To prepare for water-bath canning:
- Wash jars and lids in hot soapy water; rinse with warm water.
- Fill canning pot half full of water; add jars and water to cover. Bring water to a boil, reduce heat and simmer.
- Place lids in water to cover in a small saucepan; bring water to a simmer. Simmer until ready to use.
- Remove and drain jars and lids, one at a time, as needed for filling.
To make the jam:
- Fill a stock pot ¾ full of water; bring to a boil. Working in batches, add plums and boil for 20 seconds or until skins easily peel away; remove with slotted spoon and transfer to ice water.
- Use a paring knife to slip skin off plums; cut in half and remove pits. In the bowl of a food processor, add plums (working in batches if necessary) and process until finely chopped. Measure exactly 5 cups of fruit.
- In a large, heavy stock pot, combine plums, lemon juice, and pectin; stir until pectin is dissolved. Let stand 10 minutes.
- Bring mixture to a full rolling boil (a boil that doesn’t stop bubbling when stirred) over medium-high heat. Boil for 1 minute, stirring constantly. Remove from heat; add Splenda Sweetener, stirring until sweetener dissolves. Skim off any foam. Stir in almond extract, if desired.
To can the jam:
- Ladle hot jam immediately into prepared jars, leaving ½” of headspace at top of jars. Wipe jar rims and threads. Cover with 2-piece lids; tighten bands.
- Place jars on rack in canning pot. (Water must cover jars by at least 1”; add boiling water, if necessary.) Cover; bring water to a gentle boil.
- Process for 10 minutes. Remove jars and place upright on a towel to cool completely.
- After jars cool, check seals by pressing center of lid with finger. (If lid springs back, lid is not sealed and refrigeration is necessary.)
- Store in a cool, dark place for up to 1 year. Once opened, store refrigerated and use within 2 weeks.