Like many foods in your pantry, molasses is a long-lasting ingredient with an extended shelf life. Most manufacturers stamp their molasses with a one-year "best before" date, so you might be tempted to freeze it if you only occasionally use the product. In practice, freezing is not a good idea.
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Molasses is a by-product of the sugar-refining process. When sugar cane is crushed and its juices reduced to concentrate the sugars, a number of impurities and flavor molecules are concentrated along with the sucrose. That's molasses, which is spun off from the sugar in a centrifuge. The molasses still contains usable sucrose, so the "first" molasses is usually recrystallized and spun again to extract more sugar. The resulting "second" molasses is darker and stronger-flavored, and a third extraction results in bitter blackstrap molasses. Commercial molasses blends the first and second extractions in varying proportions to provide light table molasses and darker baking molasses.
Fruit juices typically freeze to a tooth-challenging hardness, like water itself. Yet if that juice is mixed with a sugar syrup, it becomes a relatively soft sorbet. That's because sugar inhibits the freezing of liquids, helping keep them soft. In the case of molasses, the concentration of sugars is very high. Light table molasses are about 35 percent sucrose and 35 percent invert sugars, a concentration that keeps it from freezing except at very low temperatures.
Theoretically, keeping molasses in your freezer might extend its shelf life, but in practice it's not so simple. There's a high risk that your molasses will absorb undesirable flavors from the other items in your freezer, and although the molasses itself is unlikely to freeze solid, its water content might crystallize. This can have an adverse effect on the molasses when it thaws, and manufacturers advise against freezing for that reason. On a more practical note, cold and sluggish molasses can be difficult to clean out of your freezer if its container should be damaged.
Fortunately, freezing isn't necessary to ensure a long useful life for molasses. It's so concentrated that it doesn't have enough moisture to support the growth of mold or bacteria, making it safe even after extended storage. Most manufacturers simply state that the quality might deteriorate after the best-before date. To ensure the longest shelf life, always pour it rather than dipping utensils into the container. That minimizes the risk of contamination from outside elements. If the molasses should be contaminated by liquids, that can create areas diluted enough to support bacterial growth. If you use molasses infrequently, buy it in small containers and keep it in a cool, dark cupboard. Keeping the container inside a sealed plastic bag can help prevent contamination, especially from insect pests.