THE ROLE OF EXTRUSION TECHNOLOGY ON FEED SAFETY AND HYGIENE

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1 THE ROLE OF EXTRUSION TECHNOLOGY ON FEED SAFETY AND HYGIENE Mian N. Riaz, Ph.D Texas A&M University USA Pellet presses and many other feed processing technologies result in an agglomerated or pelleted feed. Recognized advantages of pelleted feed are as follows: - increased bulk density - less bridging in bins - less dust - reduced ingredient segregation - less feed waste - increased nutrient density - improved palatability - increased nutrient availability - decreased microbiological activity While many processing technologies result in an agglomerated feed, only a few have sufficient energy inputs to ensure feed safety. Feed safety is a major factor in choosing extrusion-based methods over traditional pelleting methods. Extrusion is a hydro-thermal process where the critical process parameters of retention time, moisture, and thermal and mechanical energy inputs can be varied over a wide range. The feed industry has considered a provision to mandate that all feeds are pasteurized and some form of extrusion will likely be implemented as the processing method of choice. The extrusion process can generally be divided into two basic categories; cooking extrusion and forming extrusion. Both processes affect the feed as the name indicates. Cooking extrusion elevates feed temperature to a level that often results in an expanded product (final feed bulk density less than the bulk density of the starting raw materials). Forming extrusion is usually a low temperature (often called cold-forming) process that increases product bulk density and cools the feed resulting in a feed bulk density that is equal to or greater than the bulk density of the starting raw materials. INGREDIENTS Raw material preparation for extrusion and related processes is very similar to that required for pellet mill installations. Most applications employ a post-grind step. Individual whole grains are pre-ground to reduce particle size, mixed with the balance of the recipe, and then passed through a final post-grind step to achieve the desired particle size distribution. The correct particle size is important for many reasons: - improved product appearance - reduced incidences of die orifices plugging - greater ease of cooking and pasteurization - improved retention of liquid coatings due to smaller cell size in final product matrix Paper by Dr. Mian Riaz 1

2 The process flow diagram for an extrusion application differs from the process flow for pellet press application (Figure 1) in two major areas. EXTRUSION PELLETING Raw Material Receiving Pre Grind Bins, Scale Mixers Final Grinding, Sifting Final Grinding Extruder, Dryer, Cooler Pellet Mill, Cooler Sifting, Coating Packaging Figure 1: Comparison of Process Flows for Pellet Mill and Extrusion Applications The grinding step for an extruder follows the guidelines discussed above and usually precedes a sifting operation to remove foreign material and large particles. A magnet is usually installed prior to the grinding step in all feed mill process flows to remove tramp metal. It is recommended to also include a magnet just prior to the extrusion process to prevent accidental metal from the grinding operation damaging the extruder components. The second major difference in process flows occurs after the extrusion or pelleting steps. Extruded products usually contain more moisture that pelleted products. This moisture must be removed in a drying step if moisture is greater than 12-15% by weight of the extruded product. The higher moisture levels required for most extrusion processes can lend versatility to the process and expands the feed manufacturers product possibilities. The high temperature/short time extrusion cooking process is able to accommodate a wide range of raw materials that might otherwise be discarded as unqualified material. Although extrusion does not completely eliminate toxins and other anti-nutritional or antigrowth factors, in many cases these substances or their activity is reduced to permit some level of incorporation into the recipe. Research at the University of Nebraska indicates that certain temperatures of extrusion are sufficient to reduce fumonisin levels (Figure 2). Studies have also Paper by Dr. Mian Riaz 2

3 indicated that viruses, molds, and other pathogenic organisms can be destroyed by the operating parameters employed during extrusion. However, very little published data is available on this subject and there is a need for carefully designed studies to investigate the effects of the extrusion process. Fumonisin B 1 Recovered (%) y = -0.72x R 2 = Extrusion Temperature (C) Figure 2. Effect of Extrusion Temperature on Fumonisin Levels Hardware Components An extrusion system includes a live bin/feeder, preconditioner, extrusion cooker, and die/knife assembly as shown in Figure 3. Each component is designed to accomplish a specific function in the process of cooking and forming feed products. The operating conditions can be adjusted to vary the characteristics of the finished product. The live bin/feeder provides a means of uniformly metering the raw materials into the preconditioner and subsequently into the extruder As the material leaves the preconditioner, it enters the extruder barrel. Here the major transformation of the raw preconditioned material occurs which ultimately determines the final product characteristics. The initial section of the extruder barrel is designed to act as a feeding or metering zone to simply convey the preconditioned material away from the inlet zone of the barrel and into the extruder. The material then enters a processing zone where the amorphous, free flowing material is worked into dough. Most of the temperature rise in the extruder barrel is from mechanical energy dissipated through the rotating screw. It may be assisted by the direct injection of steam or from external thermal energy sources. The screw profile may be altered by choosing screw elements of different pitch or with interrupted flights, or by adding mixing lobes configured to convey either in a reverse or forward direction. All of these factors affect the conveying of plasticized material down the screw channel and therefore the amount of mechanical energy added via the screw. Paper by Dr. Mian Riaz 3

4 Figure 3. Extrusion System PROCESS PARAMETERS Extrusion and similar agglomeration techniques have been utilized to process various feedstuffs for many years. Extrusion cooking is universally recognized as a high temperature, short time process. The higher temperatures employed during the extrusion process present an interesting challenge in the assessment of nutrient retention. During extrusion, the recipe and its constituents are subjected to a succession of almost instantaneous treatments or unit operations. These variables include moisture and temperature profiles, extruder configuration, extruder speed, and preconditioning of the material prior to extrusion. The critical process parameters could be summarized into four areas specific mechanical energy, specific thermal energy, retention time, and moisture levels. FINAL PRODUCT APPLICATIONS Many of the advantages claimed for agglomerated or pelleted feeds are really due to the form in which the feed is presented to the animal, and the fact that the feed has been subjected to a heat treatment. The relative dry treatment employed during pelleting followed by a pressing step yields a final pellet matrix that often deteriorates during transportation and handling. The process flexibility and the processors philosophy toward total quality management are the greatest factors in pellet quality. The higher energy inputs and the use of moist heat during the extrusion process allow flexibility not present in the pellet press. Low moisture or dry extrusion has been utilized in the feed industry for many years. Although applications have usually been limited to extrusion of dietary ingredients such as full fat soy, extrusions of complete diets without a pellet press have been employed. Extrusion has been used to process the following feeds: - full fat soybeans and other high oil ingredients - piglet feed and calf starters - hygienic feeds for poultry - protein by-pass feeds for ruminants - aquatic feeds - pet foods - feeds containing high levels of wet byproducts Paper by Dr. Mian Riaz 4

5 PASTEURIZATION AND SALMONELLA CONTROL: The feed industry is acutely aware of the need to eliminate the possibilities of food-borne illnesses from microbial contamination which can occur at any point along the food chain. Mandating that all feed be sterilized through processing was even considered by the United States government as a means to ensure public safety. Even though the practical aspect of such a mandate is questionable, feed processing techniques such as extrusion are in place that could fill the requirements of this type of program. As early as 1965 widespread testing indicated that moist extrusion was much more effective than pelleting in salmonella control (Table 1). Extruded feed Pelleted feed Number of samples Process temperature (ºC) Process moisture (%) Salmonella positive 0 21 Salmonella positive (%) 0 60 Table 1: Effect of Extrusion and Pelleting on Salmonella Destruction THERMAL-DEATH-TIME STUDIES y = 1E+12e x Thermal Plastic Spores E. Coli Salmonella Senftgenber Listeria Monocytogenes 1000 y = 4E+11e x y = 3E+13e x y = 8E+12e x Time (Seconds) Temperature (C) Figure 4 Thermal Death Time Studies Paper by Dr. Mian Riaz 5

6 The graph entitled Thermal Death Time-Studies (Figure 4) indicates the time temperature relationship required to destroy common pathogenic microbes found in the feed and food industries. Note that E. coli, Salmonella, and Listeria are all destroyed if a temperature of 70º C is achieved for a one second time interval during processing. The first, larger square in the graph outlines the average time/temperature possible in a DDC preconditioner (100º C for 120 seconds). The second, smaller square outlines the operating window for the extruder barrel. Note that the normal time/temperature employed in preconditioning and extrusion of pet foods and aquatic feeds will easily pasteurize feedstuffs and foodstuffs. POST EXTRUSION PROCESSING For most dry feeds, the final moisture content needs to be less than 12 percent to prevent mold and bacterial growth. Final products with moistures above 12% are sometimes referred to as semi-moist products. This group of products may have moistures levels greater than 30% and represent a category of products that cannot be processed on pellet presses. When considering a soft-moist product one needs to determine the water activity of the product. Water activity is the critical factor in determining the lower limit of available water for microbial growth. In general, if the water activity of a product is less than 0.65, no microbial growth can occur. DRYING AND COOLING The primary purpose of drying is to reduce the level of moisture in an extrusion cooked product. Many extruded products exit the extruder die at moisture levels above 18% which necessitates product drying for shelf stability. In some cases, the drying process can involve additional heat treatment of the product. One example of this is the drying at elevated temperatures to impart a baked or toasted flavor and appearance to the product. As mentioned earlier, many feeds are best processed at extrusion moistures between 23 to 28 percent. Some of the moisture is lost due to flash evaporation as the superheated product exits the die and expands. Further moisture will be lost through evaporative cooling, as the product cools during conveying or when a cooling step is employed. Pellet coolers will generally result only in a reduction in moisture levels of about 3% and further reductions in moisture levels require a drying step. Pneumatic conveying of products from the extruder to the dryer inlet reduces product moisture content one to two percent. Pneumatic systems help separate sticky products that tend to clump with belt conveyors and improve sanitation around the extruder die. COOLING After being dried the product requires cooling before it is packaged. If the product is packed or stored in bulk without prior cooling, the moisture contained in the hot air and the residual moisture in the product will condensate on the container wall as the product slowly cools in the package or storage bin. This will result in the growth of mold and bacteria in the damp areas. The cooler can be directly coupled to the dryer or can be a standalone unit. When more than five to eight percent fat is applied as a coating to feed, the product is dried, and coated before being cooled. In these instances, a vertical counter-flow cooler is used to avoid fat accumulation on the conveyors of a horizontal cooler. After drying and cooling, it is important to screen out any small pieces or fines that may have developed during the drying, tumbling, and cooling operations. Those fines may be recycled back into the raw mix prior to extrusion so that no loss of product occurs. A properly operated feed manufacturing system will average less than 3% fines for recycling. Paper by Dr. Mian Riaz 6

7 CONCLUSION The selection of processing equipment for a feed manufacturing plant is an important decision. Equipment that will give the highest operating efficiency and most versatility at the lowest total cost should be chosen. When sizing equipment for any plant, it is important to determine the rate or capacity at which the plant will be operating. The probability of future expansion must also be considered. For example, it may cost very little extra to purchase a conveyor or storage bin that is capable of twice the presently needed capacity. The equipment in feed-processing plants are required to perform the following functions: receiving, storage, grinding, mixing, conveying, extrusion cooking, drying/cooling, pumping, coating, and packaging. The objective in the formulated feed industry today is to manufacture a diet that satisfies the following i : - targets animals nutritional requirements - meets or exceeds the customers expectations - satisfies environmental concerns - provides favorable economics Although the focus in the past has often been strictly on economics, shifting trends in today s society underlies the importance of environmental issues. Feeds that are environmentally friendly are not only popular, but are being demanded by the public. The issues of today include processing a feed in a manner that will maximize animal efficiency by freeing feedstuffs for human consumption, reducing animal waste outputs, and eliminating food-borne illness in the food supply chain. As formulated feeds become more sophisticated to meet the specific physiological needs of the animal and the environmental expectations of the public, processing technologies such as extrusion will become a factor in this industry. Paper by Dr. Mian Riaz 7