Before we start in with the blueberries, a few words about the role of heat and acidity in safe canning. The whole deal with preserving is to arrest, avoid or carefully control the bacterial and enzymatic breakdown of fresh food. These include such options as freezing, drying, fermenting and canning. Canning preserves food by sealing it away from the air under vacuum and killing any bacteria trapped inside through heat, which works for the vast majority of bacterial species. Clostridium botulinum is a more complicated story. While live C. botulinum are killed at boiling water temperatures, they produce spores which aren't. The spores remain active at that temperature and thrive in an oxygen-deprived environment at between 40°-140°F, (like, say, a can at room temperature) happily producing deadly neurotoxins. The spores can be destroyed at temperatures of 240°F. However, an acidic environment with a pH of <4.6 inhibits these spores, so that the live-bacteria killing 212°F is sufficient. This pH of 4.6 marks the dividing line between high-acid foods, which can be safely processed in a 212°F boiling water bath and low-acid foods, which must be processed using a pressure canner, which can reach temperatures of 240°F.
Blueberries, along with all other berries and the majority of fruits (by the common rather than botanical definition) are classified as high-acid foods, and can be processed in a boiling water bath. The general process for canning berries is as follows:
- Obtain berries.
- Wash berries.
- Load berries into sterilized jars.
- Cover with hot juice, syrup or water.
- Seal with lid.
- Process in a hot water bath for 15-20 minutes.
Here's those steps in more detail:
1. Obtain berries.Blueberries are in season right now, and they're one of the easiest berries to pick: they lack the thorns of raspberries and blackberries, grow pretty densely and don't require stooping over like strawberries. Unless, of course, you're being hardcore and picking wild, lowbush blueberries. Good for you.
I picked this batch at Round Hill Orchard in Southampton, MA (almost on the Westfield line). If you live in Southern New England (MA, CT or RI), CISA's Farm Fresh website can help you find where to pick your own. Elsewhere, it might be worth slogging through the highly informative but awkwardly laid-out PickYourOwn.org.
2. Wash berriesPerhaps obvious, but it's important to work with clean fruit. Rinse berries in cool water and pick out any that have turned to mush. The sooner you can go from bush to jar, the fewer squishy ones you'll have to pull out.
3. Load Berries into sterilized Jars
3. Cover with hot juice, syrup or water.This is the one place you have a little creative room. According to the FDA, blueberries have a pH of about 3.7, well under the 4.6 acidity cutoff and therefore safe all on their own. However, you do need to add liquid to take up the airspace in canning. You could use water, but not only is that boring, it leaches flavor out of the berries and offers nothing in return. Many people use sugar syrup, which maintains the sweetness of the berries and helps them keep their structure to some degree. I prefer using juice, which maintains both the sweetness and the flavor outside that. I also sometimes add sugar to the juice.
I typically pair blueberries with apple juice (pH 3.4-4), but you can choose another kind you like. You can buy blueberry juice, but it's typically mixed with other juices, particularly apple and white grape. I also add some lemon juice, which helps keep the color of the berries and ups the acidity just to be safe - lemon juice (pH 2) is a stronger acid than vinegar (pH 2.4-3.4)!
pH is a logarithmic scale, which means that it increases by powers of 10. So, for example, if apple juice has a pH of about 4 and lemon juice about 2, that means lemon juice is about 102 or 100 times more acidic than apple juice. Because of this, it really doesn't take much lemon juice to significantly lower the pH. If you want to learn more about determining the resulting the pH of canning recipes, I put on my math teacher hat and made up a tutorial about finding the pH of a mixture. (coming soon!)
- light syrup is about 25% sugar; 1 part sugar to 4 parts water
- medium syrup is about 50% sugar; 1 part sugar to 2 parts water
- heavy syrup is about 75% sugar; 3 parts sugar to 4 parts water
- light juice syrup (~25% sweet); 1 parts sugar to 7 parts apple juice
- medium juice syrup (~50% sweet); 2 parts sugar to 5 parts apple juice
- heavy juice syrup (~75% sweet); 2 parts sugar to 3 parts apple juice
5. Seal with lid.Canning jars can be reused over and over again, which is a good thing, because they're expensive. You should use a new lid every time you can, though, or you risk improper sealing; the sealing compound on the underside rim deteriorates after one canning. The lids are available separately at a fraction of the cost of the jars.
The lids only need be dunked in the hot water for about 30 seconds, to warm up the sealing compound. I hold the whole thing with tongs in the water bath so I don't have to fish the lid off the bottom of the pot, then set it gently on top of the jar. Grasping the jar with a towel (it's hot!), I screw the band down as tight as I can by hand, which sets the lid in place. Then, I set the sealed jar aside until the rest have been filled and sealed.
6. Process in a hot water bath for 15-20 minutesOnce all jars are filled, they must be processed in a hot water bath. The processing time for blueberries is about 15 minutes for pints. For other canned goods, processing time will vary based not only on the pH of the contents, but also on the size of the jar, the size of the pieces, the permeability of the material, and other things. When in doubt, round up.
I typically use my 8 qt. stockpot with a vegetable steamer at the bottom for processing jars. I can fit 10 half-pints or 4 pints (quarts are too tall). It fits the necessary criteria and means a couple fewer bulky things to find a home for. If you have a lobster pot (which usually hold about 20 qts), that's also an excellent choice. When considering your own existing pots for this purpose, here are the criteria they must meet:
- has a lid
- wide enough to hold several jars
- deep enough for the jars to be totally submerged
- must meet the above criteria and accommodate something to keep the jars off the bottom of the pot
- keeps the jars raised off the bottom of the pot
- safe for use against direct heat (so, probably made of metal)
- allows for free circulation of water
You can use the sterilization bath for the processing bath. After all, it's already hot. Put your rack in the bottom of the pot and load the jars in (if you're timid about the hot water, use tongs or a jar lifter. Add water until the jars are totally submerged. Cover and turn stove back to high, if it isn't already.
After 15 minutes, turn the heat off. Lift the jars out of the canning bath and set on a towel. Wait for the beautiful metallic pop of the vacuum in the jar sucking in the lid. You'll know it when you hear it. You can also check for a good seal by pressing the center of the lid with a fingertip. A properly-sealed jar will have no give in the lid. If any of the jars hasn't sealed within an hour of canning, it probably won't. You can try re-processing it with a new lid or, if it's only one or two in a batch, stick them in the fridge and use them first. Their shelf life in the fridge is comparable to refrigerated jam, so there's no pressure to use them immediately. Those with a good seal will be safe for a very long time, but probably less fun to eat after about a year.