Homemade marmalade sometimes does not set properly and turns out runny if there was not enough pectin added during the original processing. Oranges and other low-pectin fruits may require additional powdered pectin to be added during the process, but you're unlikely to discover this until the marmalade has been canned. If one jar did not set it is likely the entire batch did not set properly. Turn the jars upside down to see if the contents slosh inside. Don't throw out the runny marmalade because it can be reprocessed in small batches of 6 to 8 cups at a time.
Things You'll Need
No-sugar powdered pectin
Hot water canner
Figure out the number of cups of marmalade that need to be remade. Four 8-ounce jars equal 1 quart, which is a simple quantity to work with.
Add 1/4 cup sugar, 1/4 cup water, 2 tablespoons lemon juice and 4 teaspoons powered pectin for each 1 quart of marmalade being reprocessed to a large kettle and mix well.
Bring the mixture to a boil on the stove while stirring continually to prevent it from burning. Open the jars and pour the marmalade into the kettle and bring the contents to a boil while stirring. Boil for one minute.
Remove the kettle from the heat and skim any foam off the top of the marmalade. Fill sterilized jars with the reprocessed marmalade to 1/4 inch from the top. Place a new lid that has been sterilized in boiling water on top of the jar and tighten with a canning ring. Do not reuse the old lids as they will not seal.
Process the marmalade in a hot water bath for 5 minutes when at an altitude of 0 to 1,000 feet. Process for 10 minutes between 1,001 and 6,000 feet of altitude and for 15 minutes above 6,001 feet.
Make a note to add 20 percent more pectin to the recipe the next time you make it. Boil the pectin mixture for 1 minute or slightly longer to ensure it will process correctly. Sterilize jars in a dishwasher if it has a sterilize setting.