One of the most ominous sounding ingredients in a fruit-flavored beverage is glycerol ester of wood rosin. Indeed, there is actually a trace of real wood rosin in many citrus-flavored sodas and other fruit drinks. There is a perfectly good explanation for the presence of wood rosin, and it has to do with the age-old idea that oil and water do not mix.

In order to achieve a pleasing and authentic fruit flavor in a water-based beverage, manufacturers often use flavoring oils derived from citrus fruits such as oranges, lemons or limes. These flavoring oils are very concentrated, and must be balanced out with sweeteners before a beverage becomes palatable. The problem is that fruit oils do not mix well with carbonated water or even natural fruit juices. The flavoring oils would simply float on top of the beverage, even after vigorous stirring or mixing.

This is where the wood rosin enters the picture. Rosin is collected from the stumps of long-leaf pine trees, then mixed with other ingredients to form a thickening agent called ester gum. Ester gum is often used to thicken or stabilize food products such as chewing gum or ice cream, but would be too thick for beverages. When the ester of wood rosin is combined with glycerol, however, it is suitable as a stabilizer.

Although water and oil do not mix, they can be blended into an emulsion. Mayonnaise, for example, would be an emulsion of eggs and vegetable oil. In the case of fruit-flavored beverages, the addition of glycerol ester of wood rosin allows the fruit oils to remain in suspension when blended with water. This means that a can of orange-flavored soda would have a consistent orange flavor, not a layer of orange oil floating on top of carbonated water. Without the ester of wood rosin as a stabilizer, the flavoring oil would eventually separate and the beverage would become unpalatable.

There is no evidence that wood rosin poses any sort of health threat to consumers. Federal regulations prevent more than 100 parts of wood rosin per million, which essentially means there is only a trace amount of actual wood rosin in a typical can of citrus-flavored soda. There are other stabilizers approved for use in beverages, but manufacturers consider glycerol ester of wood rosin to be the best and most natural option.