Vacuum-Concentrating, Part 2


In my last post, I explained how vacuum-concentrating can condense flavor well below the boiling point of water, thereby leaving aroma compounds intact. Some Modernist chefs do this with a rotary evaporator, or rotavap for short. The only problem is that a full-sized version is a $40,000 piece of research equipment. Even a small one costs over $5,000. They’re fragile, and replacement parts aren’t cheap; they can leak in at least a dozen different places, requiring time to futz around and find the leak. They’re designed for laboratories, not for kitchens.

This isn’t to say that rotavaps aren’t useful for chefs. They are one of the few ways to capture distillate at temperatures below the boiling point of water. But if you want only the concentrate, rather than the distillate, there’s a much easier way to put together a vacuum-concentrating system. The photo below shows just how to do that (click the photo to enhance the image).

To build a vacuum-concentrating system, you need a few things:

1. First, you need a vacuum pump that can handle a lot of liquid. Many cheap vacuum pumps use oil, but if you pull water vapor through that oil it will emulsify, gum up, and damage the pump. Make sure to get a water-recirculating aspirator pump with a capacity of about 10 liters. This looks like a beer cooler, but inside there’s a pump that circulates water. As the water flows by the little orifice in the nozzle, it creates a venturi effect, creating a vacuum. Because they’re sold to laboratories (which are less sensitive to price), new ones can cost more than $1,000. If you’re mechanically inclined, you can take a trip to any major hardware store and get everything you need to build your own. If you look around on eBay for recirculating aspirator pumps, however, you’ll find a lot of these for far less than the one linked to above.

Your pump should be able to pull 5-40 mbar (0.07-0.58 psi), depending on water temperature. The colder the water, the stronger the vacuum will be. To maintain a cold temperature, keep ice floating in the water bath while it’s circulating.

An aspirating nozzle, which has a little side arm that you can screw onto your faucet, is an even cheaper alternative. Vacuum strength will depend on how fast the tap water is flowing as well as the water temperature. The downside to these devices is that you throw away tens of gallons of water. That water goes down into the sewage to be reused, but it can add up. If you vacuum-concentrate a lot, a recirculating pump probably makes sense financially, but if you just want to try it, you should go with the faucet aspirator because you’ll save a few hundred dollars.

2. The next thing you need is a vacuum flask, sometimes called a side-armed Erlenmeyer flask. They come in myriad sizes, from a few hundred milliliters (about one cup) up to tens of liters or more. For home use, 2-5 liters is optimal.

3. You also need rubber vacuum tubing. Most flasks require a hose with an inner diameter of 5/16 in. You can find this sold by the meter in a well-supplied auto parts store, or online.

4. Your flask will need a size-appropriate stopper, which is sold separately. For example, a 2-liter flask takes a number 9 stopper.

5. You need a Teflon-coated magnetic stir bar. This will work in conjunction with item #6 below, and should be about 2 in long.

6. To go with the magnetic stir bar, you need a magnetic stirring hot plate, about 6-7 sq. in. Again, because this is a piece of lab equipment, it’s more expensive than you’d guess. Luckily, eBay is just brimming with them. Digital ones cost more, but analog is just fine.

This handy gadget not only heats the plate, but also creates an alternating magnetic field that causes that stir bar inside your glass flask to spin. Once it gets going fast enough, the stir bar creates a vortex, which expands the surface area of the liquid and thus increases the rate of evaporation. The vortex also encourages nucleation. When liquid is in a smooth glass flask, it tends to boil quite violently because there are few nucleation sites on which bubbles can form. In such situations, the temperature of the liquid can actually become super-heated, rising a couple of degrees above its boiling point. You may have seen this phenomenon if you’ve ever heated a mug of water in the microwave and noted that it barely bubbled at all until you dropped a spoon in it, at which point the liquid suddenly boiled all at once. When super-heating occurs inside a stoppered flask, a huge bubble can burst to the surface so violently it can actually cause the flask to jump off the plate and shatter. Stirring the liquid creates little bubbles that serve as nucleation sites, so the liquid boils steadily and more safely.

A magnetic stirring setup creates a vortex that assists boiling.

The key idea here is that the liquid in the flask can never be hotter than its boiling point, which is determined by the strength of the vacuum. This is just like boiling water on a gas burner because while the burning gas beneath it is thousands of degrees, the water in the pot is not above 100 ?C / 212 ?F. Turning the heat up higher will make it boil faster, but it doesn’t make it boil hotter, so your flavor compounds remain intact. You want this hot enough so that it boils fast enough to get the evaporation to make it worthwhile, to get the job done. If you go too fast, the pump can’t keep up and the pressure starts to rise, so then the temperature rises a little. We tend to set the hot plate to about 205 ?C / 400 ?F. If the water is cold enough in the pump, it will boil away at 26 ?C / 80 ?Fa warm swimming pool, but not warm enough to change delicately flavored liquids, such as a citrus juice. You could set your hot plate as low as 150 ?C / 300 ?F, but you’d be surprised, you almost never want it to go lower than that for a reasonable rate of evaporation.

Nathan Talks Cookbooks on

One thing we’ve learned from working with Nathan Myhrvold is that if you get him talking about a topic that interests him (and there are many, many, many topics to choose from), he’ll go on and on until someone comes and rushes him off to another meeting. You’ll probably learn something in the process, too, even if it’s a subject you think you know everything about. Recently,‘s Paula Forbes got him talking about his cookbook collection: which ones are his favorites, which he’s most sentimental about, and why he doesn’t think much of the barbecue books out there.

A few of the cookbooks Nathan keeps at The Cooking Lab. Photo courtesy of Paula Forbes.

Click here for the whole interview.

Vacuum-Concentrating, Part 1

The Lower, the Better

Concentrating flavor is one of the most basic yet important tasks in cooking. From a technical point of view, concentrating generally means evaporating off a solvent while leaving behind as many flavorful molecules as possible. In the kitchen, the solvent is usually water, but sometimes is alcohol. Rarely is it anything else; although fats and oils are edible solvents, you’ll create a spectacular fire if you try to vaporize them.

The traditional way of concentrating flavors relies on heating the liquid to its boiling point.

To get the job done in any reasonable length of time, you must raise the temperature of the solvent to very near its boiling point. The downside to this is that water boils at about 100 °C / 212 °F (the exact temperature varies with altitude and weather conditions), which is often hot enough to dramatically alter many of the flavors you’re trying to concentrate. Sometimes those alterations are exactly what you want: simmering a meat stock for hours plays a crucial role in creating the rich flavor of a traditional demi-glace, for example. But in many cases, the new flavors aren’t so delicious. As a rule of thumb, foods that people usually eat raw are likely to suffer from the high temperatures that reduction requires. When you concentrate an orange juice, for instance, you lose its vibrancy, and it ends up tasting like… well, cooked orange juice.

It turns out there is an alternative way to concentrate these delicate kinds of flavors without ruining them. Increasing pressure raises the boiling point of water (as happens in a pressure cooker), and conversely decreasing pressure lowers the boiling point of water. So the lower the pressure (the stronger the vacuum), the lower the boiling point. In fact, it’s entirely possible to reduce the pressure so far that ice-cold water will boil. A couple years ago in our research kitchen, we used a chamber vacuum sealer to freeze liquid nitrogen solid!

A rotary evaporator offers unparalleled flexibility in creating vacuum-concentrated juices and sauces.

When talking about vacuum pressure, it’s both convenient and illustrative to quantify the pressure in units of millibars (mbar). At sea level, the standard atmospheric pressure is 1,013.25 mbar and the boiling point of water is 100 °C / 212 °F. Take a trip to the mile-high city of Denver and the pressure drops to 805 mbar, and water boils at 93.7 °C / 200.7 °F. That’s not too great a difference, but a vacuum-concentration setup can reduce the pressure surrounding your pot of liquid all the way down to 55 mbar, enough for it to come to a boil at the perfectly pleasant room temperature of 20 °C / 68 °F. That moderate temperature will not destroy any delicate and fresh-smelling aroma compounds.

And more of those compounds will stay in the food, rather than being flung into the air as happens during traditional stove-top reduction. Now it’s true that lowering the boiling point of water also lowers the boiling point of other volatile molecules, so even vacuum reduction does throw away some of those aromas (which make the kitchen smell so nice). But more of them will remain where you want them–flavoring the food–than if you just turn up the burner to drive off the liquid.

It’s easy to imagine all kinds of dishes that benefit from flavors concentrated at low temperatures. At The Cooking Lab, we vacuum-concentrate granny smith apple juice, to preserve its fresh, tart flavor as well as its bright green appearance. Vacuum-reduced wine-based sauces are also interesting because you can boil out both the ethanol and water at very mild temperatures. Personally, I like to use a simple setup I have at home to prepare cocktails with vacuum-concentrated infusions and tinctures.

The results are always very different than anything you’ve had before. Actually, it’s a bit hard to describe these flavor profiles because few people have tasted anything like them before. Until now no one has come up with an easy way to vacuum-concentrate in the kitchen.

In my next post, I’ll show you how to build your own relatively simple and inexpensive vacuum-concentrating setup with the help of a little Google-fu. In the meantime, check out the table below for a range of concentrating strategies, all of which are covered in Modernist Cuisine. (Click on the table for a larger version suitable for printing.)

Bill Gates on Nathan Myhrvold and Modernist Cuisine

Can Science Improve Cooking? from bgC3 on Vimeo.

News of the “Former Microsoft CTO Publishes Giant Cookbook” has been heralded around the world since the launch of MC. But there’s no denying that, despite having a PhD and working with Stephen Hawking after turning just 23, Nathan Myhrvold got his start when Bill Gates hired him as Microsoft’s first chief technology officer. Watch Gates as he explains Modernist Cuisine and gives a little insight into his former colleague and lifelong friend.

For more from Gates’s perspective, check out the article and slide show on the gatesnotes.

Mayuri: Where we get our spices

Uwajimaya is one of our favorite places to pick up all kinds of produce, but when we need an exotic spice or two, we head over to Mayuri, an Indian grocery store near The Cooking Lab in Bellevue, Washington.

The spice aisles at Indian grocers can be daunting at first, but they become easier to navigate if you know what you’re looking for.
Spices come in bags, in jars, and in bulk. Cardamom, black onion seeds, and mace are all important to Indian cooking.
Pomegranate seeds are delicious with lamb!
Certain products may vary in their spellings, reflecting common usage in the region in which they were grown. Ajowan, ajwan, and ajwain are all popular spellings of the same seed we like to sprinkle on our caramelized carrot soup.
A little research goes a long way. Look up alternative names and spellings while making your grocery list.
You can often find vinegars, oils, and fragrant waters not found in chain grocery stores when you explore ethnic markets.
Don’t be afraid to ask for help! We found our aloe juice in the refrigerator!
Tamarind, an important ingredient in Indian cuisine, can come in many forms. Make sure you know what type of product is best for the dish you are making.
You can find ghee in many grocery stores these days, but if you want variety, it’s at an Indian grocer!

Chris Young to Speak at Chicago Ideas Week

MC coauthor Chris Young will speak at Chicago Ideas Week about a topic we have a lot of ideas about: food. Even more fittingly, he will be presenting the Modernist Cuisine perspective at Chicago’s Museum of Contemporary Art. Chris will discuss how food “shapes our cities, our culture, and our bodies.” The line-up of speakers includes:

  • Colin Archipley: Co-Owner & CEO, Archi’s Acres
  • Karen Archipley: Co-Owner & Marketing Director, Archi’s Acres
  • Faith D’Aluisio: Former Award-Winning Television News Producer
  • Peter Menzel: Photojournalist, What I Eat: Around the World in 80 Diets
  • Kevin Pang: Reporter, Chicago Tribune
  • Ryan Poli: Executive Chef/Partner, Tavernita
  • Chris Young: Chef-Scientist & Coauthor, Modernist Cuisine

The event is Saturday, October 15, 2011, from 11:30 a.m. to 1 p.m. Click here for tickets and more information.

For information about other MC events and appearances by the authors, head over to our events page.