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The Science of Collagen—and How to Make Mean Duck Confit—from Cooking For Geeks

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Duck confit tastes entirely different from duck cooked almost any other way. Cooking it is all about converting tough collagen proteins—pound for pound tougher than steel—into gelatin.

While this isn't a fast chemical reaction, it's a simple one to trigger: hold the meat at a low temperature for long enough, and the collagen proteins denature and eventually hydrolyze.


154°F / 68°C: Collagen (Type I) Denatures

An animal's connective tissues provide structure and support for the muscles and organs in its body. You can think of most connective tissues-loose fascia and ligaments between muscles as well as other structures such as tendons and bones-as a bit like steel reinforcement: they don't actively contract like muscle tissue, but they provide structure against which muscles can pull and contract.


The most common type of protein in connective tissue is collagen, and while there are several types of collagen in animals, from a culinary perspective, the main chemical difference between the different types of collagen is the temperature at which they denature. In cooking, collagen shows up in two different ways: either as discrete chunks (e.g., tendons, silverskin) outside of the muscle, or as a network that runs through the muscle. Regardless of its location, collagen is tough (it provides structure, after all) and becomes palatable only given sufficient time at sufficiently high temperatures.

It's easy to deal with collagen that shows up as discrete pieces: get rid of it by cutting it off. For cuts of meats that have a thin layer of connective tissue on them (called silverskin, presumably because of its somewhat iridescent appearance), cut off as much as possible and discard it. Beef tenderloin cuts commonly have a side with this layer; trim off as much as possible before cooking.

Chicken breasts also have a small but noticeable tendon connected to the chicken tenderloin. Uncooked, it's a pearlescent white ribbon. After cooking, it turns into that small white rubber-band-like thing that you can chew on endlessly yet never get any satisfaction from. Generally, this type of collagen is easy to spot, and if you miss it, it's easy to notice while eating and can be left on the plate.


However, for the other kind of collagen found in some cuts of meat-collagen that forms a 3D network through the muscle tissue-the only way to remove it is to convert it to gelatin via long, slow cooking methods. Unlike muscle proteins-which in cooking are either in a native (i.e., as they are in the animal), denatured, or hydrolyzed state-collagen, once hydrolyzed, can enter a coagulated (gelled) state. This property opens up an entirely new world of possibilities, because gelatin gives meats a lubricious, tender quality and provides a lip-smacking goodness.

One piece of information that is critical to understand in the kitchen, however, is that hydrolysis takes time. The structure has to literally untwist and break up, and due to the amount of energy needed to break the bonds and the stochastic processes involved, this reaction takes longer than simply denaturing the protein.


Hydrolyzing collagen not only breaks down the rubbery texture of the denatured structure, but also converts a portion of it to gelatin. When the collagen hydrolyzes, it breaks into variously sized pieces, the smaller of which are able to dissolve into the surrounding liquid, creating gelatin. It's this gelatin that gives dishes such as braised ox tail, slow-cooked short ribs, and duck confit their distinctive mouthfeel.

Since these dishes rely on gelatin for providing that wonderful texture, they need to be made with high-collagen cuts of meat. Trying to make a beef stew with lean cuts will result in tough, dry meat. The actin proteins will denature (recall that this occurs at temperatures of 150–163°F / 66–73°C), but the gelatin won't be present in the muscle tissue to mask the dryness and toughness brought about by the denatured actin. Don't try to "upgrade" your beef stew with a more expensive cut of meat; it won't work!


"Great," you might be thinking, "but how does any of this tell me whether I need to slowcook a piece of meat?" Think about the piece of meat (or fish or poultry) that you're working with and consider what part of the animal it comes from. For a land-based animal, those regions of the animal that bear weight generally have higher levels of collagen. This should make sense: because the weight-bearing portions have a higher load, they need more structure, so they'll have more connective tissue. This isn't a perfect rule of thumb, though, and cuts of meat generally have more than one muscle group in them.


For animals like fish, which don't have to support their weight on land, the collagen levels are much lower. Squid and octopus are notable exceptions to this weight-bearing rule, because their collagen provides the equivalent support that bone structures do for fish.

Duck Confit, Oven method

The secret to duck confit is in the time and temperature, not the actual cooking technique. The upshot? You can make duck confit in a slow cooker or in an oven set at an ultra-low temperature. The fat that the duck is cooked in doesn't matter either; some experiments have shown that duck confit cooked in water and then coated in oil is indistinguishable from traditionally cooked duck confit. Regardless, definitely skip the exotic block of duck fat; duck legs are expensive enough as it is.


Arrange duck legs in an oven-safe pan and cover with oil. Place in oven set at 170°F / 77°C for a minimum of six hours. (200°F / 95°C will work, but avoid anything hotter to prevent steaming the meat.)

The duck legs will become more tender with longer cook times. I've cooked batches of 36 duck legs overnight using a large pot held at temperature in an oven. If you do cook a large batch, remember that the core temperature needs to get to about 140°F / 60°C within two hours. In this case, heat the oil up to ~250°F / 120°C before placing the duck legs in it. This way, the hot oil will impart a solid thermal kick to get the cold legs up to temperature faster.


• Traditional recipes call for duck fat instead of olive oil. One advantage to the duck fat is that, upon cooling to room temperature, it solidifies, encasing and sealing the duck leg in a sterilized layer of fat, somewhat like how some jams are preserved with a wax seal. If you were living in France a century ago, this would've been a great way to preserve duck legs for a long winter, but with the invention of refrigeration and modern grocery stores, there's no need for the duck fat to store the meat safely for the few days it might last. Use olive oil. It's cheaper and healthier.
• If you pour off the oil and liquid into another container, a layer of gelatin will separate out on the bottom once it cools. Use that gelatin! Try tossing it into soups.


Cooking For Geeks just might get every geek you know excited about cooking. Written by Jeff Potter, a software engineer turned food writer, this cookbook is for anyone who likes to understand how things work. The best-selling book comes packed with outstanding, well-thought out recipes recipes—and give readers enough information to enable them to go off-recipe. Indeed, when you think about it, recipes are nothing but code.

Jeff Potter has done the cubicle thing, the startup thing, and the entrepreneur thing, and through it all maintained his sanity by cooking for friends. He studied Computer Science and Visual Art at Brown University.


Cooking for Geeks
By Jeff Potter
Copyright 2010 Atof Inc. All rights reserved.
Published by O'Reilly Media, Inc.

Cooking for Geeks is available for purchase at Amazon.