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.