Function of starch in extruded products 1. Expansion It is largely the starch which determines the amount of expansion, and therefore the bulk density Degree of expansion is greatly affected by the amylose famylopectin ratio higher amylose materials tend to allow greater expansion, but the amylopectin gives structure to HOLD the expansion But other factors also affect expansion eg both corn and rice are quite high in amylose, both expand well. However, corn alone tends to provide a relatively small number of large bubbles. The addition of rice tends to encourage the formation of more but finer bubbles providing a finer texture