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Apologies if I've phrased this wrongly. I don't try to do this very often: that is, boil a sauce to evaporate the liquid (I understand this to be known as 'reducing the sauce'); however, when I do, I frequently end up with a skin forming on top of the sauce.

I have two questions related to this: this first is, is this expected / normal, or am I doing something wrong? The second: assuming I'm not, should I try to scrap the skin off, or just stir it back in?

As an aside, is there a guideline time for this (for example, 5 minutes per 10ml)? I usually leave it for too long, or get bored early and end up making a standard sauce using flour and butter.

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    I've usually seen this happen as it cools, not necessarily as I'm reducing it ... because I stir it frequently when I'm reducing it, so make sure I don't scorch it. – Joe May 6 '19 at 15:57
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Not an expert at this, but I suspect the skin that forms on top is just the upper layers that gets slightly drier from being exposed to open air. By not being submerged like the rest of the sauce, fluids on top evaporate at a different ratio causing a more solid "skin" to form on top.

It can also happen on casseroles or sauce heavy dishes that go in the oven, and is probably an important part of gratin technique.

I don't think there is anything wrong with it other than potentially being unsightly. If you find it inaesthetic or unpleasant you can remove it at the expense of wasting a portion of the sauce.

You can however mix it back in without issues, or prevent its formation by stirring gently or from time to time so that the top layer never forms.

Math experts here can probably give you a more informed answer, but setting a ratio of evaporation per unit of time is hard because it depends on a lot of different factors, like the type of sauce, chemical composition, the total volume being cooked, area of exposure to air (wider or narrower recipient, lid/open air etc.), temperature/flame intensity, among others.

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It happens because of lecithin which is found in butter. Lecithin helps to dissolve the lipid (fat) phase and water phase together.

Water phase evaporates rapidly at surface leaving behind lecithin and lipid dissolved in lesser concentration of water, this becomes the skin.

Reagitating either mechanically or by heating will make it uniform by dissipating in the entire solution. Or you can just throw it away.

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