Like many home preservers, I've often wondered at the USDA's recommendation that one always use commercially bottled lemon juice when following tested recipes. I understand the underlying logic: You need a certain level of acidity to ensure safe conditions, and bottled lemon juice is guaranteed to meet a minimum acidity level of 4.5%. Consequently, in tested recipes where that level of acidity is sufficient (most say 5%, but since all USDA tested recipes refer to bottled lemon juice, I assume they were rounding up), all bottled lemon juice will ensure safety. But the elephant in the room has always been: Are lemons naturally acidic enough, or must we rely on the bottled stuff, which is often adulterated with sulfites?
Linda Ziedrich at A Gardener's Table also wondered, and decided to do a little research into the matter. (It helps that she's married to a chemist.) It turns out that, even though acidity will vary from lemon to lemon, pretty much all lemons (excluding Meyers) exceed the minimum acidity required for safe use in tested recipes -- sometimes by quite a lot.
Of course the USDA could never recommend using fresh lemons specifically due to the variability of acidity in the fruit. But knowing that the variability is irrelevant so long as it stays above safe levels dispels any fears for me, and I intend to use fresh lemon juice from now on. Moreover, in many cases where we use lemon juice, it's less for safe acid levels and more to make pectin set.
Linda's experimentation and learnings are fascinating, so do go read it to get the full story. And I'm just dying for the sequel when she explains how to titrate juice to test for acidity. Aren't you?