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When I’ve got five or six racks of chicken plus a couple of duck carcasses, I break out my biggest pot and a pair of tin snips. Since smaller pieces mean faster collagen extraction, I break the frozen carcasses into pieces (if they’re too tough, I use the snips) and add them to the pot. I add enough cold water to cover the bones and bring to a boil.

Now here’s the screwy part. Collagen is not the only thing in the pot. Many other water-soluble substances will emerge from the bones once the water hits a boil. They will collect at the top of the pot and, since their presence serves to reduce the surface tension of the water, once water vapor starts to rise from the bottom, there’s going to be a whole lot of foaming goin’ on. Ever seen foam riding the waves on a windy day at the beach? Same stuff.

As soon as it does, drop the heat so that you maintain a low boil.

Many stock makers, wishing to discard this foam, turn to slotted spoons, ladles, you name it. I use one of those little square nets they use to catch neon tetras down at the pet store. As soon as I get a big netfull I unload it by turning it upside down under cold running water. After five minutes or so of defoaming, stop and watch. Odds are you’ll be in the clear and can go ahead and toss in a couple of quartered onions, a couple of carrots split down the middle, at least three ribs of celery broken in half, and a palmful of black peppercorns. No green herbs yet. And no salt.32

THE WHOLE BIRD

One of the reasons I always buy whole chicken is to get the rack: the carcass, complete with wings, rib cage, and backbone.32 There are, of course, other reasons to buy whole:

• The less processing a food has undergone, the cheaper its per-pound price will be.

• All things being equal, whole birds will keep longer than pieces will.

• Many meat cutters hack the pieces apart instead of taking the time to separate them at the joints.

So in buying a whole bird you pay less money and get all those great pieces to make stock with. I make stock only a few times a year, so I bag, tag, and freeze the bones as I gather them. (A chest freezer in the basement is a wonderful thing.)

SURFACE TENSION

It was in 1751 that Johann Andreas von Segner, a German physicist and mathematician, first introduced his ideas about the surface tension of liquids. Today, we understand that molecules at the surface of a liquid attract each other to create something that’s been referred to as like a skin or a stretched membrane. (It’s because of this tendency that some insects are able to stand on the surface of a pond.) Water molecules are so attracted to each other that when presented with a different environment, such as air, they will shape themselves into spheres to expose as few molecules as possible to that environment. It’s easy to find an example of this phenomenon right at home in your kitchen. Check out a slowly dripping faucet. As the drops of water form, they sag or stretch out into almost a teardrop shape before falling.

Surface tension explains why pure water cannot be bubbled or persuaded to foam. And this goes not only for water but also for any pure liquid. To coax water into foaming, you have to break the tension by adding something that can work its way into the water.

Now here’s where the simmering comes in. You want to drop the heat as low as you can and still have a few stray bubbles breaking the surface. It’s not that boiling won’t do the job, it’s just that all that turbulence would break things up so much that you’d end up with a very cloudy stock.

How long to simmer? That depends on the volume of bones and water. I try to keep mine going for at least 8 hours, but then I’m greedy for the most gelatin I can get. You’ll know it’s over when you reach in with tongs and can easily crush the bones.

Big point: the more the water fills with gelatin, the slower the gelatin is extracted—the water gets “full” so to speak. So