@TheFoodGeek – Another canning Q for you: is there a min amount of sugar needed for safe processing of fruit butters?
When you are canning for long-term storage, you have two basic types: low-acid, and high-acid. Most of your jams, jellies, and marmalades are high-acid. Things like soups and butters are low-acid.
For safety, you are concerned with killing bacteria; botulism is a quick way to ruin an otherwise good dish. And while high sugar concentrations will keep bacteria from thriving, that’s not what you are concerned with in canning. Instead, you kill off all the bacteria with heat and, when appropriate, acid.
If you have a high-acid food, then you can effectively kill off the bacteria at sea-level or higher boiling temperatures. With low-acid foods, you have to use higher temperatures, so you need to use a pressure canner to get the cans up to the proper temperature.
Butters tend to be high-acid recipes, so you should be able to get by with the simpler boiling method instead of the pressure canner. Just don’t skimp on the lemon juice or vinegar. If you have a good pH meter and know how to use it properly, you’re looking for a pH of 4.6 or lower. If not, stick with tested recipes or store in the refrigerator or freezer.
The National Center for Home Food Preservation has a great site for learning all about various methods of preserving foods. I tend to check there before I embark on a canning project that I haven’t done before.