Why heat milk and use it to temper eggs instead of mixing cold milk and eggs and slowly cooking the whole thing?

Question

My understanding of tempering (similar to the answer here) is that you're attempting to add egg to a hot mixture slowly enough so that you don't curdle the egg, so that it incorporates in the mixture instead of making, say, scrambled eggs inside of whatever dish you're meaning to be making. You would accomplish this by taking a small part of the hot mixture, add it to your egg mix while stirring quickly, and when the overall temperature of the egg has risen adding the entire egg mixture to the rest of the hot mixture.

But why is this necessary? Why couldn't you, say, mix the egg with the other ingredients while everything is still cold, and then slowly raise the temperature of the whole mixture? For example, when mixing a custard why can't you mix the sugar, vanilla, eggs, and milk together, then then raise the heat slowly in a saucepan?

Answer 1

Why couldn't you, say, mix the egg with the other ingredients while everything is still cold, and then slowly raise the temperature of the whole mixture?

You could (and when cooking by sous vide, that's exactly what you do).

It's the "slowly raise the temperature" that gets you.

Sous vide guarantees that not one bit of the mixture will ever get above the set temperature.
But when the mixture is heated in a pot, the bottom surface of the pot will almost always get hotter than the maximum desired temperature of the food, so any mixture that stays in contact with the bottom of the pot for more than a small fraction of a second will be raised to too high a temperature. The egg proteins will instantly denature, and you'll end up with lumps of scrambled egg.

Some people can get good results, but it requires very gentle heating, continuous stirring and scraping, and a lot of practice.

For us mere mortals, it's so much easier to heat the mixture without the egg, and then when it is at the appropriate temperature quickly stir in the egg and it won't get hot enough to congeal. Mixing the egg with some of the liquid first makes it thinner and easier to combine with the rest of the liquid.

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The key principle in all of this is how protein cooks.

All cooking is a combination of temperature and time, and some foods can be cooked quite well either at low temperatures for a long time or at higher temperatures for a shorter time.

For protein, temperature is by far the dominant factor.
It can be cooked for hours at too low a temperature and still be raw, or can be cooked for one second at too high a temperature and be overcooked.

This is very evident with eggs.
Consider breaking an egg into a hot frying pan. As soon as the egg hits the oil, some of it instantly turns white (it's already cooked), because the part of the egg that was immediately in contact with the heat was raised above its cooking temperature. The rest of the egg remains liquid for a while because it is more isolated from the heat and is still below that temperature.

The whole point of tempering is to ensure that none of the egg reaches its critical temperature before it has been thoroughly incorporated into the other ingredients.

Answer 2

Beside the technical challenges Ray Butterworth mentioned, it's also the best way to move forward if you have to heat the milk anyway.

1. For some applications, you want scalded milk. Then you wait until your milk is no longer super hot, and start tempering the eggs.
2. Some flavors have to be steeped in the hot milk. Vanilla beans are the classic example, but if you're making e.g. sage ice cream, then the process is basically cooking a tisane in the milk instead of water.
3. If your milk isn't pasteurzized, you have to cook it first.

Another reason is speed. If you're using large amounts of sugar (again typical for ice cream), it's very slow to dissolve them in cold milk. And even if you don't, you're much quicker if you heat the milk to the proper temperature and add the eggs, than by using some super gentle heating process on a milk-egg mixture.

Answer 3

One problem with mixing first and then heating is that different types of protein have different denaturation temperatures. So the protein molecules with low denaturation temperature like casein aggregate together while the less temperature sensitive proteins like ovalbumin are in the liquid phase. And then when the temperature rises further the ovalbumin molecules aggregate together unless you stir. Personally I use an immersion blender with metal shaft instead of stirring by hand.