An unexpected use for gels: freeze-filtering.
[recommended pre-read: Gels]
Lets say you want to clarify a stock and produce a clear flavoursome consommé.
Traditionally, this is achieved by adding an egg-white mixture to the stock, and slowly heating it. With heat, the egg proteins unravel and reconnect to form a very loose ‘gel’ that traps large molecules (impurities) but lets water flow through the mesh. The set ‘raft’ floats to the surface and is removed.
As an aside, a cooked egg is a gel. The heat unfolds the egg proteins, which then bond with themselves and create a mesh trapping the water in the egg (which makes up 90% of the white) and thereby setting it. The process is irreversible. Scrambled eggs are a gel too. If you heat too long though, the mesh tightens and instead of holding water, it is squeezed out (syneresis). This leads to dry scrambled eggs and a puddle of water on the plate. Adding ~10% milk and some finely chopped butter will get in the way of egg protein bonds and delay the onset of syneresis.
Anyway, that’s one way to clarify. A more elegant way is to take the gelation further – to actually use syneresis.
The process is dead easy, but takes time (unsupervised).
If it is a meat stock, which already contains gelatin, then freeze it solid, place the block on a perforated sheet lined with cheesecloth and place this over another pan to contain the drips. Place the assembly in the fridge and wait for the block to thaw (at least 24 hours, it can take 3 days, just ignore it). The result will be a solid mass on the top tray, and a beautiful clear liquid in the bottom tray. The weeped liquid is the clarified stock.
It works because as it cools but before it freezes, the gelatin forms a gel that traps not only water, but also larger molecules such as impurities and fats. As the temperature drops below 0C, the water starts to freeze, and it expands as it does so. This neatly breaks apart the gel mesh. When the solid frozen gel is placed in the fridge, the ice begins to melt, and the liquid water drips through the broken gel filter, taking flavour molecules with it. The result is a clear flavourful liquid. Because the process occurs at fridge temperature, the fats don’t melt, and the resulting liquid is clear and fat-free. The gelatine doesn’t melt either, and impurities remain trapped.
How elegant is that?
If the liquid doesn’t already contain gelatin, just add some (0.5%). So you can clarify fruit juices, produce crystal clear carrot juice (Wylie Dufresne), or a clear chocolate consommé (blend with water then clarify; Tony Iuzzini). You can even make a fat-free brown butter consommé (simultaneously decadent, delicious and guilt-free). It can be fun thinking of outrageous things to clarify.
The idea has been attributed to Prof. Gerd Klöck, who described it in 2004 (H. McGee). A beautiful modernist technique that you can do at home.