How to Thicken Homemade Jam

Follow the recipe for the jam. You can find quantities for common jams written on a flyer inside most brands of commercial pectin. Recipes are also available in reference works like the Ball Blue Book of Canning.

Place a measured amount of fruit, pectin, spices and an optional 1 teaspoon of light oil like canola oil in a large saucepan. Bring to a boil over medium heat.

Add the measured amount of sugar and boil for at least 2 minutes. Reduce the heat if the jam begins to froth or boil over.

Boil the jam gently, stirring constantly until the sheeting test or freezer test detailed below indicates that the jam is thick enough to set properly. (Note that extended boiling destroys some of the fresh fruit flavor of the jam but adds caramel flavor and gives a thicker, smoother tongue feel.)

Perform the sheeting test by stirring the jam and then holding the stirring spoon sideways above the jam pot so that the content of the spoon runs from the side of the spoon. If jam falls in vertical drips, like water from a spoon, then it's too thin and will not set. If 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 plate 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's not yet thick enough, the jam will smear without the top of the jam wrinkling. 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 or the instructions below.

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 sterilize the old jars. Prepare canning jars and new lids.

Heat the jam over medium heat until it reaches a steady boil while being stirred. Continue to boil the jam until it passes the sheeting test or freezer test. (Note that extended boiling will destroy some of the fresh fruit flavor of the jam but adds caramel flavor and gives a thicker, smoother tongue feel.)

Perform the sheeting test by stirring the jam and then holding the stirring spoon sideways above the jam pot so that the content of the spoon runs from the side of the spoon. If jam falls in vertical drips, like water from a spoon, then it's too thin and will not set. If 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 plate 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's not yet thick enough, the jam will smear without the top of the jam wrinkling. The thicker the wrinkles, the harder the jam will set.

Sterilize jars and rings by washing them carefully and then boiling them in a canning kettle for 10 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 to fill the hot jam jars. Leave 1 inch of headspace at the top of each jar. Pint jars normally pinch inward at the right spot for headspace, while engraved jelly jars have the engraving stop at about the right spot for headspace. Overfilled jars do not seal properly.

Wipe the top of each jar with a clean new 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 center of each lid to hold it in place while you attach the rings to the jars. Do not tighten the rings—you need to let heated air escape from the headspace.

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 1 inch above the top of the tallest jar. Bring the canning kettle to a boil and boil for 10 minutes or for the time specified by the recipe. Longer boiling reduces the fruit flavor but adds some caramel flavor.

Remove the jars from the boiling water using a jar lifter or tongs, and place them on a cooling rack. Let the jars cool completely before touching them. You should hear a "pop" sound from each jar as the vacuum created by the cooling headspace sucks the lid onto the jar to create a firm seal.

Test each jar by pressing down on the center of the lid after the jar has cooled. If the jar sealed properly, the center will not pop. If it does pop, the canning failed. In the case of a failed canning, refrigerate those jars and eat them first. Label properly sealed jars with their contents and the date canned, and store them for up to five years.