In the last post, we focused exclusively on flour and the role of its starch in baked goods. Today, we’ll explore how starch’s chemical properties make it useful in desserts such as buttercream, meringue, and custard. We’ll also consider how other ingredients affect starch in these recipes.
Starch is a drier.
As we discussed in the last couple posts, starch absorbs water. This makes it useful for extending the shelf life of products such as baking powder and powdered sugar, so these ingredients are mixed with starch. Without starch, for example, baking powder would react and lose its leavening power as soon as it came into contact with moisture from the air. Instead, that moisture is absorbed by the starch, and the baking powder remains fresh. Similarly, in powdered sugar, cornstarch keeps the sugar dry so it doesn’t doesn’t dissolve and clump.
Starch can also stabilize products such as buttercream frosting. American buttercream contains powdered sugar, which already contains some cornstarch, but extra starch can help the buttercream hold in humid environments. The starch absorbs moisture from the air, preventing the sugar from dissolving into a syrup that would thin the frosting.
Starch prevents egg protein structure from forming.
As we discussed in the last post, starch granules swell but do not gelatinize in low-moisture environments. The enlarged granules act as physical barriers to protein network development and ultimately tenderize the baked good. In egg-based products, such meringue and custard, starch similarly prevents protein coagulation.
Meringue
As you may recall, meringues are made from egg whites. When we beat air into the whites, we force the proteins into a structure that accommodates both air and water. Meringues also contain sugar, which draws water out from the proteins to stabilize the structure. Soft meringues, which are often used for topping pies, contain less sugar, so their raw egg proteins are loosely connected around a lot of water molecules. When the meringue is heated, the proteins draw together, the water cooks off, and the meringue shrinks. The resulting meringue is difficult to slice, and the edges of the pie are exposed.
To remedy this, Shirley Corriher suggests adding starch to the meringue. As with baked goods, swollen starch granules interfere with the proteins and prevent them from tightening when they’re baked. The resulting meringue is easy to slice and does not shrink. And since the proteins are not packed as tightly, the texture is more tender.
Custards
We can also use starch in custards, which are made from eggs, sugar, and milk or cream. As with meringues, swollen starch granules prevent the egg proteins from clumping together, and that makes all the difference in the cooking method. Custards without starch, like crème anglaise and other stirred custards, are heated carefully over a double boiler to ensure that the eggs thicken but do not curdle. However, if we add some starch to that same mixture to make pastry cream, we can cook it over direct heat without a thought for the eggs. The starch prevents the proteins from coagulating no matter how hot they get. If pastry cream clumps, it’s because of the starch, not the eggs.
The addition of starch also changes the texture of the custard. Custards without starch dissolve in the mouth smoothly, while those made with starch have more bite from the swollen granules.
Factors that affect thickening power
Egg yolks
If you use starch to thicken a custard with egg yolks, it’s important to boil the custard. Egg yolks contain amylase, the same enzyme that breaks glucose units off starch for yeast to eat. If these enzymes are not deactivated, they will continue to do their job and break down large starch molecules. The fragmented starch does not trap water effectively, so the custard will thin as it sits. Boiling the custard ensures that it reaches temperatures high enough to denature and deactivate the amylase so the custard stays thick.
Sugar
As we discussed in the last post, sugar pulls water away from starches. With less water available, starches need more energy to gelatinize, so they will not thicken until they reach higher temperatures. In fact, high concentrations of sugar prevent custards from setting altogether. To work around this, some recipes gelatinize the starch with only a fraction of the sugar, then add the remainder after the custard has thickened.
Acid
Custards also remain thin if they contain too much acid. As we discussed in the context of candies, acids separate individual links in a carbohydrate chain. Like amylases, acids break down large chains of starch, so acidic custards do not contain the large starch molecules necessary for thickening. In lemon custard recipes, the lemon juice is added to the filling after the custard has thickened. If it were added earlier, the custard would never set.
Stirring
However, in lemon custards, the lemon juice must be stirred in before the custard cools. If we wait too long, the gelatinized network of starch molecules, swollen granules, and trapped water sets. And if we stir the custard at this point, we rip through this structure and thin the custard. For the same reason, flavorings should be stirred into pastry cream before it cools.
When we add raw starch to a cold liquid, however, it’s important to stir well. We want to separate individual starch granules so that they do not clump when we heat them. If the starch granules are stuck together, the ones on the surface swell and seal together when they’re cooked, leaving the granules inside dry and unable to thicken. The result is a lumpy, thinner custard.
Conclusions
Starch is crucial for custards, and it’s also useful for stabilizing toppings like buttercream and meringue. But some starches work better than others for specific applications. Although we’ve been describing starches broadly so far, starches’ behavior also depends on factors such as their ratio of amylose to amylopectin. In the next post, we’ll describe some of the differences between these two types of starch molecules and how to choose between them.
References
Corriher, S. O. Bakewise; Scribner: New York, 2008.
Corriher, S. O. Cookwise, 1st ed.; William Morrow and Company, Inc.: New York, 1997.
Figoni, P. How Baking Works, 3rd ed.; John Wiley & Sons, Inc.: Hoboken, 2011.