By Tom Nease and Paul Tedman

January 27, 2003

Aquatic feed extrusion has seen many changes in the last decade for a variety of reasons. Formulation changes, physical characteristics of the feed and higher production rates are just a few of the changes the industry has endured. Two specialized equipment processes, the PDU® and SAS® , were developed by Extru-Tech, Inc. to specifically address problems encountered in aquatic feed manufacturing. Although these processes are not brand new, they continue to be utilized in new applications around the world.

The PDU, short for Product Densification Unit, was initially developed for processing extruded, sinking aquatic diets. High carbohydrate, low fat, sinking diets can be challenging to make using a standard cooking extruder at high capacities. Shrimp feed, for example, must have good stability in water, yet it must be dense enough to sink in water after it is extruded and dried. In the typical cooking extrusion process, moisture, thermal energy and mechanical energy are added to dry ingredients in order to cook the carbohydrates, sterilize the ingredients, and to force the mass through the die where it is cut to size. As the mass exits the die, expansion will likely occur because of the pressure differential between the inside and the outside of the extruder barrel. If the moisture inside of the extruder barrel is heated above 100 C, then there will be a rapid transformation of the moisture to water vapor as it exits the die. This rapid flash will cause the extrudate to expand.

Many cooking extruders are often incorporate a vent near the mid section of the extruder barrel to help reduce expansion (See Figure 1.) With the vent arrangement, the majority of the cooking process is done prior to the vent. The vent, which is at atmospheric pressure, allows for some moisture and its associated heat content to be released from the material prior to the material being recompressed and forced through the die plate.

Some extruder manufacturers also incorporate a vacuum pump at the vent, further reducing the pressure at the vent below atmospheric pressure. Lowering the pressure below atmospheric conditions will allow additional moisture to be removed from the extrudate.

There are several problems associated with using a vent. The residence time that the material is at atmospheric pressure is very short, usually a fraction of a second in most cases. This short time period means that minimal moisture and heat can escape from the mass. Since only a small portion of the moisture and heat is removed, expansion can still occur once the material is recompressed and forced through the die plate. In addition, it is not uncommon for extrudate to escape from the vent, creating a problem with housekeeping around the processing equipment.

The theory behind the PDU process is the same as the theory for the vent described above – remove moisture and its heat content from the mass then recompress the material to form the pellet. The difference is that two extrusion machines are used instead of one. The cooking extruder is solely used to cook and sterilize the product, with the extrudate exiting the cooking extruder in a rope form. From the cooking extruder, the extrudate is transferred to the PDU by a belt conveyor. This allows the material to be exposed to atmospheric conditions for longer time periods so more moisture will flash from the mass prior to recompressing the extrudate and forming the pellets. Figure 2 illustrates the typical arrangement for a PDU used in conjunction with a cooking extruder. The retention time the rope is exposed to atmospheric conditions is generally between two and five seconds, which is much longer than that associated with a vented head.

The advantages when using the PDU is additional mechanical and thermal energy can be added to the extrudate in the cooking extruder. This can be used to increase the specific amount of mechanical and thermal energy added to extrudate or to increase the overall production throughput. The use of the PDU has also allowed some processors not to change the screw configuration when they switch from floating to sinking diets, greatly reducing production downtime.

The PDU is similar to a single screw cooking extruder, except that it is designed for low shear so minimal mechanical energy is reintroduced into the extrudate. The machine is easy to operate, with the process variables limited to screw speed, barrel cooling, die area, and knife speed. The addition of the PDU also greatly simplifies the operation of the cooking extruder. With the conventional cooking extruder, the operators must continually monitor/adjust the process variable to make sure the product will sink in water. The PDU greatly reduces the chance of producing a feed that will float.

The SAS process, short for Sphere-Izer™ Agglomeration System, was developed several years ago to improve both the quality and production efficiency of starter feeds. Typical starter feeds are produced by first making a standard extruded pellet of substantial size, normally greater than 4 mm in diameter. Once the pellets are dried and cooled, they are crumbled using a corrugated rolling mill. Following the sizing mill, the particles are sifted and classified into size ranges. With this method, typical on-size products are usually in the range of 50% or less, with the balance being smaller and larger particles.

The SAS process is much different than the typical starter feed manufacturing process. The SAS™ process is designed to produce more uniform and nutritionally homogeneous particles than a traditional crumbling system. A uniformly mixed and pulverized formulation is passed through a low shear, low temperature extrusion process where it is conditioned with water as well as other possible liquid additives and then compressed through a special die to form extruded strands. These strands are then transported to the Sphere-izer™. This machine, by cyclonic motion, sizes and shapes the strands into pellets with lengths about the size of the strand in diameter. The SAS™ will produce finished feeds in a size range of 0.3 to 1.2 mm diameter with >85% "on-size" product. The low processing temperatures required are suitable for minimizing nutrient damage, production of medicated feeds and utilization of other temperature sensitive ingredients.

First, the raw ingredients used in the process can be much different. In typical extrusion, starch is required to bind the ingredients together in order to form a durable pellet. Starch contents of 15 to 20% are the norm. With the SAS process, low-starch formulations can be successfully used, the starch required for traditional extruded products does little to bind the product together because the temperature of the process is low, usually in the 40° C range. The particles are bound by using natural binders from fish meal, fish soluble, gluten, other organic ingredients, etc..

Preparation of the formulation mixture includes micro-pulverizing to a specified particle size range to ensure proper agglomeration and pellet uniformity. To ensure a trouble free operation this formulation mixture is passed through a rotary sieve to remove any particle larger than specified for the finished product size. The grind size is also an important factor in insuring each pellet has a uniform nutritional balance unlike crumbled feeds that can vary in each individual pellet.

The product agglomeration is accomplished by using a Product Densification Extruder (PDU). This is a "low-shear/low-temperature" extruder designed to form the materials into a continuous strand without subjecting it to excessive energy and temperature conditions. The PDU also utilizes a dual shaft conditioning cylinder to incorporate water, oils and fats, and slurries into the dry materials before agglomeration. The agglomerated strands are then fed into the Sphere-izer™ for sizing and forming to the desired size. Additional liquids, powders, etc. can be applied in the Sphere-izer™ if the formulation requires such additions.

Continuous drying of the small diameter starter feeds present problems when considering the traditional horizontal continuous bed dryers and vertical dryers. The small diameter products cannot be handled in a static bed or moving bed perforated tray dryer because of several factors. The SAS™ product will create a bed of product that will not allow air to pass through thus the product will not dry completely. To eliminate this problem a vibrating bed/fluid bed dryer is utilized for drying and cooling these products. The fluid bed dryer forces air through perforations in the vibrating bed with enough velocity to suspend the particles in the air flow and keep the product moving and exposed to the heated air. The cooling portion of this unit does the same only using ambient temperature air.