My mother always put a quarter of a tablet on the top of the waxed paper on marmalade and jam before storing it. I would like to know what the tablet was and did this prevent or reduce the amount of mould etc. She kept some jams for years! Is there a modern equivalent?
We would guess that your mother may have been using Campden tablets in her jars of jam. Campden tablets (sodium metabisulphite) are usually used in home brewing and wine-making to kill certain bacteria and inhibit fermenatation of sugars by yeast. It is not something we would recommend for home preserves. First, one tablet can treat up 1 gallon of wine must must or 20 gallons of water and so even a small part of a tablet would be far too strong for a jar of jam. Second it would probably need to be added to the jam to really be effective and could affect the flavour of the jam.
To keep home-made jam or marmalade you need to make sure that the jars are properly sterilized before using. Wash the jars in warm, soapy water then rise them with and dry them with a clean tea towel. Transfer the jars to a cool oven (140c/275F) for 10 minutes before filling with the warm jam. If you are using a ladle to transfer the jam from the pan to the jars then dip the bowl of the ladle in the jam just after it has finished boiling to sterilize the ladle. You can use special wax discs on the surface of the jam to help reduce the exposure of the jam to air. Some people also suggest a thin layer of an alcoholic spirit (eg whisky) can be poured on the surface of the jam to reduce the chance of the jam spoiling, though we have not tested this method and it may not be suitable for everyone to eat.
Home-made jam should be stored in a cool, dry place away from direct light and used within 12 months of making. Once opened the jar should be stored in the refrigerator and used within one month. We would suggest discarding any jars of jam that have mould growing on top.