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Screw capped tubes - autoclaving - your technique? - (Feb/22/2010 )

My former posts were all revolving around tubes that become contaminated. Liquid agar is poured into screw-capped tubes and they result in contamination. My autoclave has been validated, and I have found that autoclave tape stuck inside the tubes during a cycle does not come out with the typical color change, meaning steam isn't penetrating through the tube screw cap.
I was taught to screw on tight then unscrew a quarter or half turn, but that isn't working. Screw caps twisted on one full turn only also do not result in steam entrance. I feel I must simply place the screw cap on without turning, but wanted to see what techniques others use...
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-J Bardo-

Try not putting the cover on at all -- put tin foil over the open tubes, and wrap the caps separately in loose foil. After autoclaving, you can flame the necks of the tubes and put the sterile caps on aseptically.


I just turn them loosely and have never had any problems. . . maybe your lids fit more tightly than others usually do?