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Custard can be made with or without starch, cooked by stirring on the stovetop or baked in the oven. Making custard recipes from scratch can be difficult for some, and problems differ by how they’re thickened - starch-based (thickened with starch and eggs) or without (thickened entirely by eggs); I have included the most commonly asked custard problems with solutions. If it is not for you, try using instant pudding mix or already made pudding from the grocery store! There is also Bird's Pudding Mix, which is also good. Go to Baking - How to Test for Doneness.
If you have additional questions, please post them in our baking911.com Forum, and we will be happy to answer them.
Won't thicken: Egg yolks have a starch digesting enzyme called alpha-amylase. In order for a successful gelling of a starch in the recipe, the enzyme has to be killed by cooking the custard almost to boiling (a little less than 212 degrees F). Otherwise the left-over enzymes digest all of the nice firm starch gel and your custard is nothing but liquid. (All other custard cooking methods without starch can curdle if cooked beyond 185 degrees.)
An undercooked custard may initially appear thick but will slowly turn to soup as the amylase enzyme attacks the starch and breaks the custard down, usually as it sits under refrigeration. A good guideline is to cook for 1 to 2 minutes after bubbles appear in the custard, stirring constantly.
SARAH SAYS: You can reboil refrigerated custard if you discover it hasn’t gelled.
Weeping or Synersis: When a stirred cornstarch thickened recipe weeps, it is usually a sign of slight undercooking or overcooking. In the early stage of cooking, the water is held rather "loosely" by the corn starch granules, and when the mixture cools, the water simply runs out. To stop weeping, just be sure to bring the corn starch mixture to almost a full boil over medium heat and, stirring constantly, for 1 minute.
Thinning: Starch granules absorb water when heat is introduced, and they start to swell and bump into one another, thickening the mixture. The solution reaches its thickest point just past the gelatinization stage, which occurs between 175 and 205 degrees F. The starch granules begin to leak amylose and amylopectin starches into the liquid. The mixture thickens further because the long amylose chains form a web that traps the swollen granules.
As the cooking temperature reaches somewhere near boiling (near 212 degrees F), the starch granules swell to their maximum size and burst open. Here most of the starch molecules escape into the recipe, releasing the water that had been absorbed by them to escape back into the mixture, resulting in thinning.
SARAH SAYS: Use Instant Tapioca; it is further processed to avoid bursting granules even at high heat. It also less likely to have its thickening powers impaired by acids, such as lemon juice.
BASIC CUSTARDS WITHOUT STARCH
In general, egg-based puddings and custards can curdle if cooked beyond 185 degrees, unless a thickening starch is present. Recipes such as crème anglaise are done when the mixture registers 175 to 180 degrees F, but when making the base for ice cream, the temperature for doneness can be 180 to 185 degrees F for maximum thickness.
SARAH SAYS: For these recipes, sometimes the instructions will tell you to cook the mixture until a thin film adheres to a metal spoon dipped into the custard, which is about 180 degrees F. (The term is called nappé, the French term to describe the consistency of a sauce, especially a custard sauce, is thick enough to coat the back of a spoon and hold the shape of a line when a finger is drawn through it.)
Curdling: Sweet custards (without starch) typically thicken between 160 degrees F and 180 degrees F, well below the boiling point of 212 degrees F. If you go past that point, the egg proteins lose their shape and can no longer hold liquid, so a baked custard like crème caramel will appear curdled and runny, and a stirred custard sauce like Crème Anglaise may have bits of scrambled egg in it.
SARAH SAYS: When cooking a basic custard on the stovetop, stir the mixture and then check its temperature frequently to ensure that it doesn’t go above 180 degrees F. Remove baked custards from the oven when they have just a slight wobble in the center when nudged; residual heat will continue to cook them until fully set.
When egg mixtures such as custards or sauces are cooked too rapidly, the protein becomes overcoagulated and separates from the liquid leaving a mixture resembling fine curds and whey. If curdling has not progressed too far, it may sometimes be reversed by removing the mixture from the heat and stirring or beating vigorously. To prevent curdling, use a low temperature (cook over a double boiler or bake in a waterbath), stir, if appropriate for the recipe, and cool quickly by setting the pan in a bowl of ice or cold water and stirring for a few minutes.
Question: I overcooked my stovetop custard, and it formed lumps (curdled). Can I fix it?
SARAH SAYS: To find a fix custard for lumps, use an immersion blender. A quick pulse with it will effectively break them down, restoring to a desired creamy texture, even when refrigerated.
If lumps are starting to form during the making of your recipe, remove the custard from the stove and immediately pour it into a bowl. Pulse the mixture with an immersion blender for about 30 seconds, taking care not to overprocess, otherwise you will thin the mixture. Process the custard through a fine mesh strainer, to remove any remaining lumps, back into a clean pot and continue cooking your recipe.
Burning: The constant stirring with a stove top recipe maintains an even temperature so the protein bonds don't form too early, which are allowed to set undisturbed in baked versions. Make sure you scrape the sides, corners and bottom of the pan while stirring.
SARAH SAYS: I like to use a flat whisk when I am making stirred stovetop custards because I can get into the corners of the pan more readily.
Cracking: You can also see overcooking or exposure to too much heat in an cracked
cheesecake. Overheated eggs shrink when cooled causing with a large
crack running through its center or tiny cracks all over its top.
SARAH SAYS: A water bath can help prevent this, but an overbaked recipe is an overbaked recipe.
Curdling: I adapted the Libby's Classic Pumpkin Pie Recipe, with my additions for a creamier filling! I added some cornstarch to the recipe to help prevent the eggs in the recipe from overcooking. If a recipe contains starch thickeners (flour or cornstarch) it gives more lead way between success and failure of the egg-rich mixture, but problems can still occur. The starch molecules slow protein coagulation, making the egg proteins more resistant to overcooking and curdling (clumping).
SARAH SAYS: Another way I solved this problem was to bake the pumpkin pie in a tart pan so the recipe's filling has less of a tendency to overbake because it bakes evenly. See my Easy Pumpkin Pie Tart Recipe.
Filling seeps through pie crust: Custard can seep through the pie crust, ruining it. Moisture proof the crust. Prebake the crust and allow it to cool. One way is to brush the bottom with melted chocolate and let it harden. Fill the pie and chill.
Gritty, sandy, or grainy texture: Custard can be gritty, sandy, or grainy in texture. I have discovered that there isn't much research on why this happens, especially with Pumpkin Pies, which are technically custard pies. So, I set out to see what causes this. SARAH SAYS: as of 11-2013 Further research on my part shows that grittiness, sandiness, or graininess in custard pies, such as pumpkin, is probably caused from lactose crystals especially forming when evaporated milk (or any milk product) is used, a major ingredient in the Libby’s Pumpkin Pie recipe. Lactose is the only carbohydrate found in any quantity in milk is also peculiar to milk and is named lactose or “milk sugar.” "Lactose is one-fifth as sweet as table sugar, and only one-tenth as soluble in water, so lactose crystals readily form in such products as condensed (and evaporated or powdered dry) milk* (and ice cream) and can give them a sandy texture." (On Food and Cooking, Harold McGee, Scribner, 2004, page 18) As the moisture from the pie evaporates during cooking, especially during overooking, this triggers crystals to form in the pie filling, hence a gritty, sandy, or grainy texture. Sugar crystallization can also occur, and a small amont of corn syrup will help counteract this.
Research: *“Milk also contributes to smoothness, thanks to its high concentration of milk solids (such as calcium salts and lactose), which are even more effective than fat at controlling crystal size. Condensed, evaporated, or powdered dry milk, like milk, these ingredients have lots of milk solids, so (ice) crystals stay small. But they’re also brimming with lactose (milk sugar)....But beware, if there’s too much of these products, lactose crystals will form and you’ll end up with a sandy-textured (recipe)". ("Getting the Best Texture in Ice Cream," Fine Cooking, by Shirley Corriher, 12-3-2013). In a solid state, lactose appears in a crystalline form.
Also, when custards are overheated, the egg proteins over-coagulate and grainy curds occur in the cream - or, what you feel as a grainy texture in the pumpkin pie filling. A small amount of cornstarch (or flour) added to the recipe helps with this. The starch molecules slow protein coagulation, making the egg proteins more resistant to overcooking and curdling (clumping).
So, I changed the Libby's Classic Pumpkin Pie Recipe and included cornstarch, and a mix of cream and milk, in place of the evaporated milk, plus some corn syrup, to ensure that the recipe stays creamy and smooth. I made a note with my adapted Libby's Classic Pumpkin Pie Recipe.
Shrinking and cracking: Overbaking will cause the egg proteins in the recipe to overcoagulate from too
much heat which eventually shrink (and crack) when cooled.
Thin crust forms: If refrigerating just cooked or baked custard, let it cool about 5 - 10 minutes and then cover it with plastic wrap. Make sure it touches the surface of the custard to prevent the milk proteins from forming a thin crust on top when refrigerated.
SARAH SAYS: Pierce a piece of plastic wrap large enough to cover the custard, in about a dozen places with the tip of a sharp knife or a toothpick; place pierced plastic wrap directly onto surface and refrigerate to set.
Weeping or Synersis: are more often used with reference to pie meringues or baked custards, plus cheesecakes. If custard is overcooked, the more tightly the proteins join together. They becoming thicker, curdling and squeezing out all the water which you see evidence of coming from little tunnels in the custard, called synersis.
SARAH SAYS: Do you ever have to pour water from a baked cheesecake after is has been refrigerated? It is because it is weeping due to the cake being overcooked.
White stringy part of the egg is in my smooth custard: Before making custard, the chalazae or white stringy part of the egg should be removed. They are small white strings that are attached to the egg. After cooking with stirred custard, catch any left-over strings by straining it through a fine mesh strainer. By doing so, you'll greatly improve the custard's texture.
QUESTION: Egg yolks seem to “cook” when we mix the sugar and yolks to make pastry cream. How do we prevent this?
SARAH SAYS: When combining egg yolks and sugar, make sure that you mix together immediately, otherwise the yolks will quickly dry out from the undissolved sugar, called "burning". This will leave you with hard bits in the finished recipe i.e., crème anglaise, pastry cream, lemon curd, etc.