Jam can ONLY be made in small batches at a time (about 6 cups at a time). DO NOT increase the recipe or the jam will not "set" (jell or thicken).
Preparing the equipment: Before you start preparing your jam, place canner rack in the bottom of a boiling water canner. Fill the canner half full with clean warm water for a canner load of pint jars. For other sizes and numbers of jars, you will need to adjust the amount of water so it will be 1 to 2 inches over the top of the filled jars. Wash jars, lids, and rings in hot soapy water; rinse with warm water.
Sanitize the jars, lids, and rings. Never plunge room temperature jars into rapid boiling water or they may crack. Place the jars in a large pot. Add 1-inch of water to the bottom, cover securely, and bring to a boil for 10 minutes. Keep the jars, lids, and rings in the hot water until they are ready to by used.
Preparing the blackberries: Gently wash, stem, and drain the blackberries (removing any stems, cores, dried-up berries, and leaves).
Crush the blackberries with a potato masher, food mill, or use a food processor (If using a food processor, pulse to very fine chop) to lightly crush them. For best results, crush 1 cup at a time. DO NOT PUREE. Jam should have bits of fruit. Sieve all the pulp to remove the seeds. You can keep some seeds in for effect, but I would not recommend for blackberry jam since the seeds are hard on the teeth.
Making the jam: Measure the exact amount of sugar into a separate bowl; set aside. NOTE: Reducing sugar or using sugar substitutes will result in failure for the jam setting up. To use less sugar, you must purchase and use the pectin for Less Sugar or No Sugar.
Measure the exact amount of prepared blackberries (juice) into a large 6 to 8-quart saucepan. Stir in 1 package/box fruit pectin into the blackberry fruit mixture. Add the 1/2 teaspoon butter to reduce foaming, if desired.
Over high heat, bring mixture to a full rolling boil, stirring constantly. A full rolling boil is a boil that does not stop bubbling when stirred. Quickly stir in the sugar and return again to a full rolling boil and let boil exactly 1 minute, stirring constantly.
If you bring it back to a full boil fairly slowly (on medium heat rather than high) that will help reduce foaming.
Remove from heat and quickly skim of any excessive foam/bubbles.
Place jam into the jars: One jar at a time, ladle the hot prepared jam into the hot, sterilized jars. Fill to within 1/8 inch of top of jar. Wipe rim of jar or glass with a clean damp cloth. Immediately place a hot lid and ring on top of the jar; tightly screw the ring on the jar.
Processing the jam: Place jars on the elevated canner rack. Lower rack into the canner with the hot water. Add more boiling water, if needed, so the water level is at least 1 inch above the jar tops. Pour the water around the jars and not directly onto them. Cover the canner with a lid. Process in a boiling water canner for 10 minutes (start time when the water returns to a boil). Adjust processing time according to altitude chart below.
After 10 minutes, remove jars with a jar lifter and place jars upright on a towel or cooling rack to cool completely. Leave at least one inch of space between the jars during cooling. Avoid placing the jars on a cold surface or in a cold draft. Let the jars sit undisturbed while they cool, from 12 to 24 hours. Do not tighten ring bands on the lids or push down on the center of the flat metal lid until the jar is completely cooled.
After jars cool, check seals by pressing middle of lid with your finger (if lid springs back, lid is not sealed and refrigeration is necessary). Put any unsealed jars in the refrigerator and use first.
Label jars and store the sealed jars in a a cool, dry, dark place up to 1 year. Refrigerate any open jars up to 3 weeks.
Yields 9 cups.
Altitude Adjustments for Boiling Water Bath Canner:
Altitude In Feet - Increase Processing Time
1,001 to 3,000 - 5 minutes
3,001 to 6,000 - 10 minutes
6,001 to 8,000 -15 minutes
8,001 to 10,000 - 20 minutes
There are a number of circumstances that affect whether or how well your jam sets:
- Pectins are somewhat sugar amount specific. Also liquid pectins do not set up as well as powdered. Use only the type of pectin called for in your recipe.
- Powdered pectin and liquid pectin are not interchangeable in recipes. The preserving books seem to confirm that the reason liquid and powdered pectin may not be interchangeable is that the liquid version is always added after boiling but most types of powdered are added to the raw fruit or juice.
- Never EVER double a jelly or jam recipe. For some reason, this effects the setting.
- If your jam does not set, chances are it contains too little pectin.
- An imbalance between the pectin and the acid in your jelly. Adding a little lemon juice helps the pectin, and also helps create an environment hostile to bacteria.
- High humidity in the kitchen can cause problems with jam and jelly.
- Finally, you can defeat the whole purpose of adding pectin if you boil the mixture too long - overcooking causes the pectin to break down and lose its thickening capacity.
Complete Guide To Home Canning, United States Department of Agriculture, Preparing and Canning Jams and Jellies.
National Center for Home Food Preparation, Using Boiling Water Canners, Elizabeth L. Andress, Ph.D., Professor and Extension Food Safety Specialist,Department of Foods and Nutrition,