Cooking for Geeks: Real Science, Great Hacks, and Good Food (68 page)
Read Cooking for Geeks: Real Science, Great Hacks, and Good Food Online
Authors: Jeff Potter
Tags: #COOKING / Methods / General
One of the greatest travesties regularly foisted upon the American dinner plate is overcooked chicken. Properly cooked chicken is succulent, moist, and bursting with flavor — never dry or mealy. True, the potential of contracting salmonellosis from undercooked chicken is real; besides, raw chicken is just gross. But I’m not suggesting undercooking chicken — just cooking it correctly.
The “problem” with cooking chicken “correctly” is that, from a food safety perspective, ensuring pasteurization (sufficient reduction of the bacteria that cause, say, salmonella) requires holding the chicken at a high enough temperature for a sufficiently long period of time. “Instant” pasteurization can be done at 165°F / 74°C, but at this temperature the actin proteins will also denature, giving the chicken that unappealing dry, mealy texture. However, pasteurization can be done at lower temperatures, given longer hold times. Sous vide is, of course, extremely well suited for this: so long as you hold the chicken for the minimum pasteurization time required for the temperature you’re cooking it at, you’re golden. Even if you hold it too long, as long as it’s below the temperature at which actin denatures, the chicken will remain moist. Another win for sous vide!
Time at temperature chart for poultry.
As with fish, you don’t need a recipe in the traditional sense to try out sous vide cooking with chicken. Here are some general tips:
- Chicken has a mild flavor that is well suited to aromatic herbs. Try adding rosemary, fresh sage leaves, lemon juice and black pepper, or other standard flavors in the bag. Avoid garlic, however, because it tends to impart an unpleasant flavor when cooked at low temperatures. When adding spices, remember that the items in the bag are held tightly against the meat, so herbs will impart flavors primarily in the regions that they touch. I find that finely chopping the herbs or puréeing them with a bit of olive oil works well.
- As with other sous vide items, allow space between the individual items in the vacuum bag to ensure more rapid heat transfer, or place individual portions in separate bags.
“Wait a sec,” you might be thinking, “this ‘sous vide’ thing...how’s it different from a slow cooker?” I thought you’d never ask!
They’re not actually that different. Both hold a reservoir of liquid at a high-enough temperature to cook meat but not boil water. Sous vide cooking has two advantages over traditional slow cooking, though: the ability to dial in a particular temperature, and to minimize the amount of variance that occurs around that temperature.
With a slow cooker, your food cooks somewhere in the range of 170–190°F / 77–88°C. The exact temperature of your food and the extent to which that temperature fluctuates aren’t so important for most slow-cooked dishes. This is because slow cooking is almost always done with meats that are high in collagen, and as discussed in
Chapter 4
, these types of meat need longer cook times in order for the collagen to denature and hydrolyze and transform into something palatable. However, this isn’t true for cuts of meat that are low in collagen, such as fish, chicken breast, and lean cuts of meat. For these low-collagen items, cooking needs to denature some proteins (e.g., myosin) while holding other proteins native (e.g., actin). The difference in temperature at which these two reactions occur is only 10°F / 5°C, so precision and accuracy are important. Sous vide wins hands down. It’s not even close.
Try cooking ducks legs both ways. Seal up two legs and cook them sous vide at 170°F / 77°C. Meanwhile, prepare a second set of legs in a slow cooker. Cook for at least six hours and then examine the difference.
Sous vide duck legs.
Slow-cooked duck legs.
The geeky way to think about cooking is to consider the addition of heat to a system. Adding heat isn’t a spontaneous thing: there will always be a heat gradient, and the difference between the starting and target temperatures of the food will greatly affect both the cooking time and the steepness of the gradient.
This is one reason to let a steak rest at room temperature for 30 minutes before grilling: 30 minutes is short enough that bacterial concerns are not much of an issue, but long enough to lower the temperature difference between raw and cooked steak by a third. You can use a water bath to the same effect for vegetables: reduce the heat delta by holding them in a moderate-heat water bath (say, 140°F / 60°C) for 15 to 20 minutes, and then steam or sauté them.
Yes, you can cook vegetables sous vide, too, but because vegetables don’t begin to cook until relatively high temperatures — typically above 185°F / 85°C — and even then take a while, it’s easier to cook them with traditional techniques.
I often cook steak tips at the same time that I preheat bok choy, Swiss chard, or other hearty greens, using the same water bath for both the steak and the veggies. This works because the veggies don’t actually cook at the temperature that the meat is cooking at.
This technique works great for small dinner parties. I bag and seal the steak tips just before my guests show up, and once they arrive, I drop the bag of steak tips and some bok choy into a water bath set to 140°F / 60°C. Thirty minutes or so later — after catching up with my guests, sharing a beer or glass of wine, and noshing on cheese and bread — I pull the steak tips out and let them rest for a few minutes, during which time I quarter the bok choy and steam it in a hot frying pan.
Because the bok choy is already warm, it reaches a pleasant cooked texture in two to three minutes, at which point I transfer it to the dinner plates. Reusing the same frying pan, I quickly sear the outside of the steak tips, which I then cut and transfer to the plates. Total time spent while guests wait? Five minutes, tops. Number of dirty dishes? One, plus plates. And it’s delicious!
Ever wonder why some vegetables in canned soups are mushy, textureless blobs, but others aren’t? Some vegetables — carrots, beets, but not potatoes — exhibit a rather counterintuitive behavior when precooked at 122°F / 50°C: they become “heat resistant,” so they don’t break down as much when subsequently cooked at higher temperatures. Holding a carrot in a water bath at around 120°F / 50°C for 30 minutes causes
enhanced cell-cell adhesion
, science lingo for “the cells stick better to each other,” which means that they’re less likely to collapse and get mushy when cooked at higher temperatures.
During the precooking stage, calcium ions help form additional “crosslinks” between the walls of adjoining cells, literally adding more structure to the vegetable tissue. Since “mushy” textures occur because of ruptured cells, this additional structure keeps the vegetable tissue firmer by reducing the chance of cellular separation.
The normal solution to mushy vegetables is to refrain from adding them until close to the end of the cooking process. This is why some beef stew recipes call for adding vegetables such as carrots only in the final half-hour of cooking.
For industrial applications (read: canned soups), this isn’t always an option. In home cooking, you’re unlikely to need this trick, but it’s a fun experiment to do. Try holding carrots at 140°F / 60°C for half an hour and then simmering them in a sauce mixed in with a batch of sliced carrots that hasn’t been heat-treated. (You can cut the heat-treated carrots into slightly different shapes — say, slice the carrot in half and then half-rounds, versus full-round slices — if you don’t mind your experiment being obvious.)
Tempering chocolate — the process of selectively melting and solidifying the various forms of fat crystals in cocoa butter — can be an intimidating and finicky process. The chocolate must first be melted to above 110°F / 43°C, then cooled to around 82°F / 28°C, and then heated back up and held between 89°F / 31.5°C and 91°F / 32.5°C. Once tempered, you must play a thermal balancing act: too warm, you lose the temper, and too cold, it sets.
It’s not exactly correct to describe chocolate as something that “melts,” because chocolate is a solid sol, a colloid of two different solids: cocoa powder and cocoa fats. The cocoa powder itself can’t melt, but the cocoa fats that surround it can. Cocoa butter contains six different forms of fats, and each form melts at a slightly different temperature.
The six forms of cocoa fat are actually six different crystalline structures of the same type of fat. Once melted, the fat can recrystallize into any of the six forms. It’s for this reason that tempering works at all — essentially, tempering is all about coercing the fats to solidify into the desired forms.
Melting points of the six polymorphs of cocoa fat.
How do scientists tell when something is melting? Two common techniques are used: differential scanning calorimetry (DSC) and x-ray diffraction. In DSC, energy is added to a closed system at a controlled rate, and the temperature of the system is monitored. DSC picks up phase changes (e.g., solid to liquid) because phase changes require energy without a temperature change. X-ray diffraction looks at how x-rays scatter when passed through a sample: with each phase change, the x-ray pattern changes.
It’s not a matter of different types of fats; it’s the structure that the fat takes upon solidifying that determines its form. Two of these forms (Forms V and VI) link together to create a metastructure that gives chocolate a pleasing smoothness and firm snap when broken. Chocolate with a high number of Form V structures is said to be
tempered.
The other primary forms (I–IV) lead to a chalky, powdery texture. Form VI occurs in only small quantities, due to the temperature range at which it crystalizes. Chocolate that has been exposed to extreme temperature swings will slowly convert to Forms I–IV. Such chocolate is described as having
bloomed
— the cocoa particles and cocoa fats separate, giving the chocolate both a splotchy appearance and a gritty texture.
To further complicate things, the fats in cocoa butters don’t actually melt at an exact temperature, and the composition of the fats varies between batches. The ratio of the different fats determines their exact melting point, and the ratio varies depending upon the growing conditions of the cocoa plant. The fat in chocolate from beans grown at lower elevations, for example, has a slightly higher melting point than chocolate from beans grown at higher, cooler elevations.
Still, the temperature variances are relatively narrow, so the ranges used here generally work for dark chocolates. Milk chocolates require slightly cooler temperatures, because the additional ingredients affect the melting points of the different crystalline forms. When looking at chocolate for tempering, make sure it does not have other fats or lecithin added, because these ingredients affect the melting point.
Luckily for chocolate lovers worldwide, chocolate has two quirks that make it so enjoyable. For one, the undesirable forms of fat all melt below 90°F / 32°C, while the desirable forms noticeably melt around 94°F / 34.4°C. If you heat the chocolate to a temperature between these two points, the undesirable forms melt and then solidify into the desirable form.
The second happy quirk is a matter of simple biology: the temperature of the inside of your mouth is in the range of 95–98.6°F / 35–37°C, just above the melting point of tempered chocolate, while the surface temperature of your hand is below this point. Sure, a certain sugar-coated candy is known to be made to “melt in your mouth, not in your hands,” but with properly tempered dark chocolate, the sugar coating isn’t necessary (it is necessary for milk chocolate, though, which melts at a temperature lower than that of your hand).
M&Ms were developed in 1940 by Frank C. Mars and his son, Forrest Mars, Sr. During the Spanish Civil War (1936–1939), Forrest saw Spanish soldiers eating chocolate that had been covered in sugar as a way of “packaging” the chocolate to prevent it from making a mess.
How does all of this relate to sous vide cooking? Traditional tempering works by melting all forms of fat in the chocolate, cooling it to a low enough temperature to trigger nucleation formation (i.e., causing some of the fat to crystallize into seed crystals, including some of the undesirable forms), and then raising it to a temperature around 90°F / 32.2°C, where the fats crystallize to make Form V crystals.
This three-temperature process requires a watchful eye and, during the second step, constant stirring to encourage the crystals to form while keeping them small. Water baths allow for a shortcut in working with chocolate: already tempered chocolate doesn’t need to be tempered
if
you don’t get it any hotter than around 91°F / 32.8°C. The desirable forms of fat won’t melt, so you’re good to go. To melt already tempered chocolate, seal it in a vacuum bag and submerge it in a water bath set to 91°F / 32.8°C. (You can go a degree or so warmer; experiment!) Once it’s melted — which might take an hour or so — remove the bag from the water, dry the outside, and snip off one corner: instant piping bag.
Temperature versus time chart for melting and tempering chocolate.
If you’re going to be working with chocolate on a regular basis, the sous vide hack will probably get tiring. It works, but if you have the dough to spend, search online for chocolate tempering machines. One vendor, ChocoVision, sells units that combine a heat source, a motorized stirrer, and a simple logic circuit that tempers and holds melted chocolate suitable for everything from dipping fruit to coating pastries to filling chocolate molds. Of course, if you have a slow cooker, thermocouple, and temperature controller...
My local grocery store recently started carrying specialty bars of chocolate infused with unusual ingredients: curry powder and coconut; plums, walnuts, and cardamom; even bacon bits. These exotic chocolate bars also carried exotic price tags, so I thought: how hard can it be to make these? With sous vide, it turns out it’s downright simple.
Place tempered chocolate in a vacuum bag. Use chocolate in bar form; chocolate chips might not work if they aren’t as well tempered.
Add your flavorings. Try almonds or hazelnuts (at about a 1:2 ratio — one part nuts to two parts chocolate by weight). Your ingredients should be dry. Any water in them will cause the chocolate to seize up.
Seal, drop in a water bath set to 92°F / 33.5°C, and wait for chocolate to melt, which may take an hour or two.
After the chocolate is thoroughly melted, work the bag to distribute the chocolate and flavorings. You can use a rolling pin to work the fillings around if using something like nuts.
Let bag rest on counter to cool.
Once cooled, snip the bag open and peel it off the chocolate. You can break the bar up into pieces.
Try using coffee beans (yum), candied grapefruit rind, dried fruits such as cranberries, or a mix of toasted nuts (almonds, pistachio, and pecans, and maybe a pinch of cayenne pepper).
...or How to Void Your Warranty in Three Easy Steps
Once you have a sous vide setup, you also have most of the tools needed to do flash pickling. In the culinary world,
flash pickling
refers to submerging a food item in a liquid-filled container, evacuating the container, and then repressurizing the container. Unlike traditional pickling, which requires time (or heat) to coerce the pickling liquid into the food, flash pickling is instant, hence its name.
Cucumbers being flash-pickled in gin using a consumer jar sealer attachment.
Under vacuum, microscopic air pockets in foods like sliced apples and cucumber wedges lose their air. Upon returning to atmospheric pressure, the food expands back out to its original shape, a bit like a sponge. But because the food is submerged, liquid is pulled back in, instead of air. Why bother? Because “Manhattan apples” (use whiskey) or “martini pickles” (use gin) are just plain
awesome
. See
http://video.nytimes.com/video/2007/12/04/magazine/1194817116911/the-edible-martini.html
for a video of Dave Arnold talking about the process using a commercial vacuum sealer.
The pros, who use commercial vacuum chambers, can just drop the food into the liquid bath and clamp down on the lid. For the rest of us, though, generating a sufficiently strong vacuum isn’t so easy. But if you have a vacuum food sealer and don’t mind voiding your warranty, there is a way.
I should have just written an entire chapter called “Voiding Your Warranty.”
Consumer vacuum sealers have a pressure switch that triggers them to stop pumping and start sealing, meaning that they stop short of creating a strong enough vacuum to create a good pickle. But if you disable the pressure switch, the unit should continue to pump indefinitely, or until the motor burns out.
To make a DIY flash-pickling system, start with a consumer vacuum sealer. You’ll need a toggle switch and an extra piece of wire, along with a screwdriver and wire cutters.
Start by popping open the vacuum sealer. It should look something like this.
PHOTOS USED BY PERMISSION OF CARL HILL-POPPER
Locate the pressure switch (highlighted on left). Cut one of the wires that runs back to the circuit board and interpose a toggle switch (highlighted on right). Cut a small hole in the plastic and mount the toggle switch so that you can flip it from the outside.