Filtering liquids with gelatin

Have you ever accidentally frozen Jell-O? Maybe you put it in the freezer to make it set faster, and forgot about it? If so, you probably noticed that when it thawed, it started to weep liquid. The technical term for this is “syneresis” and, used deliberately, it can be a powerful tool in the kitchen.

Gelatin is made up of protein molecules that like to get tangled up with each other. When they do, they trap water between them, turning liquids into solids. When you freeze this gel and thaw it again, the gelatin network allows water, sugars, acids and flavour molecules to leak out, while trapping larger particles.

In other words, it acts like a giant filter, making your liquid crystal clear.

While restaurants like the Fat Duck use this technique extensively to clarify meat stocks (which contain gelatin naturally) it can be used with just about any liquid. Simply weigh it and add 0.5% of that weight in gelatin, warming gently to dissolve. Let it set in the fridge, then freeze it. Once it’s frozen, put it back in the fridge in a colander, with a container underneath to catch the liquid that weeps out. It’s a slow process – depending on the temperature your fridge is set to, it may take a couple of days – but the results are worthwhile.

You can read more about the technique in this eGullet thread. A similar technique using agar (which has the benefit of being vegetarian) is discussed here and here, but I haven’t gotten around to trying it out yet.

On Friday, I’ll show you a fun use for this technique, and explain what I’m filtering in the picture up top.

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