Bumping

Reprinted by permission from Jeff Renner

When is a boil truly "a boil?" As you heat the water it sounds like it’s boiling. But it might not really be boiling water yet. Some have described this roaring sound as "a group of marbles rioting." But what makes that sound? Jeff offers this explanation:

There is really no mystery; what is going on is really very simple. The wort in the bottom of the kettle is hotter than that above it. It boils, bubbles of steam (gaseous phase of dihydrogen monoxide) are formed, they rise, reach the cooler wort, and condense back into the liquid phase, never reaching the top for you to see. Eventually, the entire wort reaches 100 ºC, steam bubbles don't condense and they rise all the way to the surface for you to see.

The "rioting of marbles" is called "bumping," and is caused by superheated (100+ ºC) liquid water that has not changed phase into the gaseous phase due to a lack of a nucleation point. When a "cell" of this does change phase (boil), it does so all at once with a mini-explosive expansion, producing shock waves, which we hear. [Close observation in glass vessels demonstrates that it is not caused by water cells slapping back closed (implosion) after giving up the gaseous phase.]

A way to produce smoother boils (without bumping) is to use boiling "chips." I use a few bottle caps from which I've removed the plastic liners and bent in half so I don't mistakenly bottle with them later (I've done it). These provide nucleation points for the phase change with the result of smoother boils.

Jeff Renner in Ann Arbor, Michigan c/o nerenner@umich.edu