Good and safe preserving comprises multiple layers of preventive measures, counter-counter-measures and safeguards. There are a lot of bugs out there, after all, so it pays to cover every last anti-microbial base. We’ve talked sugar, we’ve also talked heat. This time let’s talk about acid.
Acid is a time-tested bug killer. It’s found in copious amounts in vinegar (5-7% acetic acid), which is the foundational ingredient in pickling. But then vinegar and blueberries aren’t what you’d call a classic combination. Citric acid on the other hand will do quite nicely, which is why a typical blueberry jam recipe calls for lemon juice. We wouldn’t have to take this step if we were making, say, orange marmalade, but blueberries are naturally low in acid, so we need to beef up their defenses.
Why is acid important when we’ve got all this heat and sugar? Because there is one particular bug out there that can take an awful lot of both. It can also live quite well without oxygen, in fact it prefers it, which is why it’s been the bane of canners ever since the science was first invented. I’m talking of course about Clostridium botulinum, a bacteria that produces a toxin which, if consumed in high enough doses, leads to a condition known as botulism. What makes botulinum toxin so dangerous is that it’s a nerve toxin, which means it affects things like motor function, speech and eyesight, eventually causing muscle paralysis, respiratory paralysis and asphyxiation.
Fortunately there isn’t much of it around anymore, only about 100 cases in the U.S. a year. Also, thanks to advanced treatments (which are made that much more effective when the condition is treated promptly), only about 5 of those cases end in fatalities. Yet you can never be too careful, and anyway, I like my jam with a tang.