Polymer science—the study of the glass and melt transitions of polymers—has been developed and used by the synthetic polymer industry for many years. In recent years, the principles of polymer science have been applied to the petfood extrusion process to better understand and predict processing effects.
There are several instruments that can be used to measure the physical changes associated with the glass transition and melt transition in biopolymers such as starch and protein during thermal processing. Some instruments are not configured to measure a sample’s transition temperatures under typical extrusion conditions. In addition, these instruments are often too sensitive to measure the “controlling” Ts and Tf for a complex blend of raw materials that often include starch, protein, fat, fiber and ash (Strahm and Plattner, 2001). The controlling Ts is the temperature at which a sufficient amount of a sample is softened to allow for particle compaction (Ts) or melted to allow for flow (Tf) through a small orifice.
One tool that can measure these transitions is the Phase Transition Analyzer, or PTA. While the PTA does not provide all the answers to questions about how petfoods change under thermal processing, it does provide many unique and potentially useful insights for the petfood extrusion and drying process.
To measure the Ts of a petfood sample, a blank die (no orifice) is inserted into the PTA, a two gram sample is loaded, and the pressure is preset to 120 bar and maintained for ten seconds. This initial compaction allows the sample to be formed into a solid pellet, thus reducing the bridging effects that can occur with high moisture samples (above 20% wet basis). The pressure is then reduced to 80 bar and the sample is typically heated at a rate of increasing temperature, usually ten degrees Celsius per minute. As the sample is heated at constant pressure, it begins to compact. This initial compaction occurs over a temperature range. The midpoint of the temperature range is assigned the value of Ts. Once the initial compaction has ended, the temperature is held constant and the blank die insert is replaced with a second die (2.0 mm orifice). The sample is recompressed and the temperature is again raised at an operator-determined rate. As the sample heating continues, the sample begins to flow through the orifice. The temperature at which the flow begins is called Tf.
Polymer science can be used to understand the expansion characteristics of petfood products. As shown by studies (Plattner, Strahm, & Herbster, 2001), a state diagram can be developed relating a product’s transition regions with the extrusion process (Figure 1). When constructing such a diagram, the process technologist must first plot the Ts and Tf curves. Plotting these curves allows one to determine where the petfood product will behave as a glass, rubber or liquid.
Transition temperature data can be extremely useful in selecting ingredients for the extrusion process. The biopolymers used for petfood products are not only inconsistent from year to year, but they can also change based on variety, storage conditions and growing location. Understanding a raw material’s transition temperatures can be useful in predicting how a given ingredient might affect the extrusion process. Figure 2 shows the melt transition curve in relationship to the extrusion process for a generic petfood. Plots of this nature would be typical of an expanded petfood process. The material is transformed from a hard, glass state into its melted state. The product then expands as it leaves the extruder. The evaporative cooling converts the product back to a rubbery state and, after drying, the product returns to a glassy state.
The PTA can also be used to assist with product development. For example, if the task is to develop a soft, chewable product, transition temperatures are used to predict a material’s final characteristics. Since the product needs to be soft, it must remain in the rubbery region (between the Ts and Tf curves) after processing.
A final area in which knowledge of transition temperatures and state diagrams can be of assistance is in process development (Plattner, Sunderland and Erdley, 2010). One of the more difficult petfood products to extrude and dry is a diet which contains a high level of fresh meat. The addition of meat adds significant moisture to the process, creating cooking and hydration issues in the preconditioner and extruder. Recipes with high levels of fresh meat are subject to significant deformation during cutting and conveying because the extruded kibbles are very soft. Fresh meat contains significant non-denatured and functional protein which holds onto the moisture and requires long retention times in the dryer. Applying the principles of polymer science creates an understanding of the processing issues occurring during the extrusion and drying of diets containing high levels of fresh meat.
The proper use of polymer science can allow the process technologist to not only understand the basic biopolymer properties, but also allows for a practical application to petfood processing. When applied properly, these principles can assist in ingredient screening, formulation development and process design in the production of petfoods.