Why does a custard become semi-solid or increased viscosity when the liquid egg and milk mixture is cooked? What is the difference between a baked versus a stirred custard?

The key to custards is the correct denaturation of the heat-denatured egg protein. A stirred custard is prepared by taking the custard mixture and slowly heating it on a burner of a range, stirring slowly to permit even heating. It will likely heat from 85F to 92F. It depends somewhat upon the amount of sugar, size and amount of egg, in the proportion to the milk. It will become a viscous mixture if heating and removal is done correctly. A stirred custard is somewhat tricky. The difference between the optimum smooth "creamy" mixture and a curdled yukky mess may be 2 to 5 degrees Fahrenheit. If all other things are equal, the speed of heating will make a difference. Generally, the faster it is heated the fewer degrees that there will between from the time it "coats the spoon" to that where it curdles.

The baked custard is placed in the oven and heated undisturbed. This allows for the slow unwinding of the egg protein and the creating of new cross-bonds and formation of a gel. Certainly if overheated these bonds will tighten and shrivel and extrude liquid and be somewhat porous.

Precisely, a stirred custard is a viscous [thick] mixture; whereas, a baked custard is a semi-solid gel. This webber must say, she has always found the stirred versus baked custard, made from exactly the same mixture, is fascinating as it indicates how truly complex the denaturation of protein is. Proteins are large complex molecules and this translation into a denatured product reenforces this complexity.

Another point regarding custards is that rarely do you see mention regarding the affect of milk. Certainly, if you look at the custard references, you will find the work by Jordan and others that indicate the type of milk does affect a custard characteristics. In experiments by students we have found that undiluted evaporated milk will give a stiff gel and a thick stirred custard. Either will denature easily and overcook easily. The point that I am making is that the milk itself is important. There is a role of calcium in the production of the appropriate denaturation to make the custard products. If you try and make such custards with water the product just looks like bad bad scrambled eggs with lots of water. If one takes the water and adds a calcium salt at the approximately concentration as milk you get a somewhat better "custard" product; although it still is not as appetizing as a "normal" custard.

Updated: Wednesday, May 23, 2012.