A batch of thick, flavourful jam is the reward for spending an afternoon preparing the fruit, cooking it down with sugar and lemon juice, and then washing and filling jars. Nevertheless, sometimes your efforts to make jam go unrewarded, leaving you with a batch of runny or loose jam. A jam needs a sufficient amount of acid and pectin to set.
The jam or jelly won't set if one or the other is absent. Both pectin and acid levels are measurable. The jam must next be cooked for a sufficient amount of time to enable the pectin molecules to bond together, assuming there is enough pectin. Don't worry, though; runny jam can be fixed, and thickening it takes a little patience. Here are some tips for fixing loose jams.
Be Patient And Wait
Even though you followed the instructions exactly and checked the jam's thickness by spreading some of the cooked jam on a cold spoon straight out of the freezer, the processed jam still seems runny in the canning jars. Now is the moment to be patient, as many jams—strawberry jam, in particular—take longer than expected to gel and cool fully. If you're not sure about the thickness, chill a single jar or leave the jars at room temperature overnight, and then check the jam the next morning.
Incorporate Cornstarch
It's usual practice to thicken jams, glazes, soups and sauces with cornflour. It can also be used as a substitute in jam recipes that call for less sugar or pectin or in fruits that naturally have reduced sugar or pectin content. One thing to be aware of is that using corn flour as a thickening agent might give the jam an unclear or milky appearance. Mix the cornflour and water to a slurry and stir it into the jam mixture. Bring to a boil and the jam should thicken almost immediately.
Cook Or Boil It Again
Pectin-free jam cooking calls for patience and skill. Even the finest homemade jammers occasionally produce a runny batch. After some time has passed and the jam is still too runny for your taste, dump the jars back into a large saucepan and reheat the mixture. Try again after sterilising and washing the jars. You may thicken the jam by adding chia seeds or commercial pectin, but you can also just reduce the jam until you reach your desired consistency.
Incorporate Chia Seeds In The Jars
Because chia seeds naturally gel, you can make a quick jam by mashing fruit and sugar with a few tablespoons of chia seeds. You can use such gelling qualities in jars of loose jam as well. Toss to mix. Transfer 1 tablespoon of chia seeds into each 8-ounce jar (if you'd like the seeds to be less noticeable, you may also purée the jam and seeds together).
Low-Oven Method
Rather than thickening the jam once more on the burner, transfer the jam onto a rimmed baking sheet and 'dry' it for one or two hours at a low temperature (let's say 95 °C). Without requiring any actual cooking, this will concentrate the flavour and lower the moisture content of the jam. While a little more labour-intensive than the other techniques, this approach is excellent for delicate jams such as persimmon or apricot.