As I mentioned yesterday, the primary purpose of jam — other than being a spread of almost unspeakable pleasure — is to preserve fruit. For sugar, as I have oft written, is as lethal to microbes as salt…albeit in a slightly different way. Salt kills microbes by osmosis, literally extracting water from them through their cell walls. Sugar doesn’t perform this same function, though being itself hungry for water (what scientists call hygroscopic) it binds up all available water, thus depriving microbes of the medium that they need to reproduce and grow.
The ancients capitalized on the preservative powers of sugar by immersing various foodstuffs in honey. It was a fairly effective technique, though not as foolproof as modern jam-making. For one, because honey is not pure sugar (about 20% of honey is water). Second, because the immersion process didn’t involve cooking, it didn’t shut down food enzymes, which degraded the preserves over time. Even so, honey immersion was the dominant food preservation method in Europe well into the Middle Ages. Because sugar cane grew naturally in the Middle East, the Arabs were onto cooked fruit and sugar preserving much earlier. A little something called the Crusades, however, prevented them from swapping recipes with Europeans at tea parties.
In time, some sugar did start coming into Europe in dribs and drabs from territories to the east. Yet it wasn’t until Europeans began producing their own sugar in the New World that jam making started in earnest, and even then it didn’t become commonplace until the late 18th century when sugar became abundant enough for the everyman to afford. Home canning was obviously the next big step in the evolution of jam making, but more on that later in the week.