Ice to "shock" the stock... - Page 2
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The truly cool trick with lobster, though, is to make crustacean butter. Chop up cooked or raw shells coarsely, heave them in a big stand mixer, and add a lot of sweet butter. Run the machine slowly until everything is stuck together, and then turn up the speed a bit and leave it running for half an hour, by which point the butter will be salmon-colored. Now scrape everything into a big saucepan and add a whole lot of water, like several inches over the top. Bring very gently almost to a boil, then shut off the heat, let cool, and chill. The butter will freeze solid on the top. Remove it in a block, then heat it gently until it starts to sizzle just a little bit and strain it very fine. You now have pure crustacean butter, which freezes in an ice cube tray and keeps for a month or so.
Now make lobster risotto and finish with a cube of this stuff. Heaven!
I realize this is extremely late, googled ice and saw this.
Some chefs say its because it hardens fat solubles to clean the stock better. Stories for days. I worked with a chef that seriously added 1, a single, all by itself, ice cube to a simmer stock.
The reason I start a stock with ice doesn't have to do with any nonsensical crap wives/chefs - tale. It's simple.
Ice machines filter water very well. You get all the crap out of the tap water. You're making a stock, if you're reducing that stock in say a pork jus, you may taste the chlorine, the fluoride, etc. If you melt ice and drink it, it will taste better than tap water drawn from the same restaurant. I'm not going to buy bottled water to make 20 gallons of chicken stock, but i may use 160# of ice. Depending on the day of the week I can empty my ice machine. Cheaper, cleaner, tastier.
I've read Thomas Keller adding ice to the stock once it comes to the simmer, this "shocks" the fat (solidifies) and catches some impurities, making skimming at this stage somewhat more efficient and improving the final clarity.
From the cookbook "Bouchon":
"Place all the bones and the feet, if using, in a 14- to 16-quart stockpot. Cover with the cold water. Slowly bring the liquid to a simmer, beginning to skim as soon as any impurities rise to the top. (It is important to keep skimming, because as the stock comes to a simmer, impurities could otherwise be pulled back into the liquid and emulsify and cloud the finished stock.
Once the liquid is at a simmer, add the ice and then remove the fat. (The ice will chill and thicken the fat and turn it opaque, making it easier to remove.) Skim off as much of the impurities as possible. (Once the vegetables are added, skimming will be more difficult.)"
Mmmm... Thomas Keller or not, fat doesn't "solidify" at simmering temperatures --say, 75-90 C.
Oils remain liquid at room temperature, and fats remain solid at room temperatures, was how my gr. 9 science teacher explained it. And it goes without saying that oil or liquid fat will always float to the top of any liquid--provided it isn't emulsified.
On the other hand, proteins start to coagulate at the mid 60's regardless if they are dead proteins or live proteins. Scum is nothing more than dead protein, has nothing to offer in terms of nutrition, flavor, or mouthfeel. The classic theory goes something along the lines of: If you can keep the stock at 65-ish for prolonged periods of time, the dead protein will clump together in larger bits of foam, making it easier to skim off. Once the temp goes above 90 the dead protein breaks down quickly into tiny bits, making it almost impossible to skim off, and clouding your stock and lending a stale and gritty flavor to it.
I don't shock my stocks, it tends to annoy them and they end up in the therapist's office. ;)
I was just posting T.K. view of it. I find that most of the fat comes off with skimming, as long as the liquid is at or just below the simmer. The rest comes off after chilling in the fridge.
I have found truth in what you say about prolonging the temp at about 65-70 C - those stocks are usually beautifully clear and skimming is a snap.