Refrigerate after opening is a way of manufacturers shifting the responsibility to the consumer. Use by dates have increased the sales of some products by up to 25%. Since when did honey go off? It was found in the pyramids and tasted OK, apparently, after 2000 years. Manufacturers are finding new ways of making jams and spreads that will drop the input costs. As pointed out in previous posts, preservatives are being removed from manufactured products because of public pressure.
So to the home made jams, the making of jam is both an art and a science. There is a relationship between acids, pectin and sugars. Fruit contains fructose and small levels of sucrose. Sugarcane sugar is 100% sucrose Sugar will not gel in a low acid environment. Think Toffee - the sugar turns to toffee with the addition of a tablespoon of vinegar (the acid). The other bit of science is a gel factor so the jam is not toffee like. This is achieved through pectin. Some fruit such as apples, citrus and raspberries are very high in pectin. By combining fruits and sugar, it is possible to achieve the the right balance to achieve "Jam-set."
The art is knowing how to manage the heat and knowing how to tell when the jam is done. This can be done by a few ways. We use the frozen saucer method: take a saucer that has been in the freezer for a few hours and place a drop of jam on it. Wait 20 seconds until the drop is cold and press a fingernail against it. The Jam is done if a skin appears to wrinkle on the surface of the jam drop. https://preserveandpickle.com/test-jam-setting-point/
We have taken to adding apples at a rate 2 parts fruit to 1 part apples. EG: 1 Kg Strawberries, 500 G Apples and 1.5 Kg sugar we add 100 ml Lemon juice for acidity. This gives about 6 X 250 ml jars of jam when finished.
Apple gives both pectin and fibre plus tempers the sweetness. We always grate the apple and add immediately so it does not go brown (oxidise). The mixture less the lemon juice is soaked for 12 hours to draw the juice from the fruit. Stir the mixture and put on the stove to cook. Add the lemon juice after the sugar is dissolved and the mixture boiling. Continue to cook at the highest setting, stirring regularly to prevent burning. Once the mixture stays on the pot side, start testing for setting. Jam needs to be cooked quickly to achieve gelling or it becomes toffee-like.
Bottle in small jars and place lids on ASAP. By turning the jars upside down, the lids are sterilised. Wait 5 minutes and turn the jars right way up and leave to cool. The jars will seal. Label once cooled. Jam is much better once it has had time to mature.
Make sure that only a clean knife or spoon is used to remove the jam from the jar and the lid is replaced as soon as possible.
The mould that forms on the jam is easily removed and does not detract from the taste of the jam. A mould layer acts as a barrier to bacteria and other yick. It is removed with a spoon by scraping. We have kept home made jam for over 5 years sealed and it was still in good condition.
Hope that this adds another dimension to your jam making. We make 900 jars per year to sell at a local co-op.