What challenges does custard face and how can they be resolved? Check out these suggestions to help custard evolve and dissolve!
Custard can be made with eggs, starch or both, either by stirring on the stovetop or by being baked in the oven. Making custard recipes from scratch can be challenging for some, and the difficulties vary depending on how they are thickened – whether it’s starch-based (thickened with starch and eggs) or without starch (thickened solely by eggs). The most commonly encountered problems with custard, along with their solutions, are included here.
Starch only custard
Starch only custards require only some sort of starch to thicken up the liquid. Although any starch can be used to thicken the milk up, like all-purpose flour is used in case of Bechamel, the most popular choice when making desserts falls on corn starch. Using corn starch is the easiest method to make custard as it doesn’t really require much skill or practice. The milk and sweetener needs to be mixed with the corn starch while cold then slowly heated up until the mixture thickens.
Upon heating, starch granules absorb water and begin to swell and collide with each other, resulting in thickening of the mixture. The solution reaches its maximum thickness shortly after the gelatinization stage, which occurs between 175°F – 205°F / 80°C – 96°C. The starch granules release amylose and amylopectin starches into the liquid, further thickening the mixture as the long amylose chains form a web that traps the swollen granules.
Tips for using starch
- The more starch we add to the same amount of liquid the thicker the custard will be.
- Mind not to add corn starch to hot liquid as it will create lumps.
- All-purpose flour doesn’t react very well to hot liquids either but to dissolve evenly it requires a somewhat warm liquid.
Egg only custard
For egg only custard, the mixture should be cooked until it reaches 175°F – 180°F / 80°C – 82°C for crème anglaise. However, when preparing the base for ice cream, the desired temperature for thickness can range from 180°F – 185°F / 82°C – 85°C.
It’s advised to cook the mixture until a thin film adheres to a metal spoon when it is dipped into the custard, which is approximately 180°F / 82°C. This consistency, known as “nappé” in French, describes the thickness of a sauce, particularly a custard sauce, that coats the back of a spoon and retains its shape when a finger is drawn through it.
In general, egg-based puddings and custards can curdle if cooked beyond 185°F / 85°C, unless a thickening starch is included. Which makes egg only custards a bit of a nuisance.
The dairy-egg ratio for egg only custard starts at about 2:1 by weight. This means that 1 cup / 240g dairy will require at least 6 medium / 120g egg yolk to thicken. No wonder starch is used in most recipes to aid the process.
Curdling
Sweet custards (without starch) typically thicken between 160°F – 180°F / 71°C – 82°C , which is a bit below the boiling point of 212°F / 100°C milk or water. If the custard is heated beyond that point, the egg proteins lose their structure and can no longer retain the liquid. Consequently, a baked custard like crème caramel will appear curdled and runny, while a stirred custard sauce like Crème Anglaise may contain bits of scrambled egg.
When preparing a basic custard on the stovetop, it is recommended to stir the mixture and frequently monitor its temperature to ensure it does not exceed 180°F / 82°C. Baked custards should be removed from the oven when they exhibit a slight wobble in the center when nudged, as residual heat will continue to cook them until fully set.
When egg mixtures, such as custards or sauces, are cooked at high heat, the protein coagulates excessively and separates from the liquid, resulting in a mixture resembling fine curds and whey. If curdling is not severe, it can sometimes be reversed by removing the mixture from the heat and vigorously stirring or beating it. To prevent curdling, it is advised to use low temperatures (cook over a double boiler or bake in a water bath), stir as appropriate for the recipe, and cool the mixture quickly by placing the pan in a bowl of ice or cold water and stirring for a few minutes.
Can an overcooked stovetop custard with lumps (curdles) be fixed?
To remedy a custard with lumps, an immersion blender can be used. A quick pulse with the blender effectively breaks down the lumps, restoring a desired creamy texture. This could work even when the custard is refrigerated.
If lumps start to form during the preparation of the recipe, the custard should be removed from the stove and immediately transferred to a bowl. Pulse the mixture with an immersion blender for about 30 seconds, being cautious not to overprocess, as this can result in thinning the mixture. Strain the custard through a fine mesh strainer to remove any remaining lumps, and transfer it back into a clean pot to continue cooking the recipe.
Starch-egg custard
Thickening issues
A starch digesting enzyme called alpha-amylase is present in egg yolks. In order to achieve successful gel formation in the custard recipe, the enzyme needs to be deactivated by cooking the custard almost to boiling (slightly below 212°F / 100°C). Otherwise, the remaining enzymes break down the starch gel, resulting in a liquid consistency for the custard. (In the case of starchless custards, curdling can occur if cooked beyond 185°F / 85°C.)
When a custard is undercooked, it may initially seem thick but gradually turns into a soupy texture as the amylase enzyme attacks the starch and breaks down the custard, especially when refrigerated. A useful guideline is to cook the custard for 1 to 2 minutes after bubbles appear, while ensuring constant stirring.
It can be noted that refrigerated custard can be reboiled if it fails to set.
Weeping or Synersis
When a cornstarch-thickened recipe exhibits weeping, it typically indicates slight undercooking or overcooking. During the initial stages of cooking, water is loosely held by the cornstarch granules, and upon cooling, the water seeps out. To prevent weeping, it is important to bring the cornstarch mixture to a near full boil over medium heat and stir constantly for 1 minute.
Thinning
As the cooking temperature approaches boiling (around 212°F / 100°C), the starch granules reach their maximum size and rupture. This allows most of the starch molecules to escape into the recipe, releasing the absorbed water back into the mixture and causing thinning.
Instant Tapioca can be used to prevent thinning if necessary, as it undergoes additional processing to prevent granule rupture even at high heat. Moreover, it is less likely to have its thickening properties impaired by acids, such as lemon juice.
General troubleshooting
Burning
An even temperature is maintained through constant stirring in a stove top recipe, preventing the formation of protein bonds too early, which can set undisturbed in baked versions. The sides, corners, and bottom of the pan should be scraped while stirring.
It is recommended to use a flat whisk when making stirred stovetop custards as it allows for easier access to the corners of the pan.
Cracking
Overcooking or excessive heat exposure can be observed in a cheesecake that has cracks. When overheated eggs cool down, they shrink, resulting in either a large crack running through the center or tiny cracks all over the top.
Using a water bath can help prevent this issue, although it is important to note that an overbaked recipe is simply an overbaked recipe.
Curdling
In order to prevent overcooking of the eggs in recipes, cornstarch can be added. When a recipe contains starch thickeners (such as flour or cornstarch), it allows for more leeway between success and failure of the egg-rich mixture, although issues can still arise. The protein coagulation is slowed by the starch molecules, making the egg proteins more resistant to overcooking and curdling (clumping).
Filling seeps through pie crust
It is possible for custard to seep through the pie crust, resulting in its demise. To prevent this, the crust should be made moisture-proof. Prebaking the crust and allowing it to cool is one way to achieve this. For instance, brushing the bottom with melted chocolate and letting it harden can be effective. Afterward, the pie can be filled and chilled.
Gritty, sandy, or grainy texture
Custard can have a gritty, sandy, or grainy texture. There is limited research on the specific causes of this phenomenon, particularly in the case of pumpkin pies, which are technically custard pies. Yet, lactose crystals, especially when evaporated milk (or any milk product) is used as a major ingredient, likely contribute to the sandy texture. Lactose, also known as “milk sugar,” is the only carbohydrate present in significant quantities in milk. Due to its lower solubility in water compared to table sugar, lactose crystals readily form in products like condensed, evaporated, or powdered dry milk, resulting in a sandy texture. Additionally, sugar crystallization can occur, and the inclusion of a small amount of corn syrup can help counteract this issue.
Shrinking and cracking
Overcooking leads to excessive coagulation of the egg proteins in the recipe, which causes shrinkage and cracking when cooled.
Thin crust forms
When refrigerating freshly cooked or baked custard, it is recommended to allow it to cool for approximately 5 to 10 minutes before covering it with plastic wrap. The plastic wrap should be in direct contact with the surface of the custard to prevent the formation of a thin crust on top caused by the milk proteins.
To ensure proper contact, pierce the plastic wrap in multiple places using a sharp knife or toothpick before covering the custard. Subsequently, refrigerate the custard to allow it to set.
Weeping or Synersis
The terms “weeping” or “synersis” are commonly used in reference to pie meringues, baked custards, and cheesecakes. If custard is overcooked, the protein bonds become tighter, resulting in increased thickness, curdling, and the release of water, which is observed as small tunnels in the custard.
If ever needed to pour out water from a refrigerated baked cheesecake it is because of weeping due to overcooking of the cake.
White stringy part of the egg is in the smooth custard
Before preparing custard, it is advisable to remove the chalazae or white stringy part of the egg, which are small white strings attached to the egg. After cooking with stirred custard, any remaining strings can be caught by straining the custard through a fine mesh strainer. This process can improve the custard’s texture.