Thermal processing techniques such as extrusion generate complex chemical and physical changes in ingredients to produce final products with desired characteristics. There are many technical tools that an extrusion technologist can use to enhance his understanding of these changes. The concept of polymer science as it applies to the pet food extrusion process is one such tool that can provide valuable insight. However, measuring the glass and melt transitions of the complex recipes used in pet food production have proven difficult, if not impossible. The Phase Transition Analyzer, a closed-chamber capillary rheometer, may be one useful tool for measuring the "controlling" glass and melt transitions of complex recipes. Information gained from the Phase Transition Analyzer contains the basis for enhancing our understanding of the extrusion process as well as the potential for predictive control of the extrusion system based on raw material properties.