Introduction: How-To: Gelatin/Agar Filtration

Filtration (separation of solids from liquids) is a common task in the kitchen. It's used to make tea, coffee, and to clarify soup stock.

Typically, liquids with large amounts of suspended solids are clarified by successive filtrations. Large chunks are removed manually by skimming, and smaller bits can be removed by passing the material through a sieve. But if you want a crystal clear consomme, you need to take more drastic steps. Usually, a mixture of egg whites and meat is used to collect the smaller particulates into a "raft" which can be skimmed or strained off.

In addition to not exactly being vegetarian or vegan friendly, this technique can hardly be used to clarify other cloudy beverages, like juices or broths made with unusual ingredients.

Enter "gelatin" filtration.

This is a simple three-step process that you can use to clarify any liquid.

Step 1: The Science

Gelatin and agar-agar both consist of long chains of molecules that can be detangled and dispersed in hot water. Upon cooling, the long chains can intertwine with each other. The network of entangled chains entraps pockets of water, resulting in a jiggly semisolid.

If the gel is frozen, the pockets of water will form treacherously sharp crystals, destabilizing the gel network and allowing the liquid to flow out (once thawed). This is an an example of "syneresis," a fancy word for the removal of fluid from a gel.

Incidentally, this is why cryogenics isn't feasible at the moment. Cells are one example of a gel-like structure. As water crystals form, they burst cells, resulting in an unpleasant mess upon thawing.

Anyway, if the thawing is done over a coffee filter, the residual gelatin/agar and solids will be left behind, and the crystal clear liquid will drip through to be collected.

I found the recipe for this in the Khymos Hydrocolloid Recipe Collection, which I would highly recommend for anyone interested in molecular gastronomy.

Step 2: What You'll Need

Get together the following:

- Agar-agar or unflavored gelatin
- A liquid to be clarified
- A microwave-safe container
- Ice cube trays (optional)

I used agar agar for this filtration, which (according to the recipe collection) gives up its liquid much faster than gelatin. You can find it in stick or powdered form at any Asian market and some well-stocked gourmet or health food stores.

The liquid we'll be clarifying is V-8, since it has an absurdly large amount of suspended solids.

Step 3: Gel Preparation

Measure out some liquid, and heat it to boiling in the microwave.

While that's heating, measure out your gelling agent (agar or gelatin).

For 1000 milliliters (1 liter, ~4 1/4 cups) of liquid, you need between 3/4 and 1 3/4 grams of agar - OR - 5 grams of gelatin.

The agar I bought was in 10 inch sticks which weighed about 10 grams, so I just sawed off half an inch.

Once the liquid is boiling, put the gelling agent into the liquid and let it sit for a few minutes to hydrate. Bring the liquid to a boil in the microwave again, stirring occasionally until the gelling agent is completely dissolved.

Let the liquid cool to room temperature.

Step 4: Freeze!

Like it says, freeze that bad boy.

I dispensed my liquid into ice cube trays because I needed the container I boiled the stuff in, but that isn't crucial.

Step 5: Thaw and Collect!

Take a colander, sieve, or some other porous container and line it with a coffee filter. Place the frozen gel onto the filter, and put the whole assembly onto a container to collect the clarified liquid. Go to work/sleep/wherever, and let it thaw slowly in the refrigerator.

If your refrigerator is, like mine, permanently stuck just above freezing, stick the assembly in a large ice chest with some ice to keep it chilled.

Or, leave it on the counter and monitor its progress every 20 minutes or so. Don't leave it out overnight, though - you don't want to let it spoil!

Once the mass has been completely defrosted, you're done! Collect the clear golden liquid and enjoy in the manner of your choosing.

I got about 275 ml of clarified V-8. Other liquids, like soup stock or lemonade should result in more collected liquid, as they contain less solids to begin with.

Remember - you can do this with any liquid. The sky's the limit! Want to have a crystal clear asparagus broth? Cheese water? Essence of blueberry? This is the way to do it.

Bon appetit!
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