Spiced Purple Plum Jam

Instructions

• Submerge 6 empty half-pint jars on a rack in a large pot of water. Cover and bring to a rolling boil, then reduce heat to low to keep jars warm until ready to fill.
• Tumble plums, lemon zest, and lemon juice into a wide, nonreactive 8-quart pot. Whisk together sugar, pectin, cinnamon, nutmeg, and cloves in a small bowl, then stir into pot until combined.
• Bring to a vigorous boil over high heat. Reduce heat to medium-high; cook, stirring frequently and adjusting heat as needed to maintain a gentle boil, until volume has reduced by at least one-third, 20 to 25 minutes. Remove pot from heat and check for set.
• Working with one jar at a time, remove empty jars from canning pot. Using a wide-mouth funnel, carefully ladle jam into jars, leaving 1/2 inch for headspace. Use a clean wooden chopstick to work air bubbles out of jars. Check headspace again and add more jam if necessary to bring to 1/2 inch from the top.
• Wipe jar rims, apply lids and bands (not too tightly), and return jars to canning pot; cover pot and return water to a rolling boil. Process 10 minutes. Turn off heat, remove pot lid, and let jars stand in the cooling water 5 minutes to help ensure a good vacuum seal.
• Move jars to a folded kitchen towel or wooden cutting board to cool completely before checking seals. Any unsealed jars should be refrigerated and eaten promptly. Store sealed jars in a cool, dark place. They are shelf-stable at least 1 year.