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Ask The Food Lab: Can I Make Stock in a Pressure Cooker or Slow Cooker?
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"Can I make stock in a pressure cooker or a slow cooker?"
Traditional stocks are made by slowly simmering bones, meat, and aromatics on the stovetop for many hours. I've heard that using a pressure cooker can help speed this process up. Is this true? Likewise, if I'm going to be out of the house all day, can I throw my bones and aromatics in a slow cooker and expect the end results to be as tasty?
Most of the time, I'm the kind of cook who enjoys doing things the long way—provided the long way is still easy. Simmering a big stock pot full of chicken bones and vegetables that I've saved up in my freezer for a few months is really easy. I cover it with water, set it on a burner, bring it to a simmer, then let it cook all day, occasionally stopping by to breathe in the aroma wafting out my kitchen door. I'm convinced that cooks who insist that a stock must be skimmed of excess fat and scum religiously are really only saying that so they have an excuse to stand by the pot and inhale*.
*I have other friends who make excuses to stand by the pot and inhale in completely different contexts.
That said, there are also times when I'm in a hurry and I want that great chicken stock NOW. Likewise, there are days when I need to step out for a while and I don't want to leave an unattended pot simmering on the stovetop. This is when I think about pulling out the pressure cooker or the slow cooker. But how do the results compare?
To answer this question, I made three batches of chicken stock using identical starting ingredients (chicken carcasses, onions, carrots, celery, bay leaves, and peppercorns) cooked using three different methods:
- The traditional method: on the stovetop, slowly simmered and skimmed for 5 hours.
- The pressure cooker method: cooked under high pressure in a pressure cooker for 1 hour.
- The slow cooker method: cooked in a slow cooker on the low temperature setting for 8 hours.
After each batch was cooked, I reduced the resulting stock down to the same volume in order to account for any moisture that was lost (the stovetop method lost nearly 30% more volume than the other two methods due to evaporation). I refrigerated each broth overnight so that I could get a good gauge of how much gelatin was extracted through each method (the firmer a stock sets up in the refrigerator, the more gelatin it contains), then took back-lit photos of each broth under identical lighting conditions to judge clarity and overall color, and finally had a group of tasters taste each broth, judging them on flavor, body, and overall preference.
Both the standard stock and the pressure cooker stock received high marks across the board, with the pressure cooked version taking a very slight lead over the standard version in the body department (flavor scores were within 1% of each other). The slow cooker broth fared considerably worse than either, with a paler color, thinner texture, and less flavor.
So how does broth in a pressure cooker achieve excellent results in a fraction of the time that it takes using the traditional method?
When simmering meat and bones for a stock, there are two end goals: flavor and body. Flavor mostly comes from meaty bits (either whole pieces of meat used in the stock, or from bits of meat stuck to the bones) and aromatics. The flavorful compounds found in, say, a little bit of chicken meat or an onion can be extracted relatively quickly and are largely temperature-related: they get squeezed out as soon as muscle proteins are heated and contract, or plant cells rupture and spill their contents. Once those compounds are extracted, extended cooking doesn't change things much.
Body, on the other hand, comes from the conversion of connective tissue (mainly collagen) into gelatin through the application of heat in the presence of moisture. This gelatin gives the broth a rich, mouth-coating texture. The process is dependent on both temperature and time, and the two factors are inversely related: the hotter you cook your broth, the faster you'll convert collagen into gelatin.
Using a standard stovetop method, this temperature is limited by the boiling temperature of water—212°F (100°C) at standard atmospheric pressure,** and if you care about the clarity of your broth, you'll have to cook at even lower temperatures—in the 180 to 190°F range)—in order to prevent fat, minerals, and other gunk from emulsifying into your stock and clouding it up.
**You mountain-dwelling folk where water boils at lower temperatures might find that your stock takes even longer to gain body than us sea-levelers.
A pressure cooker solves both of these problems. Not only does the higher pressure achieved inside a pressure cooker allow you to heat water to a higher temperature (up to around 250°F, or 120°C), but it also prevents the water from boiling, leading to less agitation. The end result? Rapid body and flavor and great clarity.
On the other end of the spectrum, the batch of broth made in the slow cooker had a less body and flavor than that made on the stovetop because its temperature is a little too low to convert collagen or extract flavorful compounds effectively. Perhaps given a much longer cook time, it would have eventually acquired the right level of body, but most likely, that flavor would never improve.
The standard method and the pressure cooker will both deliver good results. Want your broth finished in just about an hour? Grab the pressure cooker. If you, like me, enjoy having an apartment that smells awesome and an excuse to spend your Sunday morning in your pajamas walking in and out of the kitchen, stick with the traditional method. Just leave the slow cooker for the slow cookin'.
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About the author: J. Kenji Lopez-Alt is the Chief Creative Officer of Serious Eats where he likes to explore the science of home cooking in his weekly column The Food Lab. You can follow him at @thefoodlab on Twitter, or at The Food Lab on Facebook.