Polypropylene containers are autoclavable. The recommended autoclave cycle for empty containers is 121°C at 15 psi for 20 minutes. Care must be taken to allow free air circulation into and out of vessels during the autoclave cycle, especially during the venting and cooling stages. If the container is not properly vented, collapse or implosion (sometimes confused with melting) can occur. When autoclaving bottles and carboys, the cap threads must be completely disengaged from the container; the cap can be set loosely over the mouth opening at a rogue angle to ensure the threads don’t inadvertently engage. Once the container is completely cooled, the cap can be aseptically tipped into place and tightened down.