Every modern research lab autoclave has a “liquid cycle”—a cycle just for processing liquids, growth media, and so on.  Nonetheless, more often than not, if you fill a bunch of 1L bottles up to 900mL and process them on the “liquid cycle” they’re going to come out like this:

The glass isn’t cracked; where did the liquid go?  Some evaporated, but most of it boiled up out of the flasks and fouled the inside of the autoclave.

What’s the solution?  Just tighten the lids so nothing escapes?

NO! Never, ever autoclave a sealed container; it becomes a bomb.

There are two safe, effective ways to address boil over.  They’re cheap and easy:

• Use bigger bottles
• Chose the right autoclave cycle

Running Research Lab Autoclave Liquid Cycles in the Real World

“Dry cycles” tend to either pull a vacuum on the chamber, or permit one to naturally form.  Either way, lowering the chamber pressure lowers the boiling point in the chamber (due to the aptly named Boyle’s Law).  Liquid more readily vaporizes at lower pressure, and you end up with dry (or, at least, dryer) loads.  A “liquid cycle” never pulls a vacuum and makes a point of venting more slowly, so a vacuum will not form.  Ideally, this means little evaporation and no boil up.

“Ideally.”

Sadly, this is the real world, not the ideal whiteboard of a physics classroom.  Pressure and temperature vary throughout the cycle, especially as the cycle ends.  You can expect some evaporation (most labs assume 10% to 20%) and expansion as gases precipitate out of the fluid.  Frothing is almost inevitable, and is especially the case with broths, growth media, and agars, and most pronounced with LB agar.

A good way smooth these temperature and pressure fluctuations is to take your bottles of liquid, set them in a tray or tub, and pour some water into the bottom of the tub. Water absorbs and releases heat more slowly than air or steam; having your bottles rest in a water bath acts as a buffer and smooths out the temperature and pressure ramps.