Vegetable glycerin is used in some recipes to help maintain moisture, keep water and oil-based ingredients mixed together or add sweetness. It's usually used in relatively small amounts but still makes a difference in the outcome of the recipe, so it's best to substitute something else for the vegetable glycerin rather than simply skipping it if you can't find it in your local stores.
Other Types of Glycerin
The best substitute for vegetable glycerin -- which is usually made from palm oil, coconut oil or, less often, soybean oil -- is other types of glycerin. Animal-based glycerin is made from an animal fat, like beef tallow, so it isn't vegetarian. Synthetic glycerin is made from corn syrup, cane sugar or a petroleum-based ingredient called propylene. Both animal-based and synthetic glycerin can be used in the same amounts as vegetable glycerin in recipes and will have the same effect in the recipe. They will also have the same number of calories, with a little more than 4 per gram.
Another potential substitute for vegetable glycerin is corn syrup, as this is one substance used to make glycerin. The corn syrup won't have quite as strong an effect in the recipe as glycerin, but it will provide some of the same benefits. It's usually used in the same amounts as glycerin to avoid adding extra liquid to the recipe. This is one of the better substitutes to use if glycerin is being used at least partially as a sweetener. Corn syrup is also approximately 100 percent carbohydrates, with 4 calories per gram, so it is slightly lower in calories than glycerin.
Vegetable oil is another potential substitute for vegetable glycerin, for the same reason as corn syrup is sometimes used -- it is one of the sources of vegetable glycerin. Likewise, it's usually used in the same amounts but with less of an effect because it contains other substances as well as glycerin. This is one of the better options for a substitute if vegetable glycerin is being used mainly to help maintain moisture. It will increase the calories, however, as vegetable oil is basically 100 percent fat, with 9 calories per gram.
If you're using glycerin as a sweetener, you should be aware that it isn't a low-calorie sugar substitute. It actually has more calories than sugar and is only about 60 percent as sweet. Consuming a lot of glycerin may cause a laxative effect, but the small amounts typically used in food won't cause this effect.