Put the pureed blueberries in a 4-quart capacity slow cooker. Place a lid on the pot and turn it on to low. After it has cooked for 1 hour, remove the lid and give it a stir. From this point forward, you will want keep the lid slightly cracked. I have found that propping it open with a wooden spoon or chopstick gives just enough room for the evaporating steam to escape.
This butter will need between 4 and to 8 total hours total in the slow cooker. The time varies depending on how hot your slow cooker cooks. Check the butter at least once an hour to track the progress.
In the final hour, add the sugar, lemon zest and juice, and spices. If you want to speed the evaporation, remove the lid and turn the cooker up to high. If you do this, make sure to check and stir the butter every 10 minutes to prevent scorching.
When the butter is nearing completion, prepare a boiling water bath and 3 regular-mouth 1-pint/500 ml jars according to the process (see below). Place the lids in a small saucepan, cover them with water, and simmer over very low heat.
Once it has cooked down to be as thick as ketchup and spreadable, determine whether you like a chunky or smooth butter. For a smoother texture, puree the butter using an immersion blender (or in batches in a blender); for a slight chunkiness, leave it as it is.
Turn the slow cooker off and ladle the butter into the prepared jars. Wipe the rims, apply the lids and rings, and process in a boiling water bath for 10 minutes.
The sealed jars can be stored in a cool, dry place for up to 6 months.
How to Process:
If you're starting with brand-new jars, remove their lids and rings. If you're using older jars, check the rims to make sure there are no chips or cracks.
Put the rack into the canning pot and put the jars on top.
Fill the pot (and jars) with water to cover and bring to a boil. I have found that this is the very easiest way to heat up the jars in preparation for canning because you're going to have to heat up the canning pot anyway. Why not use that energy to heat up the jars as well?
Put the lids in a small saucepan, cover with water, and bring them to the barest simmer on the back of the stove.
While the canning pot comes to a boil, prepare your product.
When your recipe is complete, remove the jars from the canning pot (pouring the water back into the pot as you remove the jars) and set them on a clean towel on the counter. There's no need to invert them; the jars will be so hot that any remaining water will rapidly evaporate. Remove the lids with tongs or a magnetic lid wand and lay them out on the clean towel.
Carefully fill the jars with your product. Depending on the recipe, you'll need to leave between 1/4 and 1/2 inch/6 mm and 12 mm of headspace (that's the room between the surface of the product and the top of the jar). Jams and jellies typically get 1/4 inch/6 mm, while thicker products and pickles get 1/2 inch/12 mm.
Wipe the rims of the jar with a clean, damp paper towel or the edge of a clean kitchen towel. If the product you're working with is very sticky, you can dip the edge of the cloth in distilled white vinegar for a bit of a cleaning boost.
Apply the lids and screw the bands on the jars to hold the lids down during processing. Tighten the bands with the tips of your fingers to ensure that they aren't overly tight. This process is known as "fingertip tight."
Carefully lower the filled jars into the canning pot. You may need to remove some water as you put the jars in the pot. A heat-resistant measuring cup is the best tool for this job, as it won't transfer heat to your hand.
Once the pot has returned to a rolling boil, start your timer. The length of the processing time will vary from recipe to recipe.
When your timer goes off, promptly remove the jars from the water bath. Gently place them back on the towel-lined countertop and let them cool.
The jar lids should begin to ping soon after they've been removed from the pot. The pinging is the sound of the seals forming; the center of the lids will become concave as the vacuum seal takes hold.
After the jars have cooled for 24 hours, remove the bands and check the seals. You do this by grasping the jar by the edges of the lid and gently lifting it an inch or two off the countertop. The lid should hold fast.
Once you've determined that your seals are good, you can store your jars in a cool, dark place (with the rings off, please) for up to a year. Any jars with bad seals can still be used -- just store them in the refrigerator and use within 2 weeks.
Recipes reprinted with permission from Food in Jars (c) 2012 by Marisa McClellan, Running Press, a member of the Perseus Book Group.
Properly-handled sterilized equipment will keep canned foods in good condition for years. Sterilizing jars is the first step of preserving foods.
Jars should be made from glass and free of any chips or cracks. Preserving or canning jars are topped with a glass, plastic, or metal lid, which has a rubber seal. Two piece lids are best for canning, as they vacuum seal when processed.
To sterilize jars, before filling with jams, pickles, or preserves, wash jars and lids with hot, soapy water. Rinse well and arrange jars and lids open sides up, without touching, on a tray. Boil the jars and lids in a large saucepan, covered with water, for 15 minutes.
Use tongs when handling the hot sterilized jars, to move them from boiling water. Be sure the tongs are sterilized too, by dipping the ends in boiling water for a few minutes.
As a rule, hot preserves go into hot jars and cold preserves go into cold jars. All items used in the process of making jams, jellies, and preserves must be clean. This includes any towels used, and especially your hands.
After the jars are sterilized, you can preserve the food. It is important to follow any canning and processing instructions included in the recipe and refer to USDA guidelines about the sterilization of canned products.