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.
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.