Low Heat

• Pastry creams, or custards that use starch as a thickening agent, contain amylose, a polysaccharide that gives fat structure. Egg yolks contain alpha-amylase, a protein enzyme that breaks down amylose. Alpha-amylase loses its effectiveness when heated to low-simmering temperatures for two minutes, or around 180 to 185 degrees Fahrenheit. The starch in sweet custards binds with the fat and proteins when it gelatinizes, so you can heat them past the coagulation point of the egg yolks, or 158 degrees Fahrenheit. To keep starch custards from separating, cook them until they barely bubble, then maintain the simmer for a timed two minutes, stirring constantly to keep the simmer in check. Remove the custard from the heat as soon as two minutes elapses.

High Heat

• Basic custards, or those that don't use starch or gelatin to set, rely on an emulsion to keep them together. Emulsions consist of a dispersed phase, such as the fat in custard, and a continuous phase, such as the water, held together with an emulsifier, such as lecithin. Lecithin loses effectiveness at around 180 degrees Fahrenheit, causing the continuous and the dispersed phases to destabilize, or causing the custard to separate. Basic custards thicken between 160 and 180 degrees Fahrenheit, perilously close to the separation point. To prevent a basic custard from separating as a result of too much heat, cook the custard to 180 degrees Fahrenheit over a double boiler until it thickens and sets, using an instant-read thermometer to monitor the temperature and a whisk to control the temperature increase by slowing it down if needed.

Enzymes

• Gelatin-set custards have more stability than basic and starch-set custards, but they have an Achilles heel: enzymes, specifically those used to tenderize meat. Bromelain, papain and actinidain, the enzymes found in pineapples, papaya and kiwifruit, respectively, do the same thing to gelatin-set custards they do to tough cuts of meat: break them down. Gelatin-set custards separate within a few hours if they contain the aforementioned fruits, and you can't save them once they do. To keep gelatin-set custards that contain fruit from separating, cook kiwifruit, papaya, pineapples, guava, mango and figs before adding them. Canned fruits undergo pasteurization, so you can add them straight from the can.

Prevention Options

• Sometimes you need a little extra help keeping your custard together, especially when environmental conditions, such as an excessively hot kitchen, interfere with the custard's emulsion stability. You can add one of two ingredients to prevent custard separation: granular soy lecithin, which holds custard together under extreme temperature fluctuations, and corn starch, which binds to proteins and give them stability. Add 1 teaspoon of granular lecithin to any custard when mixing for separation prevention, or add 1 tablespoon of corn starch for every cup of custard to improve stability. Lecithin is more effective than corn starch at stabilizing custard because you need less of it and it doesn't change the custard's texture or mouthfeel.

Custard Pies

• Shrinking naturally occurs in all baked goods as a result of moisture loss during cooking, but it's highly visible in custard pies because the custard and the crust shrink away from each other. Although you can't eliminate the separation between the custard and the crust, you can minimize it by blind-baking the crust before filling it and letting the pie cool to room temperature after baking it before placing it in the refrigerator.