The invention provides a biodegradable protein/starch-based thermoplastic composition. The composition is particularly useful in preparing low ratio expanded foams. Natural cellulosic fibers such as grass fiber, wood fiber, chopped straw, bagasse, etc. function as reinforcement filler. A metallic salt hydrate is added to improve mechanical properties of the protein/starch-based thermoplastic. The composition is processed by conventional methods, such as extrusion and injection molding, into packaging material or articles that are low density, high compressive strength, tensile strength, and good resilience.