- The Washington Times - Tuesday, March 21, 2006

Cooks use starches to thicken sauces and gravies, to set pies and custards, to prevent proteins from curdling — and even in cakes. Most cooks have flour and cornstarch in their pantry. So do we really need any other thickening starches? The answer, according to food scientists like me, is yes. And I will tell you why.

Starches look like fine powders. They are not composed of cells or crystals, though, but are a semi-crystalline structure scientists call “granules.” These granules contain starch molecules packed together, layer on top of layer, like an onion.

When we stir starch granules into cold water, a little water begins to seep into the granules. As we heat the starch and water, more and more water seeps in. At 90 degrees, a starch granule can hold more than a hundred times its weight of liquid.

Between 150 and 212 degrees, depending on the type of starch, the granule may hold more than a thousand times its weight. At this point, the granules pop, and starch rushes out into the solution. This is when the sauce or gravy thickens.

When making a sauce, we can’t see the starches swelling. So we may give up and add a little more cornstarch dissolved in cold water to hasten the job. Then, all at once, the magic temperature is reached and the starch granules pop and the result is food glue … not exactly what we had in mind. One of the secrets to perfect sauces, then, is to wait until they reach a gentle boil before adding any more starch.

All starches can absorb a large weight of water, but the granules are different sizes and swell to different sizes. For example, cornstarch swells to about 20 times its original volume before it pops, while potato starch can swell to 100 times its original size.

Huge granules give potato starch interesting characteristics. Because of the large granules, it takes much less potato starch to thicken something. Since these large potato starch granules are full of moisture, a pastry chef I know, Bruce Healy, substitutes a small portion of potato starch for flour in his pound cake to make a moister cake. The amount he uses (about a 10 percent substitution), just loosens the extremely tight structure of a regular pound cake, although visually, the structure looks the same. However, another kind of cake — Danish sand cake — sometimes makes use of such a large amount of potato starch (equal amounts of flour and potato) that the large starch granules give the cake a coarse texture.

How much starch should we use for any given recipe? In general, the amounts of flour per cup of liquid for a sauce are:

Thin sauce: 1 tablespoon flour per cup

Medium sauce: 2 tablespoons flour per cup

Thick sauce: 3 tablespoons flour per cup

But other starches are completely different. For example, to thicken 1 cup of liquid with grain starches:

Cornstarch: 1 tablespoon plus 1 teaspoon per cup of liquid for a medium sauce

Flour: 2 tablespoons per cup for a medium sauce

Rice starch: 1 tablespoon plus 1/2 teaspoon per cup for a medium sauce

Yet to thicken 1 cup of liquid with root and tuber starches, we need:

Arrowroot: 1 tablespoon plus 1 teaspoon per cup of liquid

Potato starch: 21/4 teaspoons per cup

Tapioca starch (available at Asian markets): 1 tablespoon plus 1 teaspoon per cup

Quick tapioca pudding: 1 tablespoon plus 1/4 teaspoon per cup

How can we know which starch to use for a recipe? Each granule contains two kinds of starch molecules. They are amylose (a long, straight, bar-shaped molecule), and amylopectin (a little branched molecule with points sticking out in many directions like a multipointed star).

When amylose reaches the temperature at which the granule pops and the starch molecules rush out into the sauce, both the long, bulky amylose molecules and the puffed, empty granules contribute to the thickening.

When the starch cools, amylose molecules bond to each other to make a firm, slightly opaque gel. Amylopectin molecules and the empty granules contribute thickening, but the little amylopectin molecules do not join together to form a solid gel. So these two starches have very different characteristics.

Amylose makes sauces and gravies that are clear when hot but cloudy when cooled and very thick when cold; in fact, they are thick enough to cut with a knife.

Sauces or gravies made with high-amylose starch freeze badly, forming a dry spongy mess in a runny puddle. Amylopectin makes sauces that are crystal clear, hot or cold, but never get firm enough to cut with a knife. High amylopectin sauces freeze and thaw beautifully.

How do we know which starches contain a lot of amylose or a lot of amylopectin? Ordinary grain starches, such as wheat and corn, are relatively high in amylose, containing about 26 percent, while root starches, such as arrowroot and tapioca, contain only about 17 to 21 percent. Potatoes are a tuber, not a true root, so potato starch falls somewhere in between with about 23 percent amylose.

Some types of cereal starches called “waxy” (for example, waxy cornstarch, which is not regular cornstarch) are very high in amylopectin (99 percent). These high amylopectin starches and modified starches (man-made starches that are altered to have specific characteristics) are mainstays of the frozen food industry.

So sometimes we need one kind of starch and sometimes another. Coconut cream pie must be firm enough to cut, and it doesn’t matter if it is opaque.

For cherry pie, we want a clear thickener because it would be a shame to have a cloudy covering on bright red cherries. Yet sometimes we can have our cake and eat it, too. In cherry pie we can use mostly tapioca starch and add just a little cornstarch to make it thicker but not enough to cloud the appearance.

As home cooks, we have a number of starches available. Cornstarch and flour are usually in the baking section of our supermarket, and arrowroot is in the spice area. Most large markets also sell potato starch, but it is usually kept with the Jewish baking ingredients, near the matzo.

A good place to find a variety of starches is in Asian markets. They sell tapioca starch (not pearls, as used in tapioca pudding), rice starch, arrowroot and many others.

Selecting flour as the starch for the delicious fillets en papillote recipe that follows was an easy choice. I wanted a rich, thick, opaque sauce, not a clear one, and I had no intention of freezing it. So I needed a grain starch. I further elected to make the sauce with a roux because sauces made with a roux are generally a little more stable.

This is an impressive company dish. Cut around the top edges of each browned parchment packet, but leave it with the top covering each packet. Show guests how to roll back the top with their fork (and avoid getting burned by the steam) and let the wonderful aromas waft out.

I usually serve broiled tomato halves seasoned with vinaigrette and topped with buttered bread crumbs, along with asparagus that has been grilled or broiled for only a few minutes and seasoned with olive oil, salt, a pinch of sugar and lemon zest. (Do not use lemon juice, which will discolor the asparagus.) I have tried this recipe with all cream, but it was too rich, causing the sauce to break, so it is important to use a little milk. Be sure to butter both sides of the parchment so that the packets brown and make a handsome presentation.

Fillets en papillote

2 tablespoons melted butter for greasing parchment

4 scallions, chopped

2 medium shallots, chopped

1 teaspoon fresh thyme or 1/4 teaspoon dried thyme leaves

1 bay leaf

4 tablespoons butter, divided

3 tablespoons all-purpose flour

½ cup dry white wine

3/4 cup heavy or whipping cream (see note)

1/3 cup milk

6 ounces crabmeat, picked free of shell

1 cup diced raw shrimp

4 thin mild fish fillets, such as flounder, sole, orange roughy or tilapia (about 1 pound)

Tear off 4 8-by-11-inch pieces of parchment and brush on both sides with 2 tablespoons melted butter. Set aside.

Briefly saute chopped scallion, shallots, thyme and bay leaf in 3 tablespoons butter in a large skillet. Add flour and cook on low heat, stirring constantly, for several minutes. Remove from heat. Remove and discard bay leaf. Whisk in wine, a little at a time, and return to heat for a minute, whisking constantly. Whisk in cream and milk. Simmer, stirring constantly, until thick.

Saute crab and shrimp in 1 tablespoon butter just until shrimp is pink, about 1 minute. Stir crab and shrimp into cream sauce.

Place a fish fillet on each piece of parchment and top with one quarter of sauce. Fold parchment over and tightly roll up edges to seal. Bake in preheated 400-degree oven for about 8 minutes to heat through fish and lightly brown the parchment. Makes 4 servings.

Note: This is a typical French cream sauce, a lower-fat version can be made with all milk. If you prepare and assemble ahead, cool the sauce completely before spooning it over the fillets and refrigerate immediately. Increase the baking time to 10 minutes.

Food scientist Shirley O. Corriher is author of “CookWise: The Hows and Whys of Successful Cooking” (William Morrow).

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