On 20/01/2019 13:31, Cursitor Doom wrote:
On Sun, 20 Jan 2019 14:20:22 +1100, FMurtz wrote:

But this one breaks the seal by pressure on opposite sides of the lid
and then takes little effort.

ISTR that pre 1991 when we had that epidemic of food tampering and the
jars were at that time not vacuum sealed, there were *still* some lids
that were a serious PITA to off. So it has do more than just releasing
the vacuum.

Whether it's vacuum sealed is just a matter of how the food is
prepared/preserved. You're thinking of "tamper evident" seals, which
don't even have to be hermetic/airtight, as in the case of some plastic
milk bottles which just have a plastic rip cord you have to remove.

--
Max Demian