Rum extract is a cooking ingredient made from rum. It has a concentrated rum flavor, without the high alcohol content associated with real rum. Depending on the company which makes it, this ingredient usually contains a small amount of alcohol, although alcohol-free versions are also available. Like other extracts, rum extract keeps essentially indefinitely as long as it is stored in a cool, dry place out of the light.
There are two basic kinds of rum extract: natural, and imitation. The natural type is made with real rum, and it has a full, rich, complex flavor much like that of actual rum. Imitation versions are made with artificial ingredients, and tend to have a much simpler, less interesting flavor. As a general rule, imitations are significantly cheaper than real versions.
People use rum extract in cooking for a variety of reasons. For one thing, the flavor is concentrated, so when a cook wants a rum flavor without disrupting the liquid balance of a food, rum extract can be used. It also tends to be less expensive than actual rum, and for people who do not drink, it may be preferable to buy a small bottle of extract for a recipe rather than a bottle of rum which will never be used. It also keeps for extended periods of time, making it a shelf-stable addition to the ingredient library.
If you have a recipe which calls for real rum and you want to use rum extract instead, you can convert the recipe. As a general rule, for every two tablespoons of dark rum in a recipe, one tablespoon of extract can be used. For every five tablespoons of light rum called for, one tablespoon of extract is usually sufficient. Because significant differences in liquid levels can emerge when doing these conversions, some cooks like to add water to make up for the missing liquid.
Cooks can also convert the other way, using real rum instead of rum extract. However, this can get complicated, especially in cakes, where the amount of liquid plays a critical role in how the cake bakes. Too much liquid can interfere with the finished texture of the cake, creating an unpleasant mess rather than the desired product. The higher alcohol content of true rum can also interfere with the cooking process.