Gelatin Filtered Consomme (gelatin filtration)
Consommé is a stock that has been clarified to the point where it contains no fat or solid particles. There are several methods for creating a crystal clear consommé and gelatin filtration is the molecular gastronomy method of doing so.
The classical clarification technique involves floating a protein raft on the stock but many things can go wrong in the process and requires great culinary skills to prepare a perfectly clear consommé using this technique.
The gelatin filtration method for creating consommé is much easier and has bullet proof results but requires more time (it can take up to 3 days!) and occupies room in your fridge. The end result is also different from the consommé prepared in the traditional way.
Gelatin filtered consommé has a less rich mouth feel and does not gel when chilled as it has no gelatin content in it. Because gelatin filtration requires no heat, this method is more versatile and can be used to create consommés of heat sensitive materials like fruit juices. If you are adding gelatin or agar agar, you can dissolve it by heating it in a small amount of water or the liquid you are trying to clarify.
The gelatin filtration method to prepare consommés was created by Harold McGee. The gelatin filtration method consists of freezing a stock or other solution containing gelatin and then letting it thaw in the fridge in a fine strainer lined with cheesecloth. The resulting liquid is a perfectly clear consommé containing only water and flavor molecules.
Why gelatin filtration works
When the stock with gelatin content is placed in the freezer, it converts the bulk water into ice crystals but the water associated with solutes (gelatin, fat and flavor compounds) remains unfrozen.
When the frozen gelatin-containing stock is allowed to thaw in the fridge, the gelatin and other solutes concentrate in the unfrozen associated water. The gelatin forms a stable network through cross-linking which acts as a filter of large particles of fat or protein allowing water and smaller flavor compounds to pass through.
As the thawing process happens in the fridge, bulk water melts slowly passing through the gelatin filter which retains the large particles but lets the smaller flavor molecules be carried by the water. The melted water is never in contact with the gelatin network for long enough to start dissolving the gelatin network.
The resulting liquid is a clarified stock or consommé with just water and flavor molecules with no gelatin content.
Gelatin Filtered Consommé Preparation
1- First of all you need a stock with good gelatin content. If the gelatin content is not high, add some gelatin. For liquids with no gelatin, you should add about 7 g (1/4 oz) of unflavored gelatin powder per cup of liquid. Sprinkle the gelatin powder over half of the cold liquid and let it hydrate for 1 minute. Then boil the rest of the liquid and add it to the cold liquid with gelatin and mix. The duration of the filtration process can be reduced from about 2 days to 8 hours if agar agar is used instead of gelatin. If using agar agar, simmer the stock, add 0.15% of agar agar and stir to dissolve. Let it cool down in a flat container.
2- Place the stock with gelatin or agar agar content in the freezer using a flat container so the depth of the liquid is about 2 or 3 cm (1 inch).
3- After a few hours, once the liquid is jellified but not hard frozen, remove the container from the freezer, cut the gel in small 2 or 3 cm (1 inch) cubes and place in slightly larger container or tray so the gel squares are not completely touching each other. This is just so you can later easily break the frozen liquid into smaller pieces and conveniently place them in the strainer.
4- Place the gel cubes in the freezer and wait until they are completely frozen. This may take between 12 to 24 hours depending on your freezer and size of the cubes.
5- Line a large fine strainer with cheese cloth and place on a container that can hold the volume of consommé you are preparing without touching the bottom of the strainer.
6- Place the frozen gelatin-containing stock in the strainer and let it thaw in the fridge. This process may take about two days.
7- Collect the resulting consommé and discard the gelatin, fat and larger particles left in the strainer.