Through the years, I have had many calls about lumpy gravy and an amazing number of “Why wouldn’t my scalloped potatoes cook? The potatoes are rock-hard after two hours.” So, how do potatoes cook? As the potatoes heat, the starch granules in them take in liquid to swell and soften to “cook.” With both lumps and non-cooking potatoes, we are dealing with starch.
How Starch Works
Starch granules contain two kinds of starch (amylose and amylopectin) packed together layer on top of layer like an onion. When you stir starch granules into cold water, a little water begins to seep in. As you heat the starch and water, more and more water seeps in. Somewhere between 66°C/150°F and 100°C/212°F (depending on the type of starch), the granule may hold hundreds of times its weight in liquid. When the granule can swell no more, it pops. Starch rushes out into the liquid. This is when your sauce or gravy thickens.
Thinning
Both the starch particles and the big, empty, jellyfish-like granules contribute to the thickening. You don’t want to stir too vigorously, or you might smash these big empty granules and the sauce or gravy will thin.
Lumps!
If you spoon dry starch (flour or cornstarch) into a hot liquid, the starch on the outside of each lump will swell instantly to form a gel coating, keeping the lump dry for you to bite into. To avoid lumps, the three classic methods of preparing starch-bound sauces spread the starch out grain by grain. In a roux, the starch is blended with fat. In a slurry, the starch is stirred into cold water to disperse it. In a beurre manié, equal portions of butter and flour are worked together. Then, when this mixture is added to a hot liquid, the butter melts and disperses the flour grain by grain—no lumps.
Starch Will Not Swell–Rock-Hard Scalloped Potatoes
Potatoes or rice can remain rock-hard after prolonged cooking. Starch will not swell if it is acidic. If you have a lot of sour cream in your scalloped potatoes or add something acidic, your potatoes may not soften. You can cook the potatoes with heavy cream or milk and then, after they soften, stir in sour cream. If you have a lot of very acidic tomatoes in a paella, the rice may never soften.
Pat and Betty, chefs for Reynold’s Wrap, were developing whole meal (meat, vegetable, and starch) recipes that were wrapped in individual packets for each member of the family and would cook in 20 to 25 minutes. To simplify the recipes, they used salad dressings to season instead of using salt, pepper, herbs, and oil. They had a recipe in which the potatoes would not cook. They thought the potato pieces were too big, but when they got them down to the size of rice and they were still uncooked, they called me.
The problem was the really acidic vinaigrette salad dressing that they were using. When they switched to a milder dressing the potatoes cooked perfectly.
This is also the reason that lemon pie recipes that are very lemony will instruct you to heat the starch, water, and sugar until the starch thickens and then add the lemon juice and egg yolks.
Starch in Other Thickening
You may have seen a rice pudding with the rice in a dense layer on the bottom and the custard on top. This was probably made with refrigerated leftover rice. When cooked grain starches are chilled, the starch crystallizes into a firm network. When reheated, the crystals will melt and the starch will soften. But, such a starch will never exude starch to thicken. So, in a rice pudding made with this rice, the rice cannot contribute any thickening.
The pudding is not thick enough to hold up the rice so it sinks to the bottom. A good restaurant should use their leftover rice. They can sprinkle a few tablespoons of cornstarch over their rice, then blend it in with the other ingredients for a regular rice pudding. If you use leftover rice for my Golden Brown Puffed Cheesy Rice Spoon Bread (see recipe), the tablespoon of cornstarch is vital.
Starch Prevents Curdling
To the cook, starch can be a great hero by preventing curdling. Proteins (egg or dairy proteins) can join when heated, exposed to acid or even air, and can become a curdled mess.
For example, if you were converting a high-fat recipe with heavy cream into a low-fat recipe with skim milk or low-fat yogurt, you can get curds. The proteins in the cream unwound with heat but they were coated with fat so they did not join to form curds. But low-fat milk or yogurt has loads of proteins and no fat to keep them apart so you get a mass of curds.
Most home-use starches like cornstarch, flour, tapioca, potato starch, or arrowroot can prevent this curdling. The exact mechanism is not known. It may simply be that the starch swells enough to be “in the way” and prevents proteins from joining. A friend of mine who taught classes in low-fat cooking would stir a tablespoon of cornstarch into a quart of low-fat yogurt and then use that to make creamy sauces and quiche.
Starch Prevents Shrinkage of Meringues
Egg white proteins in meringues shrink when you cook a meringue. Not only does the meringue pull away from the edge of the pie, it becomes difficult to cut. Starch can prevent this. Stir 2 teaspoons cornstarch into 1/3 cup cool water and heat until it thickens to a paste. After adding the sugar, near the end of beating the meringue, add this starch paste a teaspoon at a time. It will interfere with the egg white proteins’ tight coagulation and will make a meringue that shrinks less and cuts without pulling or tearing.
Pie to Soup Overnight
In raw egg yolks there is an enzyme, alpha-amylase, that loves starch. If a pie filling like coconut cream pie is not heated to a FULL boil after the egg yolks are added, this enzyme will wipe out your starch and thin the filling overnight in the refrigerator. The custard is thick and when you boil it, it sticks and goes blop! blop! But, you must get the entire custard over 82°C/180°F to kill all of the enzymes.
Custard Thinning
If you stir a custard after it has cooled and set–say to stir in Grand Marnier–some of the starch bonds will be broken and the custard will thin. 
