Compositions and methods for making biodegradable plastic products are provided. The compositions include a destabilizing agent that makes the finished plastic biodegradable. The compositions also can include an oil-based carrier, such as cottonseed oil, that allows the biodegrading agent to be metered into a continuous plastic manufacturing process. The compositions further can include an additive that allows the biodegrading additive to combine with the carrier; an optical brightener to improve the appearance of the finished plastic product, and to indicate the presence of the composition in the finished product; and a pigment, so that the composition can act as a liquid coloring agent.