From the HBD Archive
From: MANSFIEL@ECS.UMASS.EDU
Subject: about two-liter bottles
Date: 1989-07-20 16:23:00 GMT

Two-liter soda bottles are made of polyethylene-terephthalate (PET). This
material is permeable to both carbon dioxide and oxygen. If beer (or Coke for
that matter) is stored in PET for any length of time longer than, say, a month
or so the beer will lose its carbonation and will become oxidized. If you
have ever seen coke in 2-liter bottles on sale real cheap it's possible that
it has been around awhile and is on the flat side.

As for the "rupture strength" of glass bottles, there is not a simple answer.
Glass has reasonably good tensile strength, but it is a brittle material and
fracture machanics are a very important consideration. Imperfections in any
material such as scratches, voids, cracks, etc. can dramatically increase the
stresses in the material in the vacinity of the flaw. The more brittle a
material is, the more dramatic these increases in local stresses can be. As
a result, the amount of pressure a glass bottle can hold depends on depends
on the flaws in the bottle as much as it depends on the tensile strength of the
glass.

It should be noted that when one uses glass for bottling that only containers
designed to hold pressurized contents (beer, soda, champagne, etc) should be
used.

Personally, I prefer to use new (unscratched) bar-type brown glass bottles.
The brown glass isn't any stronger, but it prevents at least some amount of
light from getting to the beer. Though it is unlikely that scratches on the
surface of the bottles are goingto cause problems under normal circumstances,
using unscratched bottles may provide a larger margin for error in case of
accidental overpriming or contamination at the time of bottling.

Todd Mansfield
Univ. of Massachusetts
MANSFIEL@UMAECS

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