Applying Polymer Science To Understand Drying
Applying Polymer Science to Pet Food Production
Brian Plattner & Galen Rokey
Introduction
Polymer science, the study 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 pet food extrusion process to better understand and predict processing effects. Strahm (1998) gives a more complete review of the concepts of glass and melt transitions and their possible applications to extrusion processing.
Figure 1: Schematic Diagram of the PTA.
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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 (Figure 1). While the PTA does not provide all the answers to questions about how pet foods change under thermal processing, it does provide many unique and potentially useful insights for the pet food extrusion and drying process.
Measuring Transition Temperatures
Figure 2: Sample Compaction and Flow During Testing in the PTA.
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To measure the Ts of a pet food 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 percent 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 (Figure 2). 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.
Figure 3: Typical PTA data.
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Figure 3 shows an example of data generated using the PTA. The displacement is tracked as the sample is heated. As the particles begin to compact the Ts transition is indicated by a change in sample displacement. The Ts, as determined by the PTA, is reported as the average of Ts (onset) and Ts (endpoint). Tf, which is detected by the displacement transducer, is indicated by the flow of the sample through the die orifice.
Applying Polymer Science to Pet Food Processing
Figure 4: State Diagram for Pet Food Extrusion
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Polymer science can be used to understand the expansion characteristics of pet food 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 4).
When constructing a state diagram, the process technologist must first plot the Ts and Tf curves. Plotting these curves allows one to determine where the pet food product will behave as a glass, rubber, or liquid.
The raw material enters the extrusion process at low moisture and temperature. It is then metered into the preconditioner and mixed with water and steam. The moisture and temperature addition result in the material moving from a glassy state thru the rubbery region and entering the melted region just before exiting from the preconditioner. As the material enters the extruder, additional process moisture and energy is added in the form of direct steam injection and mechanical energy input from the extruder’s main drive motor. This additional energy drives the material further into the melted region creating a melt that has relatively low viscosity. The temperature of the process can be predicted through a mass and energy evaluation of the process (Riaz 2000). As the product leaves the extruder, some of the moisture flashes off as the product goes through evaporative cooling. As the product continues to dry and cool, it transitions back into its glassy state.
Figure 5: Expansion and Collapse
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The perpendicular distance between the point representing the moisture and temperature of the product just behind the die of the extruder (point 3 in Figure 4) and the melt transition line is often called the melt penetration. In other research (Strahm 1998), it has been shown that the melt penetration can be related to the amount of expansion, which results as the product passes through the extrusion die. In general, the larger the melt penetration, the more expansion is achieved resulting in a lower bulk density. Greater melt penetration results in a larger driving force for steam generation and expansion. It also results in a lower melt viscosity and less resistance to expansion. This combination of higher driving force and lower resistance results in greater expansion.
In some cases the product remains in the melted region after expansion. If the temperature and moisture condition of the product after expansion is above the melt transition curve, the product will not have the strength to maintain shape and size after expansion. In these instances the product will collapse, resulting in a much heavier final density (Figure 5). The point at which the product must flash past to ensure no collapse will be dependent on the recipe components and extrusion processing conditions.
Ingredient Selection and Screening
Figure 6: Comparison of Corn Varieties Grown in the Same Location.
Transition temperature data can be extremely usefully in selecting ingredients for the extrusion process. The biopolymers used for pet food products are not only inconsistent from year to year, but they can also change based on variety, storage conditions, and growing location. Figure 6 illustrates the differences in flow point for three different varieties of corn (Plattner, Strahm, & Rausch 2001).
Understanding a raw material’s transition temperatures can be useful in predicting how a given ingredient might affect the extrusion process. Figure 7 shows the melt transition curve in relationship to the extrusion process for a generic pet food. Plots of this nature would be typical of an expanded pet food 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.
Figure 7: Expanded Pet Food
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The effect of replacing a major component of a recipe with a new ingredient having a much higher melting point can be predicted. Figure 8 compares the melting points of the two formulations. Formula 2 (illustrated by the red squares) has a much higher melting point than the original formulation.
Figure 8: Potential Process Map with Modified Formulation
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If the same set of extrusion conditions were used for processing, one can see that the product never crosses the melt transition line for Formula 2. This would indicate that at current processing conditions there is not sufficient energy to create flow, resulting in a blockage of the die orifices.
To create a successful extrusion operation, the processing conditions are adjusted through additional energy and/or moisture addition so that the product (just before the die) is melting and ready for expansion. If the process were adjusted as shown n Figure 9, then the product would be able to expand. The new process is shown by the green-colored curve. When comparing the two processes, it is obvious that the extrusion moisture level increased from 22 to 33 percent while the product temperature increased from approximately 130ºC to 155ºC. These conditions would result in a more expensive produce in terms of overall energy required for cooking as well as an increase in the energy required for drying the final product. Using polymer science to screen ingredients and formulations in this manner can assist in choosing the correct ingredients for a given process.
Figure 9: Actual Process Map for Modified Formulation
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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. Figure 10 shows a potential state diagram for this product development exercise.
Figure 10: State Diagram for Recipe Development
If the product in question is at a temperature of 25(C and shelf stable moisture content of less than 11 percent it is apparent that it will be well below the Ts curve as indicated by the star. According to this state diagram the product would have a very brittle and hard texture, which is much different than the requirement. To change the texture the product needs to be moved from its glassy state to the rubbery region. This can be accomplished by adding significant moisture (>20 percent) or raising the temperature to above 60(C. Neither of these changes would seem to be a good process choice.
A far better option would be to alter the recipe components to lower the transition temperatures. Adding ingredients such as sugar, glycerin, propylene glycol, or corn syrup solids can dramatically lower the Ts curve. In addition, adding mold inhibitors and reducing the pH of the formula will allow the product to remain shelf stable at elevated moistures.
Figure 11 shows what could happen if the proposed changes were made. First the Ts curve would shift down and to the left with the addition of sugar and glycerin to the recipe (from Ts to Ts - new). These ingredients are considered plasticizers, which lower the transition temperatures of the recipe. Secondly, by lowering the pH and adding a mold inhibitor, the final moisture content can be increased to a range between 17 to 20 percent and still remain self-stable for an extended period of time. This moisture change is indicted by the green star in Figure 11. The combination of lower Ts of the recipe and higher final product moisture content gives the final product the attributes required. It is now located in its rubbery region and therefore would have a soft, pliable and chewy texture as required.
Figure 11: State Diagram for Modified Recipe
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 pet food 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.
Figure 12: State Diagram for Extrusion Process
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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. As mentioned previously, a product that exists in its rubbery form, between the Ts and Tf curves will a have soft pliable texture. When force is applied, the material can be deformed or reshaped; however, it still has enough rigidity to hold its own weight. Above its flow temperature a product will resemble a liquid. At temperature and moisture combinations close to the Tf curve the material will act as a viscous liquid. As the material moves further into the melted region the viscosity thins dramatically. Kibbles which remain above their flow temperature exhibit soft dough-like textures that will compress and deform under their own weight. Deformation can also occur as cutting blades throw the product into the surrounding knife hood. The force of hitting the wall of the hood will cause the soft kibbles to flatten and adhere to each other creating clumps. As the kibbles enter the dryer and are stacked on top of each other, large clumps can be formed. Figure 12 shows a high meat inclusion pet food
Figure 13: Post Extrusion Processing of High Meat Pet Food
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Figure 14: Drying Curve for High Meat Pet Food
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The raw material enters the extrusion process at low moisture and temperature. It is then metered into the preconditioner and mixed with chilled, emulsified fresh meat and heated with direct steam injection. The moisture and temperature addition result in the material moving from a glassy state thru the rubbery region and entering the melted region just before exiting from the preconditioner. As the material enters the extruder, additional moisture and energy are added in the form of direct steam injection and mechanical energy input from the extruder’s main drive motor. This additional energy drives the material further into the melted region creating a melt that has relatively low viscosity. As the product leaves the extruder, some of the moisture flashes off as the product goes through evaporative cooling. Figure 12 shows that the material is still well above its melt transition temperature even after this cooling step. Thus, the product could be susceptible to deformation.
To minimize and avoid these process issues it is helpful for the process technologist to understand how to apply polymer science and a state diagram to the pet food process. To keep the product from significant deformation the transition from the melted region to the rubbery region must be controlled (Figure 13). Once the product is below the melt transition curve it will still be deformable; however, it will have enough structural integrity to support the combined weights of other kibbles thus reducing the potential for deformation and clumping.
Drying the kibbles will move them below the melt transition line. The data in Figure 13 indicates that the product moisture needs to be lowered to around 15 percent at temperatures below 80ºC to reach the rubbery region.
Figure 15: Segmented Drying Curve
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One way of lowering the moisture content is to use a predryer with a shallow bed depth. This would allow the moisture to be reduced while maintaining a shallow enough bed to keep the product from clumping and deforming due to the surrounding weight of the kibbles. A drying curve for the product must be developed to properly size a predryer. A sample of this product was collected just off of the extruder and a drying curve was developed by measuring the moisture loss from the product over a period of nearly an hour (Figure 14).
The drying curve indicates that it takes approximately 30 minutes for the average product moisture to drop below the targeted 15 percent level. If a shallow bed depth is maintained throughout the predrying process to prevent clumping, the predryer becomes quite large and therefore expensive.
Figure 16: State Diagram for Extrusion & Drying of Pet Food
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When developing a drying curve for a given product, the moisture content is usually depicted as the average moisture of the kibble. However, at any particular location along the drying curve, the moisture at the surface of the kibble will be less than the average, while the center of the kibble will have a higher moisture level due to the slow migration from the center to the outside. Figure 15 shows a segmented drying curve. Each curve shown on this chart depicts a different layer or segment of the kibble. The bottom-most curve is the outside surface, while the top curve reflects the center of the kibble.
Figure 17: Properly Dried and Cooled Kibbles
These curves show that the surface of the kibble reaches 15 percent moisture after about 11 minutes of drying. At this point the surface of the kibbles has transitioned from the melted state to a rubbery state. If we look back to Figure 14 we can see that the average moisture content of the kibble is still near 28 percent which tells us that the bulk of each kibble is still well above its melt transition point. But since the surface properties are now in the rubbery region, the kibbles are strong enough to hold the stacked weight of the surrounding kibbles without deformation. Since the kibbles can now be stacked the size, footprint, and overall capital investment of the predryer is reduced.
A more accurate state diagram can be constructed using the extrusion conditions shown in Figure 12, with the drying curve of the product surface shown in Figure 15. This combined state diagram is depicted in Figure 16. As shown in this state diagram, the easily deformable product is carefully dried in a predryer until the surface has reached the rubbery region. Then, a more traditional pet food drying step reduces the moisture to a shelf-stable level and the product is cooled prior to packaging. This allows the final kibbles to be free flowing with limited deformation (Figure 17).
Conclusion
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 pet food processing. When applied properly, these principles can assist in ingredient screening, formulation development, and process design in the production of pet foods.
Plattner, B., J. Herbster, and B. Strahm. 2001. Phase Transition Analysis: Predictive Control of Extrusion. Pet Food Industry. Volume 43(10):15-16. Watt Publishing, Inc. Mt. Morris, Illinois.
Plattner, B., B. Strahm, and K. Rausch. 2001 The Phase Transition Analyzer and its Impact on Extrusion Processing of Foodstuffs. 2001 ASAE International Conference. ASAE Publication. St. Joseph, MI.
Plattner, Brian, Robert Sunderland, and Keith Erdley. 2010. Applying Polymer Science to Extrusion and Drying of Pet Foods. Published on the Petfood Industry website, December 2010
Riaz, M. N. ed. 2000. Appendix in Extruders in Food Applications. Technomic Publishing Co.,Inc. Lancaster, PA, 17604.
Strahm, B. 1998. Fundamentals of Polymer Science as an Applied Extrusion Tool. Cereal Foods World. 43(8): 621-625.
About the Authors:
Brian joined Wenger Manufacturing upon graduating from Kansas State University in 1997 with a Bachelor of Science Degree in Biological and Agricultural Engineering. He went on to earn his Professional Engineering License in 2003, and in 2010 he was appointed as an Adjunct Faculty member of the Grain Science and Industry Department at Kansas State University. During his tenure at Wenger he has held numerous positions including Process Engineer, Test Run Coordinator, Technical Center Manager, and currently he holds the position of Processing Engineering Manager. He is responsible for process specification in helping customers specify new lines, improve existing ones, and works directly with the Wenger Technical Center as well as with the Wenger Design Engineering Group to develop improvements and new innovations for Wenger’s line of extrusion and drying equipment.
Galen is the Process Manager for the Applications Group within Wenger Manufacturing, Inc. After completing a course of study in the Chemistry Option of Grain Science and Management, Galen graduated from Kansas State University in 1973 with a Bachelor of Science Degree. He joined Wenger Manufacturing in 1973, and in 1983 became the Manager of Wenger’s Technical Center. Galen brings 38 years of laboratory, extrusion process, and research experience to his current position. He travels extensively, both domestically and internationally, assisting clients with process issues and providing both classroom and hands on training regarding the latest in extrusion and drying technology. A past member of the American Association of Cereal Chemists and the Institute of Food Technologists, Galen has authored numerous publications regarding the extrusion process. He was the recipient of the Alpha Mu Distinguished Service Award in Extrusion Technology from Kansas State University in 1990.
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