A melt-extruded substrate (e.g., film, nonwoven web, etc.) that contains a thermoplastic starch formed from a starch and plasticizer is provided. The starch and plasticizer are melt blended together in the presence of a weak organic acid (e.g., lactic acid, formic acid, acetic acid, etc.). By selectively controlling certain parameters of the melt blending process (e.g., extrusion temperature, content of the components, etc.), the present inventors have discovered that the starch may be hydrolyzed in a highly efficient manner to form compositions having a comparably lower weight average molecular weight, polydispersity index, and viscosity, which are particularly suitable for use in the formation of melt-extruded substrates.