Sourdough starters may be about unrestricted microbial growth, but preserves are about putting the hammer down. As mentioned in the previous post, jams put the squeeze on bugs primarily (though not exclusively) via sugar. Sugar does two things to ruin a bug party. First, it dissolves into solution and binds up water molecules, preventing microbes from using them. This in and of itself would make sugar an excellent preservative, since spoilage is really a numbers game. A few living microbes won’t do anyone any harm, it’s what they do as a group that’ll kick your carapace. Yet sugar takes moisture theft a step further, actually drawing it out of the microbes themselves, thus incapacitating and/or killing them.
The ancients discovered the miraculous preservative power of sugar when they immersed fruits in jars of honey. And while doing this did keep microbes from growing, it didn’t do much to stave off the effects of the other big players in the spoilage game: enzymes. Enzymes, as we’ve discussed, aren’t living things, they’re chemicals, tools that organisms use to execute a variety of biologically important tasks, including breaking down food into its component parts. The trouble with enzymes is that once they start working they don’t stop. They have to be deactivated, and in the world of preserving that means heat.
Here again the ancients discovered the connection. By boiling their fruits, either in honey or a severe reduction of fruit juice, they got a much longer lasting preserve. The heat shut down the enzymes, and killed any bugs that might have still been living in the bargain. We still use that basic method to this day.