How do food packaging companies decide how tight to make the lids on jars? Has this increased over time or am I just getting older?
Jackie Jones, Brighton, East Sussex, UK
One reason that food jar lids are so tight is the vacuum in the jar.
If I am making jam or chutney, I put the hot mixture into the jar, filling it to within about 5 or 8 millimetres of the top. I then put the lid on and tighten it by hand. At this stage, I can easily take the lid off again.
As it cools, the air in the space above the conserve contracts, forcing the safety button on the lid to drop. This forms a tight seal that makes the lid harder to remove. It is this seal that prevents the food going off and is essential to give it a long shelf life.
This would apply to most food packaging where it is put into jars while hot, whether in a factory or at home. As to whether lids have got tighter, it probably depends on the factory in question. I remember my mother having problems undoing jars, so isn’t just a recent occurrence and it might have an age-related aspect.
Mike Follows, Sutton Coldfield, West Midlands, UK
Lids may be harder to unscrew because of the increased use of safety seals, which rely on a partial vacuum inside the jar. There is a “button” of flexible metal in the centre of the lid that can either be raised or depressed.
Hot food is placed into the jar and the lid closed. As the contents cool, the pressure of the air left inside the jar falls and the metal on the lid pops downwards. The greater atmospheric pressure outside the jar keeps the metal button depressed and makes it more difficult to open the jar. I find it easiest to break the seal with the gentle use of a bottle opener, treating the lid as an oversized bottle cap.
Friction can also play a role. Because I am right-handed, I find it easier to close a jar – which has a right-handed screw thread – than to open it, and I can use stronger muscles.
Opening a jar demands that static as well as kinetic friction has to be overcome, which can increase if food becomes trapped in the thread, and I am using weaker muscles to turn the lid anti-clockwise. Right-handers will instinctively swap to their left hand to try to unscrew a recalcitrant lid, in order to use stronger muscles.
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