Homemade jam is easy to make, easy to store, and very tasty, but sometimes the consistency is too thick or too thin. It is very easy to fix the consistency while you are making the jam, but if you complete your canning before you realise that the jam is too thin, then you will have to re-can it in order to make it less runny.
Thicken jam while making it
Follow the recipe for the jam you are attempting to make. You can find ingredient quantities for the different common jams written on a leaflet inside most brands of commercial pectin.
Place a measured amount of fruit, pectin, any spices, and an optional teaspoon of light oil in a large saucepan. Bring to the boil over a medium heat.
Add your measured amount of sugar and boil for at least two minutes. Reduce the heat if the jam begins to froth or boil over.
Boil the jam gently, stirring constantly until the jam is thick enough to set properly. Note that extended boiling will destroy some of the fresh fruit flavour of the jam but will add a caramel flavour and give a thicker, smoother feel.
Perform the sheeting test by stirring the jam and then holding your stirring spoon sideways above the jam pot so that the content of the spoon will run from the side of the spoon. If jam falls in vertical drips, like water from a spoon, then it is too thin and will not set. If the jam falls in a wide blob, the width of two or three of the original streams, then the jam will set. The wider the blob, the harder the jam will set.
Perform the freezer test by pouring a spoonful of jam onto a clean plate. Place the plate in the freezer for 30 seconds. Remove the plate and press a finger along the pate and into the edge of the jam blob. If the jam is thick enough to set it will wrinkle up in little folds. If it is not yet thick enough then the jam will smear without having the top of the jam wrinkle. The thicker the wrinkles, the harder the jam will set.
Can the jam in a boiling water canner following the instructions on the pectin package.
Thicken jam after initial canning
Open all the jars of canned jam that you want to thicken and pour all the jam into a large saucepan. Discard the old lids. Wash and sterilise the old jars. Prepare canning jars and new lids.
Heat the jam over a medium heat until it reaches a steady boil while it is being stirred. Continue to boil the jam until it passes one of the thickening tests.
Perform the sheeting test as detailed in the first section.
Perform the freezer test as outlined in the first section.
Boil the jam gently, stirring constantly until one of the two tests indicates that the jam is thick enough to set properly.
Filling and sealing jam jars
Sterilise your jars and rings by washing them carefully and then boiling them in the canning kettle for ten minutes. Turn off the heat and then add the new lids to the canning kettle. This step can be done before you boil the jam. Once the jam is ready to be canned, use tongs to remove glass jars from the boiling water in the canning kettle and line them up next to the jam kettle.
Use a ladle and optionally a large funnel to fill the hot jam jars. Leave around 2.5 cm of space at the top of each jar. One hint is that larger jars normally pinch inward at the right spot for head-space, while engraved jam jars have the engraving stop at about the right spot for head-space. Overfilled jars will not seal properly.
Wipe the top of each jar with a clean paper towel to remove any drips or spills that might interfere with the seal. Place the warm canning lids on the top of the glass jars. Place a finger on the centre of each lid to hold it lid in place while you attach the ring to the jar. Do not tighten the rings; you need to let heated air escape from the head-space.
Place the jars into the canning kettle with the jar lifter or tongs. Add water if necessary so that the water level is at least 2.5 cm above the top of the tallest jar. Bring the canning kettle to the boil and boil for ten minutes or for the time specified by the recipe you are using.
Remove the jars from the boiling water using a jar lifter or tongs and place on a cooling rack. Let the jars cool fully before touching them. You should hear a "pop" sound from each jar as the vacuum created by the cooling head-space sucks the lid onto the jar to create a firm seal.Test each jar by pressing down on the centre of the lid after the jar has cooled. If the jar sealed properly then the lid will be in the down position and you will not hear anything. If the canning failed then you will hear a pop sound and will be able to press the dome of the lid down. Refrigerate jars that did not seal properly. Eat these first. Label properly sealed jars with their contents and the date and store them for up to five years.
Boiling sugar is very hot. Be careful when working with jam.