英语时差：蜂蜜：可作食品酵母和天然防腐剂

Leave it on the shelf too long and honey begins to crystallize. Heating the honey returns it to its original condition, but if you add water, you could ruin the honey completely. That's because honey, which is a preservative in its original state, ferments once it's diluted.

In ancient times, honey was used as a food preservative. It works as a preservative because the high concentration of sugar in honey forces the water out of any yeast or bacteria cells that could otherwise contaminate the food. This process, known as "osmosis," is also what makes salt such an effective preservative. Eventually, the process of osmosis destroys those cells by drawing out all their water--in other words, by drying them up.

So because of its high sugar concentration, pure honey will never ferment or go bad. It may crystallize but by soaking the jar in hot water you can turn it back into a liquid. Adding water, however, lowers the concentration of sugar in the honey and turns a natural preservative into an excellent food for yeast and bacteria.

You may not want a fermented jar of honey on your shelf, but ancient people turned that characteristic of honey to their advantage in creating what may well have been the very first alcoholic beverage. Honey mixed in water and allowed to ferment, produces the drink called "mead."So you may or may not want to add water to your honey. If what you want is mead, go ahead, but if you want your honey to last, . . . keep it pure.

Next time on "A Moment of Science," we'll see why honey can help baked goods last longer.