Boiling a custard made with tempered egg yolks?
I made a cornstarch custard today by heating milk sugar and cornstarch, then tempering egg yolks with a little hot milk before adding the yolks to the pot, then putting it back on the heat to boil the mixture so that the cornstarch activates as a thickener.
I did not notice any grittiness from curdling eggs. Does tempering the egg yolks first mean that egg custards can be boiled without fear of curdling? Or did the cornstarch somehow prevent curdling?
"Does tempering the egg yolks first mean that egg custards can be boiled without fear of curdling?"
First, when you say cornstarch custard, did you make a pouring custard sauce or pastry cream, which has more starch in it? Same ingredients, almost the same formula with the exception of higher amounts of starch (either cornstarch or flour), somewhat different cooking technique. It's the larger amount of cornstarch or flour, as in pastry cream, that actually allows you to bring the mixure to a very soft gentle bubbling boil, to allow the starch grains to gel, but you can't do that with creme anglaise. From your description of technique, it sounds like you were making a pastry cream-type thick custard.
For creme anglaise, tempering doesn't work that way and does not prevent coagulation; it's the amount of starch in the recipe that will allow you to gently cook the custard to a higher temp. You can and will overcook the egg yolks if you boil them in a low starch custard. There's no reason to boil anyway, the custard will have thickened adequately by the time the egg yolks reach a certain temperature, 20-30 degrees below boiling. Btw, the generally accepted method for custard calls for beating the egg yolks and sugar together with the cornstarch or flour, then tempering in the hot milk or cream, then gently heating to the point of thickening. You can certainly heat the milk and sugar, add the cornstarch slurry, then temper in the yolks. Both the cornstarch and the egg yolk act as thickener in a stirred custard, the egg yolk being the primary gel formation for it.
You cannot boil egg yolks, even after tempering them. Egg yolks start to coagulate at 175°, anything above that is scrambled or hard boiled. The tempering step is to prevent the eggs from curdling by gently heating them without shock, when adding them to the hot milk. Cornstarch starts to gelatinize at around 175° as well.
Without seeing your recipe, I'd say that your custard didn't have "grittiness" (it's called curdling or coagulation) it's because your egg yolks did not reach the maximum temperature for that to occur, as in creme anglaise, or, if you did allow it to boil, you had a high ratio of starch to egg yolk/liquid in your recipe, as in pastry cream.
Thanks for the detailed reply.
Yes, I did use a lot of cornstarch - 2 tablespoons in about 2 cups of milk. I tried heating the milk cornstarch solution in a double boiler first to get it to thicken without boiling, but it didn't thicken enough. No, I didn't check the temperature. It was on the double boiler for almost 20 minutes, the mixture was steaming and I was tired of waiting for it to thicken. Hence, the desperation that led me to temper the yolks and boil.
Do you're saying that once the cornstarch mixture reaches about 180F, it should thicken to maximum degree without having to boil? I had read online that cornstarch needs to reach boiling to fully thicken.
I was in a rush and accidentally put the cornstarch into all of the cold milk before heating it. I was also worried that not using a cornstarch slurry might have affected the thickening in the double boiler. If I don't use a cornstarch slurry and the cornstarch is mixed with all of the cold milk and heated, will it fully thicken at 180F? Or do I have to use a slurry?
"180F, it should thicken to maximum degree without having to boil?"
No, it starts to gelatinize at that temp; it needs to cook just a bit further for maximum thickness. When a slurry is added to simmering liquid, it cooks completely inside of a minute or two. Of course, that depends on the amount of cornstarch used as well. Boiling a cornstarch thickened sauce will cause the starch cells to rupture and lose their thickening ability. I find that cornstarch, when heated, loses it's starchiness much more quickly than flour; no need to boil, which is one of the reasons I use it for pastry cream. I also prefer the lighter texture it bring to the end product. This is my personal preference; I know other chefs who prefer using flour for this type of custard.
Two tablespoons of cornstarch in two cups of milk will result in a medium-thick custard, approaching a pudding-like consistency when chilled; the addition of the egg yolks was necessary for further thickening and enrichment as well.
The only reason for the slurry step is to dissolve the cornstarch (well, technically, cornstarch doesn't dissolve. The definition of a slurry is a suspension of solids in liquid; chemically speaking, a suspension is a heterogeneous fluid containing solid particles that are sufficiently large for sedimentation. The solids in the suspension will eventually settle out. If you've ever made a cornstarch slurry and let it set for awhile, you know what I mean; it needs to be agitated again before using.) It's much easier to dissolve it thoroughly in a small quantity of liquid rather than a larger quantity.
In layperson's terms, I think of making a slurry as "wetting" the cornstarch. Cornstarch also has a tendency to create a miliion small lumps when mixed with a large amount of liquid, especially if the liquid is heated, as it gelatinizes quickly, and as opposed to it's behavior when it's made into a thin paste with a small amount of cold liquid first.
Whether you initially dissolve the cornstarch in a small quantity or all of the liquid will have no bearing on it's thickening power.
So that's probably more than you wanted to know, or maybe not, but I couldn't help myself. I enjoy talking about cornstarch.