by Jeff Potter
Reasons for filtering in the kitchen can range from aesthetic (including traditional broths like consommé) to practical (needing particulate-free liquid to work with in cream whippers, as described later in this chapter—the particulate would potentially clog the system).
Which type of filter to use depends on the size of the solids. A chinois—a conical strainer—is fine for straining out spices and solids from a broth and is the standard go-to item for filtration. To mechanically mash foods and give them a finer texture, you can push them through a perforated sheet of steel. Traditional European soups, such as vichyssoise (potato and leek), pass through these to ensure a smoother mouth-feel.
A standard modern technique for making clarified liquids such as consommés is to freeze the liquid and drip-thaw it through a filter, such as a Superbag.
High-end chefs use finer filtration to achieve other effects. Straining out the solids in tomato juice to get a clear, transparent tomato water requires a much finer filter. You can also use hydrocolloids: create a gel with gelatin (e.g., stocks) or agar (e.g., Dave Arnold’s lime juice in The Easier, Cheaper Version of "The $10,000 Gin and Tonic"), and pass the gel through a filter. The gel will hold on to most of the solids, while the filter will hold on to the gel.
International Cooking Concepts sells a filter bag called a "Superbag" that’s dishwasher safe, reusable, and highly durable. For a tenth of the price, McMaster-Carr sells mesh filter bags that are FDA compliant and rated to 220°F / 104.4°C. Search for part 6805K31 on http://www.mcmaster.com.
Note
If I were to write a "cooking geek purity quiz," one question would definitely be "How many things have you ordered from McMaster-Carr?"
The McMaster-Carr product uses a stiffer material and doesn’t drain as quickly as the Superbag, however. With this size of filtration, you can quickly create flavored liquids such as nut milks (purée presoaked almonds, drop in filtration bag, squeeze liquid out) or fruit juices (purée cantaloupe, drop in filtration bag, squeeze liquid out). Try other things, such as asparagus and olives.
These finer filters can also be used for drip filtration, where the solids are rested in the filter bag and the liquids are given time to percolate out slowly. Purée tomatoes, drop them in a fine (~100 micron) filter sleeve, clamp in a storage container, and let drip overnight in the fridge to create semiclear tomato water.
Stock, broth, and consommé
Stock, broth, consommé—what’s the difference? Stock and broth are both liquids made by simmering vegetable and/or animal matter. Traditionally, stocks are made with bones, which have collagen. Most of this collagen breaks down and converts to gelatin, which gives the stock a lubricious mouth-feel and, at sufficient concentrations, causes the stock to turn into a gel when cooled. The cans of "stock" that you find in the grocery store are really broth—they don’t have the same level of gelatin that a proper stock should have.
Note
If the canned "stock" carried in grocery stores had gelatin, it would be gelled like Jell-O.
Stocks are generally more of an ingredient—not highly seasoned, usually added to a soup or dish. Broth is a finished product, and strictly speaking broths should be made without bones; they contain no gelatin and so are comparatively much thinner than stocks. From a practical perspective, in home cooking you can treat them as the same thing in most cases. Just don’t try to make a dish such as aspic that relies on gelatin using broth.
Both stock and broth contain fats and solid particulate matter from the vegetable and animal products they’re made with, giving them a cloudy appearance. A consommé is a clarified version of either stock or broth, from which the particulates and some of the fats have been filtered out. The traditional method for clarification involves creating an egg-white "raft" that is gently stirred while the broth is simmered. It’s time-consuming, and while you should try it sometime, it’s not likely to be an everyday cooking technique. An easy modern method involves using the gelatin present in a true stock to trap the particulate matter. Freeze the stock, and as it thaws, the gelatin will hold on to the particulate matter; thaw it in a filter that’s fine enough to hold onto the gelatin, and the resulting liquid that passes through the filter will be consommé.
"Filtering" by Evaporation
Okay, this isn’t really filtering in the true sense of the word, but you can separate a liquid from any compounds dissolved in it by boiling off the liquid. Think sea salt: saltwater is allowed to evaporate, leaving behind just the salt.
A rotary evaporator (rotovap) is nothing more than a fancy (and unfortunately very expensive) tool for replicating what happens in a salt flat, but it’s designed to enable better control and to allow capturing of both parts (i.e., the salt and the water after separating). It separates a solvent from a liquid or solid by gently boiling it away under a precise vacuum and temperature and then condensing the vapor in a flask, a process known as distilling. It’s like boiling water on the stove and collecting the steam that condenses on the lid, but with far more precision.
Distilling under a vacuum lowers the boiling point of the solvent (usually water or ethanol), meaning that any compounds that are heat-unstable remain undisturbed. With a rotovap, alcohol or water can be boiled off without the changes in flavor that normally come about from cooking. Chefs have made flavorings using everything from common vanilla to offbeat items such as "sea" (using sand) and "the woods" (damp dirt from the forest). Rotovaps can also be used to remove solvents from a food—removing alcohol to make whiskey essence, water to increase the concentration of fresh-squeezed juices, or both alcohol and water to make sauces such as port syrup.
Unfortunately, commercial rotovaps are expensive, and the process of distilling foods is heavily regulated. For all the details, see the Cooking Issues blog writeup at http://www.cookingissues.com/primers/rotovap/.
Basic White Stock
In a large stockpot (6 qt / 6 liter), add the following and sweat the vegetables until they begin to soften, about 5 to 10 minutes:
2 tablespoons (25g) olive oil
1 (100g) carrot, diced
2 (100g) celery ribs, diced
1 medium (100g) onion, diced
Add:
4 pounds (2kg) bones, such as chicken, veal, or beef
For bones, look for "chicken backs" in your grocery store.
Cover with water and bring to a slow boil. Add aromatic herbs and spices, such as a few bay leaves, a bunch of thyme, or whatever suits your taste. Try star anise, ginger root, and cinnamon sticks for something closer to the stock used in Vietnamese .
Simmer for several hours (two to three for chicken bones; six to eight for thicker and heavier bones). Strain and cool; transfer to fridge.
If you’re worried about leaving the stove on, use a slow cooker.
For a bit of overkill, here’s what straining a batch of white stock in various ways yields, starting with the coarsest straining and going progressively finer. (I removed the bones and vegetable matter with a ~5,000-micron spider strainer before running the stock through the 500-micron filter.)
500 micron: stuff caught by a chinois or fine strainer
...then 300 micron: stuff caught by a cotton towel
...then 100 micron: stuff caught by a Superbag mesh filter.
Drip-Filtered Consommé
Consommé made via drip-thawing stock (left), compared to the original stock (right) filtered at 100 microns. Note the transparency of the consommé—it looks like filtered apple juice.
To make a drip-filtered consommé, start with a proper stock. The gelatin is a necessary component, because it serves the same function as the egg-white raft in the traditional method.
Once the stock has cooled and gelled (leave it overnight), transfer the gelatin to the freezer and let it freeze solid. As the water in the stock freezes, it will push the impurities into the gelatin. After it’s frozen, put the stock into a filter bag or strainer lined with a cotton towel and let it drip-thaw on the counter for an hour, or in t
he fridge overnight. The filter or towel will hold on to the gelatin, and the gelatin will hold on to the smaller particles.
Make sure the container you freeze the stock in is smaller than the filter bag you use; otherwise, you won’t be able to fit the frozen block into the filter.
Place frozen stock in a strainer lined with a cotton towel. You can freeze the stock in ice cube trays, as shown here.
After an hour or two, the stock will have thawed, with the consommé in the pan and the cotton holding on to these weird blob shapes of gelatin.
Dave Arnold on Industrial Hardware
PHOTO OF DAVE ARNOLD USED BY PERMISSION OF JEFF ELKINS, HTTP://JEFFELKINSPHOTO.COM
Dave Arnold teaches at the French Culinary Institute in New York City, where he instructs students about modern techniques and equipment. He also contributes to the excellent Cooking Issues blog at http://www.cookingissues.com.
How do you get someone to make the mental leap, to think analytically, and to think outside the box, while in the kitchen?
For people who don’t naturally think this way, you can’t expect them to start organically. You just want to give them another set of tools to work with in the kitchen. So we take something that they take for granted, like cooking eggs, and then break it into a zillion little components. We set up grids where we manipulate single variables. This means that we look at two variables at once in a grid format—for example, time against temperature—and manipulate one variable to see how it affects the other.
One of the classic examples is coffee. The variables are knowable, but why is so much coffee, specifically espresso, terrible? There are plenty of people that have machines that are good enough. It’s good to think analytically. If you’re messing around with coffee and you’re changing x, y, and z, it’s the equivalent of standing in front of a big control board with a bunch of dials and then just spinning the dials. To teach someone to make good coffee, you have to teach them how to lock down all their variables and then alter them one at a time. When you’re making espresso, most people choose to alter their grinds as their variable. They find that it’s easier to lock in the temperature, the dosing, the pressure, and then manipulate grinds. It teaches them how to manipulate variables and think analytically about something.
If we’re trying to figure out the variable of temperature with eggs, we’ll just do it. I’ll use a circulator to cook 10 eggs at very precise temperatures. We’ll do it multiple times and we’ll crack them and see what the behavior is. Or we’ll teach people how to make grids to test two different variables in order to figure out something like the effect of heat on searing meat. We’ll set up a tasting grid and they can taste it. I think this helps people to pick up that skill. It’s all about control and the ability to observe.
What sort of hardware have you repurposed for the kitchen?
Basically, a chef is going to want to steal anything that can help them heat differently or homogenize or blend differently. Most of what we use that has been repurposed aren’t necessarily our own ideas. You can crib things off of other people. Everyone is using liquid nitrogen now, which is fantastic stuff.
Even stuff normally found in the kitchen we just use in a different way. A lot of people are doing interesting work with pressure cooking nowadays. We use ultrasonic cleaners and rotovaps a lot. We’ve been running some experiments on torches recently. Why do things that are hit with torches taste like torch? I’m beginning to think that it’s the component added to gases to make them smell so that you can tell when you have a leak. I think the torch flavor is due to not fully combusting all of the stinky stuff. I wanted to crisp something big, and so I fired up the roofing torch with propane, and it didn’t taste bad. I tried to shoot a regular torch through a screen to see whether we could combust any of the torch smell by capturing it on the screen and blowing it through. That also works.
How do you balance experimenting with safety?
Teach yourself as much as you can about the risks involved with any potential new endeavor. The Internet is also good for that, because there are plenty of people who’ve already hurt themselves. Do a lot of research; read a lot of things. There’re a lot of opinions out there, and what one person says may not necessarily be true. It doesn’t take too much Googling around to find out that someone has already tried to carbonate something by sticking dry ice in a soda bottle and gotten a bunch of shattered plastic in his face as a result.
You don’t want to stifle anyone’s creativity or their desire to hack around and do things, because that’s the fun of it. But it has to be tempered with a certain amount of base knowledge. Things are dangerous under three circumstances: one, if you don’t know the procedure at all. That’s what happened to the soda bottle guy. He didn’t know the procedures. Two, you’re completely frightened of something, a piece of equipment or a knife. If you decide to use it anyway, you’re more likely to get hurt. Three, when you become complacent. If you’re an inherently cautious person and you don’t become complacent, that’s the safest way to do these kinds of experiments.
What about the safety of used equipment, such as lab gear?
When I got my centrifuge, we bleached and pressure-cooked any parts that would touch any food. When I got my rotovap, I soaked that sucker in a bleach solution and then in boiling water, and then boiling water and bleach. You have biological contaminants and you have poisonous contaminants—all sorts of contaminants. I feel pretty okay that with stainless and glass I can get rid of most bad inorganic stuff, but you just have to pray that you wash enough to get rid of all the organic stuff. From a biological hazards standpoint, you’re worried about prions, you’re worried that someone has been blending up cow brains doing Creutzfeldt–Jakob research or something like that. You can’t cook it away, they’re heat-stable. Then you’re counting on mechanical washing.
I’m curious, what do you do with a centrifuge?
A lot of people buy centrifuges because they think they’re going to get awesome results with a centrifuge. What you really need to do is borrow someone else’s first. All a centrifuge does is separate things based on density.
If you’re cooking, you want a lot of product, because you want to serve a lot of people. It’s not often feasible. Unilever donated a centrifuge to us, and I had more time just to play around. Now we’re doing a lot of things like making our own nut oils, or clarifying things like apple juice, where we’re spinning it down to increase our yield. Also, you can blend olives, cured ones like kalamata, and then you spin them. It breaks into three layers. You have the best olive brine ever for a dirty martini, hands down. You have a completely flavorless middle layer you throw away. Then you have a really interesting layer of olive oil from cured olives. That’s kind of fun. Expensive, though.
We’re taking things into the kitchen that aren’t from the kitchen, not just laboratory equipment. There’s a whole group of people that make their own chocolates. They use a stone grinder from India that’s used to grind dahl. We’ve taken that, and we’re making things that have the textural properties of chocolate, which aren’t related to chocolate at all, like ketchup and mustard. Most stuff in the kitchen is going to be equipment-based, but it’s not necessarily new technology or lab technology. Sometimes it’s just learning new techniques. It’s more of an attitude.
I’ll give you another example: how are you supposed to cook mushrooms? You’re not supposed to soak mushrooms. They always tell you to wipe off your mushrooms.
I usually just do a quick wash. My take has always been that it doesn’t actually absorb that much water.
It actually does. Mushrooms are little sponges, but here is the thing: our contention has always been that it’s just going to take longer to cook. Which is true. We did a test where not only did we soak the mushrooms in slice form but then crowded the pan—all the things that you’re not suppose to do with a mushroom.
The amazing thing was not that it didn’t make a difference in cooking them, but that the ones that we had soaked an
d crowded were better. The reason is because while the soaked mushrooms are sitting there giving off their water and stewing in their own juices, they’re collapsing. It’s no longer a sponge to soak up oil, so by the time all the water had boiled off and they started sautéing they had already collapsed, and they weren’t absorbing the oil. The non-soaked mushrooms, at the end of sautéing, had soaked up all of the oil and in fact wanted more oil. The ones that had been soaked hadn’t even absorbed all of the oil. Some of the oil was still left in the pan.
So just by normal observation, because we had measured things and were trying to figure out what was going on, we realized that everything that they teach you about mushrooms is wrong. You’re not going to measure every time, but you would never pick up on stuff like that unless you were really thinking analytically about what’s going on.
I think it’s actually the key to a lot of this. I think there is a certain something that drives some people to go to lengths, when other people just kind of shrug their shoulders and end up not being as curious.
Right, and that’s why Harold McGee’s website is called "The Curious Cook." A lot of it is about curiosity and then after curiosity—and here’s where the real geek thing comes in—is the ability and willingness to actually do something about the curiosity. Go the stupid extra length. Just see whether you can do it.
The Easier, Cheaper Version of "The $10,000 Gin and Tonic"
