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I made an escalivada (eggplant, red pepper and onion in the oven, peel, cool) yesterday, and today I've added gelatin to make a nice presentation.

I used about 5gr of gelatin for 150ml heated 'escalivada juice (lots of olive oil)'.

The result was all right, but the top layer stayed liquid. The rest stayed together.

Any idea as to why the top layer stayed liquid? Does the olive oil affect the gelatin?

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In what you describe, I would expect the stew juice to exist as visible oil droplets swimming in a watery phase, and the top layer of the gelled stew juice to contain all the oil droplets/be almost pure oil. Is it what happened, or is the liquid layer the same as the juice was before? Was the stew emulsified in any way? – rumtscho May 26 '11 at 17:57
Yes, the oil droplets are visible. I don't know if the top is pure oil. The stew was not emulsified in any way. Just a shake of the pan to dissolve the gelatin. – BaffledCook May 26 '11 at 18:04
Update: I whisked (by hand) the gelatin - escalivada juice mix and it turned out just fine. Thanks. – BaffledCook May 28 '11 at 0:03
up vote 2 down vote accepted

Gelatin works by creating a very fine mesh of proteins, between which the (hidrophilic) liquid gets trapped.

A mixture of fat and water isn't a liquid. It can be either a rough two-phase mixture, with visible fat droplets swimming around in the water, or it can be an emulsion, with invisibly small fat droplets dispersed through the water. Emulsions appear smooth, e.g. milk.

When you try to use gelatine on the mixture, two things can happen. In an emulsion, the fat droplets can be smaller than the protein mesh. Then they get trapped as well as the water, and the result is gelled emulsion, just like you'd expect.

But if you have big globules of fat, they can't be trapped between the proteins. So it looks like they got squeezed out of the mesh and bubbled up to the surface, if it hadn't been there all along. On the surface, gelatine molecules couldn't find each other between globules of fat to crosslink, or maybe the motion of the globules broke any nascent links in the small amount of water between them, so it couldn't gel. So the conclusion is: you can't set soup/stew with gelatine. (I know that concentrated stock hardens from its own gelatine, but it has the fat filtered out first).

On the practical side, if you want to have thick stew juice, you have to use something else than gelatine. It is probably simplest to create an olive oil based roux with the stew juice. Or you can gel with xanthan. Both will result in a "juice" which is creamy, instead of solid wobbly blocks of juice, with xanthan retaining the authentic taste and a roux-based sauce being a novel variation.

If it is real jelly you are after, you'll have to emulsify first and use gelatine after that. Xanthan itself should emulsify well, or you can probably use lecithine. AFAIK, there is no problem to use lecithine and gelatine together, but somebody with more experience in gelling agents should confirm that. In any case, emulsifying will lead to your liquid getting quite opaque. This is inavoidable, as it has to do with the way the collection of individual micro fat droplets break and reflect light rays.

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Xanthan doesn't really gel, it just thickens, and is sort of an emulsifier. But if you're trying to gel an unstable emulsion that's heavy on fat, glycerin is probably the best choice, with lecithin being a distant second; emulsify with some glycerin flakes and you should have no trouble getting a uniform gel. – Aaronut May 26 '11 at 19:03
I don't have glycerin nor xanthan... but thanks for the explanation, I'll try emulsifying. And if I get my hands on glycerin... dynamite! – BaffledCook May 26 '11 at 20:38
I meant that you emulsify by adding an actual emulsifier, li ke lecithin or glycerin. Mechanical methods won't work in a home kitchen. I'm not even sure that there are industrial devices capable of achieving a lasting rmulsion by purely mechanical methods. – rumtscho May 26 '11 at 21:04
But it is possible that the nearest pharmacy sells food grade glycerin, so you should be able to test the emulsion easily. – rumtscho May 26 '11 at 21:06

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