How do food packaging companies decide how tight to make the lids of jars? Has this increased over time or am I just getting older?
Roger Miles, St Albans, Hertfordshire, UK
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The previous discussion about lid tightness fails to describe the root cause of the intractability of airtight jar lids. It isn鈥檛 possible to make an airtight seal between a thin metal lid and the rim of a glass jar on their own. To obtain one, a soft material is needed between the two parts.
This is achieved by a thin layer of plastic around the inside edge of the screw lid. Unfortunately, the very quality that makes the material work as a seal means that the friction between seal and glass is high. Hence, if the seal is doing its job, the lid will be difficult to turn.
The conflict of requirements has been addressed by the glass jar industry, with the manufacture of a new type of lid. This is made in two parts; a lid comprising a disc, with the seal bonded to the periphery, and the lid rim, with its thread that engages with the jar thread. It is designed so that the disc is held captive by the lid rim, but is free to rotate within it.
Crucially, the sliding friction between the two lid components is far less than that between the seal and jar rim. Problem solved. This more complex type of lid construction is no doubt more expensive to make, but the resulting reduction in exasperation across the human race is well worth it.
Sylvia Potter, Godalming, Surrey, UK
If a jar has a metal seal, the simplest solution to a stuck lid is to hold it for a few minutes under a very hot tap. This heats the metal, thus expanding it slightly, and also warms the contents, which may reduce any pressure differences.
Mark O鈥橲hea, York, UK
Further to the recent answers concerning tight lids on glass jars, anyone who has worked in the packaging industry will know how easy it is to damage the seal on glass bottles or jars.
Opening a very tight lid doesn鈥檛 require the help of the strongest person in the household. Tapping the edge of the lid against a firm surface is sufficient to break the seal and release the vacuum. The lid will then be easy to open.
This illustrates why the food safety button on lids was put there in the first place. An apparently undamaged jar could let in air if subjected to rough handling in the distribution chain. This could result in the contents spoiling.
Anthony Woodward, Portland, Oregon, US
Grip strength does decrease with age. This is measured by squeezing a handheld device called a dynamometer. Many studies in different countries around the world show declining grip strength from the age of 30 or 40.
鈥淐offee made by the cold-brew method lacks some of the acids that give regular coffee its distinctive bitterness鈥�
鈥� Editor鈥檚 note: You can find out more about the decline of strength with age, and how to fight this decline, in our recent feature on strength training (18 April, p 34).
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