Most of the time, when someone first sees a vacuum pot brewer, they think of laboratory chemistry. The vertically aligned, two-compartment contraption with water on the bottom and coffee on the top certainly presents an image of scientific mystique. Apply some heat and the water moves to the top chamber, through a tube, and mixes with the coffee. Remove the heat and the freshly-brewed coffee returns to the lower chamber while the coffee grounds remain up top, thanks to a filter nestled in place at the top of the tube.
It certainly looks different than any other coffee brewer. It works differently than most other brewers, too. This post aims to demystify the process by explaining the science (the physics and chemistry) of vacuum pot brewers.
Physics and Chemistry Overview
- Heat is a form of energy. A hotter/higher energy object will give up its heat to a colder object that has lower energy. The temperature of an object is a measure of how much energy it contains.
- When enough energy is added to a liquid, the liquid converts into a gas. When enough energy is lost from a gas, it converts into a liquid. When water has reached a temperature of 212 degrees Fahrenheit (100 degrees Celsius), it has enough energy to convert to a gas (steam).
- Liquid takes up less volume than its equivalent amount as a gas.
- The gas phase of an object is less dense than the liquid phase. When the two phases are in the same container, the gas will rise to the top.
- A gas that is trapped in a tight space, i.e. one that is under pressure, tries to eliminate that pressure. It will do this by stretching its container (think of plastic wrap on a dish that has been heated on a stove or microwave, it puffs up), moving to a place where it has more space (picture air rushing out of a balloon) or, if enough pressure is generated, it may break the container (think of a coffee can or brick that wasn’t degassed before sealing the container).
Step 1 – Lifting the Water
When heat is added to the water-filled lower compartment of the brewer (usually via flame, halogen lamp, or contact with a hot surface), the energy is transferred to the water molecules. When enough energy is transferred, some of the water molecules convert to a gas (steam). The steam rises to the top of the lower compartment and begins filling up the empty space.
Once the upper space is filled, the steam begins exerting pressure on the container wall and the pool of liquid beneath it. When the pressure exceeds that of atmospheric pressure (think of air filling the space between the ground and outer space–atmospheric pressure is the weight of all that air pushing down on the earth), the steam pushes the liquid water away to make more room for itself. The water only has one place to go, up, and so it is pushed into the upper compartment through the tube. Note: only the steam is at a temperature of 212°F. Both pools of water are much cooler in the beginning and require some time to reach the optimal brew temperature of 195-205 °F as defined by the Specialty Coffee Association of America (SCAA).
As a point of interest, this process is similar to the way that electric drip machines move water from their reservoirs to a point above the coffee bed. Electric drip machines heat the water at the bottom of the reservoir, converting some of it to steam. The steam then carries the water to the top of the machine where it escapes through the shower head.
Step 2 – Brewing the Coffee
Eventually, most of the water is moved to the upper compartment. It is held there by the steam in the lower compartment. Some water remains in the lower compartment and is a source of new steam. This new steam carries heat to the upper compartment where it will condense and transfer its heat to the pool of water.
Some brewers choose to wait for the upper pool of water to reach proper brewing temperature before adding the coffee while others begin heating the water with the coffee already in the upper compartment. Each of these methods requires its own brewing protocol because of the differences in water temperature, contact time and agitation. With either method, it is important to remember that the steam will constantly be heating the upper pool of water. Consequently, it is advisable to lower the heat input to limit the amount of heat transferred to the brewing mixture since the water can become too hot and over-extract the coffee.
The influx of new steam to the upper compartment not only transfers heat but agitates the brew. This agitation speeds up the brewing process. Thus, coffee brewed using this method takes less time than other brewing methods, except for those that use high pressure for extraction.
Step 3 – Filtering Out the Grounds
When the brewer decides brewing is complete according to his or her specifications, the heat is removed from the lower compartment. As the steam in the lower compartment cools, it condenses back into water. Since the liquid form takes up less volume than the gas, a void is left where the gas was. This void is a partial vacuum that is now at a negative pressure in the lower compartment. The coffee in the upper compartment moves into the lower compartment to equalize the pressure. The filter nestled in the upper compartment permits the water to flow down while keeping the grounds up top.
Step 4 – Drinking
The two compartments are separated from each other and the lower compartment serves as a carafe. Pour into a cup and enjoy.
What Do We Call It?
This method of brewing and the brew pot itself take their name from the creation of the partial vacuum and are thus called a vacuum pot and vacuum pot brewing. I don’t know when or where it began, but this brew method gained an additional, erroneous name: siphon brewing. It is erroneous because no siphoning occurs in the vacuum brewing method no matter what physical shape the pot takes (there are other shapes where the two compartments are not vertically aligned).
A siphon (noun) is usually a tube or pipe in an upside-down “U” shape. The “∩” is lopsided where one end is much longer than the other. To siphon (verb) is to use the tube to move liquid from a higher location to a lower location, with the liquid moving up the bend and then down the other side to the lower compartment, without the need for a constant input of energy. The short end of the ∩ is placed in the higher compartment and the long end in the lower compartment. The process begins by filling the tube with liquid (this is the only time where external energy is required) and then placing the higher (and shorter) end of the tube into the liquid. The liquid will flow freely from the upper container to the lower container. As long as the output end of the tube is below the starting location, the flow of liquid will occur on its own. In summary, siphoning is “powered” by air pressure and gravity and, apart from the initial energy to fill the tube, it requires no outside source of energy, or heat, as does vacuum brewing.
Here is a good video on siphons:
Last thoughts – Implications for the Cup Profile
The vacuum brewing method is a fun presentation of some basic scientific principles. It is also well-regarded as a method of brewing coffee. While the vacuum itself probably doesn’t impart any influence on the taste of the beverage, the method does offer two unique aspects that likely influence the taste.
First, while the coffee is in the upper compartment brewing, the heat from the rising steam allows the temperature to hold steady at the proper brewing temperature. Other brewing methods begin with properly heated water but the water quickly cools as it comes into contact with air and the coffee bed. How temperature variance while brewing influences taste has yet to be documented.
Secondly, there is always a small amount of water that remains in the lower compartment of the vacuum pot. When the brewed coffee returns to the lower compartment, it mixes with this water and becomes diluted, a process unique to this brewing method. This, too, needs exploration but it seems reasonable to guess that it is analogous to adding a few drops of water to a scotch (an adjustment to taste profile that turns out to be preferable to many.)