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.
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!
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.)