember that agar may not set when mixed with vinegar or foods high in oxalic acid, like spinach, chocolate, or rhubarb. Agar gelled liquids will stay solid at room temperature, while gelatin will eventually melt. To use agar bars, rinse them in cold water, wring them out and tear in small pieces, then add to the cooking liquid. One agar bar is equal to four tablespoons of flakes or two teaspoons of powder, and one bar or its equivalent will gel two cups of liquid. Unlike gelatin, all forms of agar need to simmer for a while to dissolve, and letting them soak in the liquid for an hour or two gives you a head start. from http://www.tcwellness.com/issues/2001/08/12.html

CHOCOLATE: Did you know that ounce-for-ounce, unsweetened chocolate has more thickening power than bittersweet? Cocoa solids are rich in starches, with unsweetened having more. When it was used in a Ganache filling recipe, it was significantly stiffer and also had a viscous, gummy quality. When made only with bittersweet chocolate, it had a pleasantly smooth and creamy texture.

Comparable amounts of bittersweet or semisweet chocolate and unsweetened chocolate plus sugar will not produce identical recipes. While a direct swap might work well enough in fudgy brownies, it could wreak havoc on a delicate custard or filling. 

DAIRY: Cream, once reduced, gives sauces a rich texture and flavor as it thickens them, but it's high in fat. To make a low-fat cream sauce, use evaporated milk mixed with a starch thickener. Yogurt is a popular soup thickener in Eastern Europe and the Middle East.  

EGGS: Egg yolks make wonderful thickeners--imparting both a rich flavor and velvety smooth texture. You can't just whisk them into a simmering sauce--they'd curdle on contact. Instead, you need to "temper" them by adding some of the hot liquid to the egg yolks, whisking the mixture together, and then adding it to the sauce. To prevent the yolks from coagulating, you need to keep the sauce below 190 degrees F, although this rule can be broken if the sauce has a lot of flour in it. Finally, never cook sauces with egg yolks in aluminum pans or they'll turn gray.  

GELATIN: (How to Use) Unflavored gelatin is an odorless, tasteless and colorless thickening agent, which when dissolved in hot water and then cooled, forms a jelly.  

Gelatin is used in such recipes as Café Brûlot Chiffon Pie Recipe, stabilized whipped cream or in cheesecakes, as well as fillings custards or other desserts to thicken and stabilize it. To paraphrase a commercial slogan, there's always room for gelatin!

Gelatin commonly found in grocery stores is pure protein derived from animals, but there are other forms of gelatin to fit all types of diet restrictions, including vegetarian and kosher. 

Gelatin comes in a granulated from the grocery store or sheet form, called leaves, available from specialty stores or online. 

I prefer to use the granulated kind because it is readily available in the supermarket. I use Knox gelatin, which is widely available and is considered the standard in the US. It comes packaged in boxes of 1/4-ounce envelopes and is also available in bulk. Leaf gelatin comes in packages of paper-thin sheets. 

Any form of gelatin used must be rehydrated, heated, melted and dispersed, before the liquid will become jelled when cooled. Granulated gelatin needs to be rehydrated in a cool liquid before being dissolved over hot water. 

Gelatin's setting power has to do with the density of the liquid being set. Gelatin is always used in a ratio to liquids in the recipe: usually 1/4 ounce of powdered gelatin is needed to set 16 ounces of liquid. To obtain a "semi-solid" consistency, increase the liquid to 32 ounces. 

It is extremely important to achieve the correct ratio of gelatin to water, and the recipe will always tell you how much to use. For example, a finished cream firmed with gelatin, should have a tender and smooth texture, yet have structure. If it is incorrect, it will be either runny or too firm and rubbery.

Unprepared gelatin has an indefinite shelf-life as long as it is wrapped airtight and stored in a cool, dry place.

PECTIN: 2 tablespoons liquid pectin = 4 teaspoons powdered pectin.  Pectin is what makes jam happen. It's a natural thickening substance found in many fruits like strawberries and apples etc. Usually fruits that are slightly underripe are highest in pectin. This is why many older recipes (ones that you don't add powdered or liquid commercial pectin) call for ripe fruit and underripe fruit.

For the pectin to set you need both acid and sugar. This is why you can NOT deviate from the recipe in jam making. Old fashioned recipes will ask you to cook the mixture to the "jam stage" which is when you spoon some mixture and let it slide off the spoon. If it is not done it will just run off, if it is cooked correctly it will slide off, but leave a "sheet" attached to the spoon.

Newer recipes that call for added commercial pectin will have very specific directions. These directions are for the purpose of setting up the pectin and must be followed to the letter. In most recipes powdered pectin is added to preserves and jams, and liquid pectin is used in jellies.

some information from foodsubs.com