Vacuum brewer danger: superheating

Yama vacuum brewer

A couple years ago, I read about a coffee brewing method called “vacuum brewing” or “siphon brewing”.  It uses two linked glass vessels, and a small heat source (an alcohol burner, for example).  Water is placed in the bottom vessel and heated.  When the water boils, the top vessel is connected and steam pressure pushes the water up to the top vessel, where it mixes with the ground coffee.  Then the heat is removed and a vacuum forms in the bottom vessel which pulls the coffee back down again, through the filter.

It sounded really interesting, but It wasn’t until last week that I actually found a vacuum brewer on sale, at the Green Beanery in Toronto.  I picked up a Yama TCA-2.

The instructions were all in Japanese (vacuum brewing apparently is popular in Japan), but from the pictures and a general understanding of how the process works, I was able to make it go.  It’s slow, but I kind of enjoy the brewing ritual.

I’ve used it a few times now, but last night something happened that caught me quite by surprise, and was potentially very dangerous.

I started a brew, and since it takes so long to boil the water with the tiny alcohol flame, I left to do some other stuff.  But it seemed to be taking even longer than usual to boil.  I went to check it out.  Just a tiny bubble popping once in a while, with a bit of loud click.  I figured it would boil in another minute or so, so I went back to what I was doing.  Moments later, I heard a loud bang.

When I went to check it out, I found the glass vessel was intact, but half the water was on the counter, and the flame extinguished.  I realized immediately what had happened: superheating.  Superheating is a phenomenon, usually associated with microwave ovens, in which water gets heated well beyond its usual boiling point, but doesn’t boil.  It usually happens when the vessel is very clean and smooth, so there are no nucleation sites where steam bubbles can form.  The danger is that the slightest bump or disturbance can cause the water to suddenly and violently flash into steam all at once.

If this had happened moments earlier when I was standing beside it, I could have been badly burned.

It was at this point that I finally grasped the significance of one step in the Japanese instructions that I hadn’t really understood.  The instructions depicted the top vessel being placed askew in the bottom vessel, so it didn’t form a seal, during the heating process.  But I was just leaving it off entirely.  The top vessel has a metal chain that hangs from the bottom tube, which is part of the mechanism that holds the filter in place (my brewer comes with paper filters, not the glass rod filter that some vacuum brewers have).  I had previously noticed that after the water started boiling, when I put the top vessel in place, the boil suddenly became much more vigorous.

This explains why the instructions say to leave the top vessel in place while heating the water.  The metal chain provides lots of nucleation sites that allow the water to boil when it should, and prevents the dangerous superheating from occurring.  So, that step in the instructions is actually important: the top vessel needs to be in place, askew, during heating, to prevent a superheating explosion.  I guess if you’re using a glass rod filter, you should provide nucleation sites in some other way.

The coffee, incidentally, is pretty good, but I wouldn’t say it rocked my world.

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