FEATURE
Dry extrusion wet extrusion VS
W
by Joseph P. Kearns, JPKearns Consulting LLC
hat is the difference between dry extrusion and wet extrusion? To start off, having worked in this industry for over 40 years both dry and wet extrusion has changed as machinery improvements resulted in the evolution
of both styles of production. Historically dry extrusion was a design for use on farms and the power source was the Power Take Off on a tractor. The idea was to process soybeans mainly to eliminate the trypsin inhibitor and increase oil availability so as the full fat soybeans could be used in “on the farm” feeds with no detrimental effect for the animals fed. It worked perfectly for this application as low production rates matched farmer’s needs. As feed mills became more available and advantageous, the system changed to be more appropriate with electrical motors and surrounding support equipment designed for feed production. These early dry extruders were in the high shear range of operation. High shear was critical for rupturing the oil sacs in the soybeans, so the oil was more readily available for its caloric value in animals, predominately chickens. It just so happened that when operating at high shear the temperature was achieved to eliminate or greatly reduce the trypsin inhibitor in soybeans, which is detrimental for chickens. It is noted and can be seen in the photos that dry extrusion has changed over the years. Insta Pro of Iowa, USA is famous for its dry extrusion equipment and two of their extruders are shown, the left without a preconditioner and the right with a preconditioner. The left extruder is designed for ingredient preparation as it has a device at the discharge end of the extruder, which allows for increased pressure development in the extruder barrel but no ability to cut and form an extruded piece. Insta Pro is historically famous for this design and its use in full fat soybean extrusion and ingredient manufacturing. The Insta ProMS3000 shown on the left is designed for processing shaped products such as pet food and fish feed. This model normally includes a 125 HP main drive motor and has an approx. capacity of 600 to 1,500 kg/hr depending on the product produced. In my opinion the addition of a preconditioner has
enhanced the ability of these dry extruders over a dry extruder with no option of adding water and or steam as one might desire. The levels that can be potentially added in this size and design of preconditioner would be limiting but I have no doubt the capabilities have been increased. Also, the addition of a die and knife allows for cutting and shaping at the discharge end of the extruder. Moisture level addition for dry extrusion would be somewhere between zero and approximately 10 percent added, and this would be in the extruder moistures of 10 to 20 percent. This statement is based my understanding of the process and the claim for less drying required as well as the appearance of some products. Lower moisture high shear and thus potentially highly expanded feeds. Wet extrusion technology has also been through tremendous changes and now can be defined as operating an extruder in the 20 percent to well over 50 percent moisture ranges. Historically it was a much lower range of 20 to mid 30 percent moisture, typical ranges for high capacity single screw extruders. Over the years preconditioning developments have greatly changed moisture and steam inputs. Simple single shafted cylinders to double diameter cylinders with differential shaft sizes; rotational directions and speeds change possibilities have greatly improved the moisture penetration and inclusion of high moisture ingredients in the extrusion process. Extruder barrels also changed to match the advancements in the conditioning cylinders. The advancements in conditioning cylinders increased the retention time, moisture input and utilisation of the steam based on these developments. Thus, the feed from the cylinder had a higher level of cook resulting in the extruder barrel modifications to increase throughput, as less cook in the barrel was needed. Capacity
26 | October 2018 - International Aquafeed