Questions about custard/ Questions à propos de crème anglaise

Pour la version en français : 

https://scilogs.fr/vivelaconnaissance/on-minterroge-a-propos-de-creme-anglaise/

 

This afternoon, a salvo of questions which are, in fact, all about cooking custards.
I have illustrated and (I hope) clear and detailed explanations in Mon histoire de cuisine (Belin, Paris), but here is something to understand.





The message:

I know that white starts to coagulate at 62°C, yellow at 68°C (I read your article on Pierre Gagnaire's website), but I think that some molecules coagulate at other temperatures; am I wrong?
A related question: why does it say to cook the custard at 85°C? Could it be for pasteurisation purposes? And why does it slice at boiling point and not at 85°C?
Is it possible to catch up with a turned custard to regain the emulsion? Will this have any effect on the molecular structure or texture?
I observed that the custard was more liquid after being "blended". Is this a destruction of the molecular structure during blending?



And my answer, question by question


Here, let's start with :

I know that the white starts to coagulate at 62°C, the yellow at 68°C (I read your article on Pierre Gagnaire's website), but I think that some molecules coagulate at other temperatures; am I wrong?


One can of course answer point by point to this first question, and I will do so, but I will also take it up differently, because I think one can be clearer.

First, the poor answer, point by point:
Yes, the egg white starts to coagulate at around 62°C.
Yes, egg yolk starts to coagulate at around 68°C.
And yes, some egg molecules coagulate at temperatures other than the two above.
But first I observe that the molecules that coagulate, in the white or in the yolk, are more precisely proteins. Each protein, each kind of protein-like molecule) coagulates at a particular temperature.

Now, as I said before, I know that the answer is not correctly given, that the explanation is not clear, so I'll take it up now.

Let's consider the egg white, since the yolk behaves in principle like it, but in a slightly more complicated way.
The egg white is 90% water and 10% protein, but several kinds of protein.
Each protein coagulates at a particular temperature.
And it is indeed at 61.8°C that the first protein in the white coagulates; the others remain in the form of balls in the white, barely caught by the coagulation of this first coagulating protein (we agree: when we say "a protein coagulates", it means that many molecules of the same type of protein "uncoil" and form a large network that traps the liquid in which they were dissolved).
Then, when the temperature is increased, a second protein coagulates, which reinforces the gel that is the coagulated blank. At this stage, there are two "nets" which trap the other molecules, and it is very soft.
And when the temperature is increased further, a third protein coagulates, reinforcing the gel that is the coagulated white, then a fourth coagulation will come, and so on, the coagulated white becoming harder and harder, until it becomes rubbery.

The same applies to the egg yolk, but with different proteins, which have different coagulation temperatures.


The rest almost follows from this


A related question: why do we say to cook custard at 85°C? Could it be for pasteurisation purposes? And why does it boil and not boil at 85°C?

First of all, let's observe that you can cook custard at any temperature you want, and I don't know where my interlocutor is getting it from: 85°C.
I am not a specialist in microbiological issues, but I know that there is above all the question of the "time-temperature couple". For example, if you cook a whole egg, in its shell, at 59°C for 15 minutes, you destroy salmonella; when you cook at a temperature higher than 59°C, you can reduce the time needed for microbiological sanitation. On the other hand, care must be taken not to go too low, because when micro-organisms are at a high but not lethal temperature, they proliferate.
This is why I so often warn my cooking friends against keeping temperatures too low for a long time.

That said, yes, you can make a custard froth when you bring it to high temperature... For a reason that I will now explain, by saying first of all that a custard that is macroscopically successful, i.e. visible to the naked eye, is actually microscopically frothy.
And I would add that, contrary to what has often been wrongly taught, a custard is not an emulsion but a suspension: it is not like in a mayonnaise, where the stacking

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