Siphon Brewing Coffee

You may have never heard of siphon coffee making. Then again, you may have been reading coffee periodicals lately and looking at the ‘meth lab’ style extraction devices that coffee ‘geeks’ are starting to talk about. The thing is, Siphons have been around for decades, they are a great way of extracting coffee and getting lower acidity in the cup. You may have heard of it under a plethora of other names - vacpots, vacuum brewed coffee, siphon brewer, siphon vacuum coffee, and all sorts of word combination’s. The thing is, coffee shops are not just espresso aficionados anymore, specialty coffee shops or ‘third world’ shops are starting to re-explore old methods and gibing them new meaning. China is a huge advocate of siphon brewing these days, they hold national barista championships that focus purely on finding a ’siphon champion’. Watch out over the next few months/years as the major ‘third world’ coffee player in Australia start to open siphon only bars, places where you will not see an espresso machine in sight!

Siphon brewing fell out of favour in the US and Canada by the 1960s, and with only a few holdovers making devices for the next few decades. Most of the major brands that used to make siphon coffee makers eased them out of production during that time, including General Electric, Sunbeam, Cory and others. Still, the brewing method maintained a hard core set of fans, maybe just in the hundreds, or dozens, and a few manufacturers continued to produce them: Bodum has continuously made a siphon coffee maker since the 1970s. Cona, out of the UK, has been making them since before World War II.

In the late 1990s, a bunch of coffee geeks started talking up the joys of siphon coffee makers, or “vacpots”, with the aid of rudimentary photos and pretty basic short video clips, a new generation of people coined on to this brewing method, if not for anything else than the show it provided.

And now, well into the 21rst century, and some 160 years after the siphon coffee maker was first invented in France and Germany, the technique is set to explode (figuratively) with almost everyone in the specialty industry taking interest.

Only recently, I saw my first ever siphon coffee maker in action. No joke, this was an eye opener for me! I really did think someone was concocting meth amphetamine right there in broad daylight. This style of extraction was pinnacle in my development in coffee. I had never tasted coffee so pure and so fresh on the tongue. I was so addicted to all things espresso at the time and this new (to me) method was chalk and cheese compared to what I was used to. I’d been reading about vacpots for a few years - In Australia published periodicals and online communities like Coffee snobs and Barista exchange - but it wasn’t until I dropped into my mate Tim’s espresso bar and tried one that I really understood the romance that is extraction through a siphon.

The sensory overload that is a siphon is just that; overloading the senses! Sight, aromas, motion, touch and hands on action. Grind the coffee, Put in in a dry chamber with a filter on the bottom; fill another chamber with water and place a burner (metho) under it; Watch as the heat transfers (siphons) the hot water over to the coffee chamber; BAM; the weightless water chamber teeters over, putting out the burner; Then the brewing begins and just like magic as the coffee infused water cools, it once again siphons back over to the starting chamber and you have the daintiest brew, ready for consumption.

So much science. So much sensory involvement. So much fun. And the taste… Straight from the cycle it is amazingly sure of itself, but let it cool for five minutes and the nuances of the coffee almost smack you as you slurp, suck and swallow.

So, How does a siphon actually work?

A vacuum style of coffee maker works on the principle of expanding and contracting  water vapour. This is what allows the device to brew a full infusion style of coffee and filter the grounds efficiently, the end result, a product similar to plunger or press pot. The romantic notions I wrote of above are in the style of a side-by-side brewer, where the two chambers are tied together with a stainless tube, enabling the liquid to dance between the chambers throughout the expansion and contraction process. The most common style of siphon has the two chambers vertically attached. This style works well, as pure gravity aids the expansion resulting in brew time, followed by contraction which gives end result.

Siphon coffee makers are made up of four parts: the bottom container where the water initially sits and the brewed coffee eventually rests; a top container that has a siphon tube attached to it (and a hole in the bottom of the vessel), where the coffee brewing takes place; a type of sealing material (usually a rubber gasket) to help create a partial vacuum in the lower vessel while brewing is taking place, and a filter, which can be made of glass, paper, metal, or cloth.

Once the siphon coffee maker is assembled, heat is applied to the lower container. As it heats up, some of the water is converted to a gas - water vapour. A gas occupies a lot more space than its liquid or solid variant, and it can expand as more heat is applied. Gas can be compressed, but only to a point, whereas liquids do not compress. The water vapour continues to expand and it seeks some relief from all the compression it’s starting to have. The only escape route out of the bottom vessel is the siphon tube traveling up to the top, but the problem is, there’s a lot of liquid blocking its way. So what does the gas vapour do? It pushes the water up the siphon tube!

This is how the brewing water “defies gravity” and gets up top past the installed filter and starts saturating the coffee grounds. Heated (though not boiling) water will continue to force itself up the siphon tube until the vapor gas in the bottom vessel can have direct access to the siphon. By design, all vacuum coffee makers do not have siphons that sit flush with the bottom vessel: there’s always a couple of millimeters of clearance (or more). This leaves some water in the bottom vessel which serves two purposes - protects the glass (if it boiled dry, glass would superheat and could crack), and has a continuing source of water to turn into - you guessed it - more steam, vapor, gas!

This is a desirable thing. Once the water in the bottom vessel is lower than the siphon tube, water vapor starts pouring up the siphon into the top chamber. This does two things - it keeps the liquid all up in the top vessel where the full-saturation brewing is taking place, and it continually heats the top vessel, maintaining near-ideal brewing temperatures (90C and 95C). As the vapor bubbles pass through the top liquid, it may appear that the active brewing is “boiling” but in fact it isn’t under normal brewing conditions. If you leave the siphon coffee maker long enough, eventually the top vessel water temperatures will reach 100C, but this takes a very hot heat source and five or more minutes of brewing time in most cases.

Once the brewing time is completed (different sized siphon coffee makers require different brewing times), the heat source is removed and more simple physics take over. Remember that gases expand when heated and that gases also compress under pressure. When heat is removed, the gases start to contract and shrink. Eventually all the gas in the bottom vessel contracts enough that it starts to create a partial vacuum of pressure (negative pressure), and it start pulling the brewed coffee back down, through the filter.

Another thing is going on as well - phase change. Some of the gas vapour starts changing back into its liquid form: water. This is almost like a miniscule turbo boost for the contraction of gas and vacuum forming, aiding the pull of the brewed liquid back down to the bottom vessel. At first it starts slowly, but as you watch, things will pick up speed and unless the coffee filter has been excessively clogged, the last half of the water travel down south will happen twice as fast as the first half did.

The effect is so efficient that once all the liquid has passed back down to the bottom vessel, air is drawn through the grounds up top, drying them out, to fill the vacuum void in the bottom vessel and balance out the pressure to normal room atmospheric pressure. For some, this is how the siphon coffee maker got one of its names - the ground coffee is literally “vacuumed” dry.