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A mixture of milk, fat, sugar and egg yolk will thicken once a certain temperature is reached, but if this temperature is exceeded by a few degrees the resulting product will not be palatable. Polysaccharides as a thickening agent may be an alternative, but I do not find this alternative to be appetizing. Perhaps compounds extracted from egg yolks or their analogs will provide sufficient thickening without sulfur, bacteria and other unpalatable and unappetizing constituents of eggs?

P. S. "Sufficient" thickening is as is enough for creme brulee.

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    I'm having trouble parsing "I would like to obtain a custard utilizing minimum ingredients of maximum nutritional value". Do you mean, you want to make a low-fat custard? You might want to edit your question to focus less on chemicals and more on what food you want to make. Chemistry is a tool of cooking, not its goal. – Sneftel Sep 30 at 10:41
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    Please clarify the reason you want to substitute the egg yolk, it's tough to answer without knowing that. (If it's bacteria that you find problematic, consider pasteurized eggs; the sulfur issue is a different question.) – Erica Sep 30 at 11:34
  • @Erica, the reason is that custard with egg yolks at least in my experience have taste and smell of eggs that I do not find palatable in a dessert. – Noir Sep 30 at 20:15
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    @Noir if you simply want a dish that is as close as you can get to creme brulee, but without an egg taste, cooking tradition has found it: you make a starch pudding with your milk. You can look around for other options, but they will not only be difficult logistically, they are highly unlikely to be closer to the original than starch pudding. – rumtscho Oct 1 at 10:59
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Your title asks what thickens an egg yolk. Your description sounds like you want to replace them in creme brulee.

Egg yolks have quite a bit of protein, though not as much as the whites. When cooked, these liquid proteins unravel and tangle up with liquid forming a gel. If cooking continues they tighten up and squeeze the liquid out making soggy scrambled eggs.

Egg yolks don't have much sulfur (the sulphur is in the whites) and bacteria isn't a risk as they are sufficiently cooked in a custard. Not overheating them is not difficult.

I submit that the premise of your question is flawed. Creme brulee is a delicate, but very rich, custard and as with most custards it's texture is defined by egg cooked with high fat dairy. You may be able to produce a firm gel with a polysaccharide but it won't have the texture of creme brulee.

Additionally, the reasons you listed for wanting to avoid egg yolks are not a concern. Little sulphur in yolks, cooked to avoid bacteria, use a water bath to avoid over cooking, etc. If your objection is nutritional consider that the fat in the egg yolks is dwarfed by the cream and sugar in the rest of the recipe.

  • Good answer, thank you. Do you not know what proteins specifically contained in egg yolks exert gelling activity? – Noir Sep 30 at 20:35
  • @Noir all of them. Raw proteins clump together when cooked, regardless of type. And, if you were to isolate only the proteins from an egg yolk, you would never get a creme brulee like texture, since it needs also all other ingredients to get as it is. – rumtscho Oct 1 at 10:57
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Dehydrate the milk and maybe try transglutaminase?

Activa YG is formulated for thickening dairy https://www.modernistpantry.com/activa-yg-transglutaminase.html

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    I think the OP's objective is not yet clear, but transglutaminase is an enzyme that bonds proteins. It's not really used as a thickening agent. However, there are all sorts of starches that might replace egg for thickening, and gelatin or agar cold work as well. – moscafj Sep 30 at 11:20
  • I know what the normal ways to to it are, my assumption is that if he finds polysaccharides unappetizing (agar, xanthan) he's also not interested in starches. Activa YG is made exactly for thickening dairy products. modernistpantry.com/activa-yg-transglutaminase.html – goboating Sep 30 at 18:22
  • This is interesting. Have you used it? If it is just gelled milk protein is the texture like yogurt? – Sobachatina Oct 1 at 14:37
  • No, have not used it. Just trying to think outside the box for OP's rather specific requirements. – goboating Oct 1 at 19:08

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