Thread Tools Display Modes
Old 05-27-2007, 07:36 PM
LilShieste is offline
Join Date: Dec 2001
Location: CA East Bay
Posts: 1,623

What is this blue plastic thing in my soda bottle cap?

I have no idea what this thing is called, so I'll describe how to find one (so you know what I'm talking about):

1) Find a plastic soda bottle
2) Take the lid off the bottle, and look inside it
3) There is a blue plastic thing in the bottom of the cap
4) What is this blue plastic thing?
4b) (You can remove it with a pencil or a screwdriver - or fingernails, if they're long enough, I suppose.)

My best guess is that it serves as some sort of barrier against mold/mildew/etc. since a lot of moisture builds up in the bottle. (Since the moisture would rise from the soda and cling to the underside of the cap - maybe it's harder for mold to thrive on this blue thing, rather than on the bare cap.)

Any ideas?

Old 05-27-2007, 07:38 PM
Full Metal Lotus is offline
Join Date: Dec 2006
Location: Edmonton, Alberta, Canada
Posts: 1,618
Its the pressure seal. The softer plastic "liner" acts as a membrane which , coupled with the pressure from the lid, acts as a gas proof barrier, keeping your soda fresh and bubbly

Old 05-27-2007, 07:40 PM
MC$E is offline
Join Date: Jun 2002
Location: Cow Country, MD
Posts: 528
I think it is just to make a better seal. Take the blue piece out of the cap from a fresh bottle, put the cap back on, tighten to a normal bottle-cap torque, and shake it up. I bet you end up with a mess.
Old 05-27-2007, 07:47 PM
Bryan Ekers's Avatar
Bryan Ekers is offline
Charter Member
Join Date: Nov 2000
Location: Montreal, QC
Posts: 59,177
As an example, clamp your jaw closed. You may notice that your teeth, having rather uneven surfaces, touch in a number of places (what dentists call an occlusion) but don't form a tight seal. You can quite easily breathe through your clenched teeth. Now imagine fitting a piece of cardboard into your mouth and biting down on it. While your teeth are hard and do not compress under pressure, the cardboard is softer, and does, conforming to the surfaces of your teeth and forming a seal. Attempting to breathe through your clenched teeth now is much more difficult. When sealing a soda bottle, a fairly rigid plastic cap is screwed onto a fairly rigid plastic bottleneck and neither surface is inclined to mold to the other. Placing a compressible gasket between then, i.e. a disk made of softer plastic or rubber, allows a tight seal around all points of contact, covering any imperfections in the cap or bottleneck that might allow the CO2 to escape and let the soda go flat as it is shipped.
Old 05-27-2007, 07:58 PM
LilShieste is offline
Join Date: Dec 2001
Location: CA East Bay
Posts: 1,623
Makes perfect sense. Thanks for fighting my ignorance, guys.

Old 05-27-2007, 08:34 PM
silenus's Avatar
silenus is offline
Isaiah 1:15/Screw the NRA
Charter Member
Join Date: May 2003
Location: SoCal
Posts: 51,524
If you look under the cap of a microbrew, you find a seal that also acts as an oxygen-scrubber, absorbing the oxygen in the headspace so it doesn't affect the beer.


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off

Forum Jump

All times are GMT -5. The time now is 03:46 AM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2019, vBulletin Solutions, Inc.

Send questions for Cecil Adams to:

Send comments about this website to:

Terms of Use / Privacy Policy

Advertise on the Straight Dope!
(Your direct line to thousands of the smartest, hippest people on the planet, plus a few total dipsticks.)

Copyright 2018 STM Reader, LLC.

Copyright © 2017