Hydrothermal Treatment of animal feed
January 01, 1993
by Teresa Acklin
By Harold N. Coles, director of engineering, Andritz Sprout-Bauer Australia, Dandenong, Victoria, Australia.
Hydrothermal treatment is the addition of heat and moisture to mash or grist before the mash is processed through a pellet mill or extruder. It is usually referred to as conditioning of material. As feed manufacturing processes become more complex, the need to understand hydrothermal treatment becomes increasingly important.
The desired end use of the processed material is the most important consideration when deciding whether or not the equipment and process employed in producing the feed are suitable. Only recently has the correct conditioning of material become paramount among the feed producer's requirements.
Today, there is a need to produce pellets with special characteristics, such as floating pellets, slow-sinking pellets and water-durable pellets. Also, conventional pellets produced for animal feeds now are composed of different types of raw ingredients. These factors necessitate a better understanding how hydrothermal treatment can be used to help produce feedstuffs with the required characteristics.
There are different types of hydrothermal treatment. Let's look at the suitability of these treatments, considering the end product's desired characteristics.
Hydrothermal treatment can be used to accomplish the following:
Conversion of starches (gelatinization).
Elimination of typsin inhibitors.
Sterilization of materials.
Production of water-durable feed.
Destruction of growth inhibitors.
Denaturing of proteins.
The first two processes that will be discussed below generally lead to the better utilization by the animal of available nutrients such as protein and improve the digestibility of the feed. For some animal species, unless these conversions are carried out, the food will not be fully utilized by the animal's digestive system.
The third process that will be addressed is the result of recent research regarding the production of water-durable pellets by hydrothermal treatment. Andritz Sprout-Bauer is in the process of obtaining the patents for this new process and machinery.
To better understand the differences in hydrothermal treatments, it is necessary to isolate the feed manufacturing processes and analyze each. The production processes are conventional pellet production, extruded feed production and water-durable pellet production.
Cereal-based feeds for poultry and pigs are composed of 60% to 70% grain, grain by-products or grain substitutes. The ideal moisture content of the feed leaving the conditioner for the pellet mill should be 16% to 18%, and the temperature should be close to 100°C. This will produce good quality pellets in terms of gelatinization, hardness and high production rates. This assumes that the initial grain moisture content was about 11% to 12%, because it is only possible to add 4% to 5% moisture to the mash in the conventional conditioner.
Types of conventional conditioners are shown in Figure 1 (in-line conditioner) and in Figure 2 (a top-entry conditioner). There are many differences between in-line conditioners and top-entry conditioners, so I will briefly describe their operating criteria.
The effective conditioning time is directly related to the distance between the steam entry and the conditioner discharge opening. The in-line conditioner makes full use of the agitator section, whereas the effective conditioning time in the top-entry conditioner is variable.
Only the in-line conditioner provides a seal between the conditioner and the supply bin or hopper, because the highest part of the conditioning chamber is not connected to the supply bin. That is, steam quickly rises to the highest point. This feature simplifies fines pelleting and solves problems associated with buildup of wet material on the supply bin discharge screw and housing.
The only disadvantage of the in-line conditioner is that it is more expensive to build than the top-entry conditioner; however, easy and efficient operation plus maintenance-free cleaning more than compensate for the higher capital costs.
Recently, formulations and raw ingredients have changed with the inclusion of high-oil and/or high-fat ingredients. This has led to another type of conditioning.
When processing high-fat or high-moisture ingredients, it is not practical or possible to increase their moisture content by 4% to 5% by using live steam. Direct steam injection, when combined with high-oil or high-moisture ingredients, leads to pellet mill problems such as slippage between the rolls and the ring die, as well as insufficient conversion of mechanical energy into heat in the die, which is necessary for the proper binding of pellets. Because live steam cannot be added, the temperature of the mash leaving the conditioner is too low (not near to 100°C) and, therefore, additional heating is required.
A Sprout-Bauer multi-unit conditioner arrangement has been used to elevate the mash temperature when processing ingredients with high initial moisture or fat contents. The arrangement is used in the production of aquaculture feeds and some special types of animal feeds.
External heating allows the mash to enter the pellet mill at a high temperature. But, indirect heating through a jacket-type conditioner is inefficient when compared with heating through steam injection. Therefore, these heating units are large compared with conventional conditioners, if the capacity of the pellet mill is to be fully utilized.
Alternatively, a conditioner operating at above atmospheric pressure, like a pressure cooker used on a Sprout-Bauer extruder, can solve most gelatinizing and sterilizing problems. However, the moisture content of the mash leaving the pressure cooker is too high for pellet mill production.
Another way to achieve full gelatinization and/or sterilization of ingredients is to use raw ingredients that have either been pre-cooked or sterilized. This approach has been adopted by some feed manufacturers.
Pre-cooking of grain and sterilizing of other raw ingredients can be done in a pressure cooker extruder, if the products are dried after the cooking process and before their inclusion in the mash for the pelleting process.
There are two types of conditioners/cookers available for extruders today. They are the high-speed atmospheric conditioner and the pressure cooker.
The conditioner used on a pellet mill is a high-speed agitator shaft unit that uses the same principle as all atmospheric conditioners (cookers) on single-screw extruders. That is, direct-steam injection is used to increase temperature and moisture content with relatively short retention times in the conditioner.
This type of atmospheric cooker is limited to adding 4% to 5% moisture to the mash from the condensing steam. Another limitation is its maximum operating temperature of 100°C. Additional water must be added to the mash or grist to achieve an overall moisture content of 20% to 26% for the extrusion process. This additional water in the atmospheric cooker must eventually be heated. This is usually done in the extruder through frictional heat and energy from the extruder screw. This is inefficient because it leads to high wear and high maintenance costs.
Alternatively, pressure cooking (Figure 3) is a more flexible and efficient method of obtaining higher cooking temperatures and moisture contents. The feed mash is exposed to pressurized steam, normally in the range of 40 kPa to 250 kPa. By pre-setting and controlling the pressure, a higher cooking temperature can be set, and the process can be precisely controlled. This higher temperature cooking corresponds to increased moisture addition of approximately 8% to 12% from the steam as it condenses on the mash to increase the mash temperature.
Also, the steam condenses on the added water, heating it to the desired cooking temperature. Therefore, steam heating of the mash and the added water greatly reduce the need for high frictional heat from the extruder screw, resulting in lower electricity and maintenance costs.
The pressure cooker used on the extruder is a slow, variable-speed unit that allows for longer cooking times of between 30 seconds and 3 minutes, compared with cooking times of less than 30 seconds in high-speed atmospheric conditioners.
There are economic advantages to be derived by using pressure cooking as opposed to atmospheric conditioning in the extrusion process. Typically, for atmospherically conditioned mash, the extruder screw must provide heat through frictional resistance equivalent to 80% of the total heat required for cooking and/or sterilizing. Compare this with the pressure cooker type extruder, where the pressure cooker provides the majority of heat, and the extruder screw provides only 20% of the total heat required for optimum cooking and density. This reduced frictional load results in lower running costs and maintenance.
Other extruder manufacturers suggest direct steam injection into the mash while it is in the barrel sections of the extruder, to reduce running costs and maintenance. This is exactly the same reason a pressure cooker is used rather than an atmospheric conditioner (cooker) for extruder operations. Is a pressure cooker still required, or is direct steam injection into the barrels satisfactory? The answer is simple; retention time is important in the cooking analysis of any grist or mash. Attempting instantaneous cooking at high temperatures compared with controlled-term cooking at a set temperature and moisture gives better control of the cooking process. Also, higher temperature and longer retention time means more uniform temperature and moisture in all particle sizes within the mash.
The pressure cooker and pressure feed screw in the cooker extruder involve higher capital costs than an atmospheric conditioner, but reduced electrical and maintenance costs compensate for this.
Conditioning the mash by means of a pressure cooker before it enters the extruder is still the best method, because it gives longer and more thorough addition of heat and moisture resulting in better control of the cooking and the extrusion processes.
Extensive research and development work have been carried out to establish a process that will provide a pellet that is durable in water for more than eight hours, without using artificial binders. Andritz Sprout-Bauer achieved this goal using a natural binder, wheat gluten. In fact, the pellets processed with the Andritz Sprout-Bauer method last much longer than 36 hours.
Wheat gluten is included in the formulation in excess of 2% and mixed with any type of starch (grain or tapioca), if required, plus fish meal and other normal shrimp food ingredients. The ingredients then are processed using the new pelleting technique and process.
The technique we have developed utilizes a conventional single-unit conditioner and pellet mill. But, the operating technique is different than conventional methods for heat and moisture addition. After the pellets are formed in the pellet mill, they enter a new process of hydrothermal treatment and are eventually cooked.
Hydrothermal treatment has come a long way since the early days of this century, when producing poultry pellets was seen as a simple process of adding a little steam to the mash as it traveled to the pellet mill. Undoubtedly, as we proceed into the 21st Century, there will be new challenges for treating raw materials and different forms or techniques of hydrothermal treatment.THERMODYNAMIC PROPERTIES OF WATER/STEAM PRESSURE TEMPERATURE RELATIONSHIP
|* Atmospheric Pressure|