Virtually any type of fruit can be used for canning jams, as long as it is completely ripe and without blemishes. Before preserving foods, the fruit should be washed and peeled; pits or large seeds should also be removed. Even if a very sweet type of fruit is used, bottling foods in this manner may require a great deal of sugar. Canning jams should be done at a rapid boil, which might make it necessary to stir the product often or skim foam from the top of the mixture.
Jam is often made from berries because they are normally very meaty and contain little juice. Even so, larger types of fruit such as pineapple, plum, or fig are often used for canning jams. No matter what fruit is used, it can be very important to extract most of the juice before preserving these foods because doing so will result in a thicker and sweeter finished product.
Fruit should be washed carefully to prepare it for home canning. While doing so, it can be inspected for insects, and any leaves or stems can be removed at this time as well. If there are any bruised spots on the fruit, these areas can be cut out with a paring knife, but extremely damaged fruit should be discarded.
Cooks may need to add around 3/4 cup (0.18 l) of sugar to each cup (0.24 l) of fruit when canning jams. Slightly more might be needed if a bitter fruit such as raspberry is being used, but a sweet fruit like pineapple might require a little less than that amount. Artificial sweetener or honey may also be used to make diabetic jam. If one of these agents is substituted, it can be a good idea to use a recipe that calls for one of these ingredients to make sure the right amount of these items are used.
The fruit should be added to a stainless steel pot, and then cooked over high heat. This can take anywhere from 15 to 40 minutes depending on the type of fruit and desired consistency. Since the fruit mixture may have a tendency to stick, it is very important that it be stirred frequently so the jam will not scorch and become unusable.
While the fruit is boiling, some foam may appear near the top of the mixture. This is a common occurrence when canning jams, especially when using blackberries or raspberries. It can be helpful to keep a ladle nearby so the foam can be skimmed off, as doing so can help ensure a thick and sweet finished product.