Aquafeed Vol 12 Issue 1 2020

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Wet rework handling during extrusion Geirmund Vik, Skala Off-spec materials are always generated in the startup and shutdown of the extrusion process and they can be used as a raw material source. For example, when the mash conditioner is turned on and filled with the meal, some off-spec materials are generated while it reaches the right cooking temperature and pressure. When the extrusion process is finished and the meal is changed, the conditioner and the extruder are cleaned with water, generating more off-spec materials.

Types of rework materials The extrusion process generates mainly three off-spec materials that can be reused: • Meal from the mash conditioner at 60-90°C with 15-30% moisture. • Pellet from the extruder at 80-98°C with 15-25% moisture. • Extruder cleanout water and wet product at 40-80°C with 15-100% moisture. The temperature and the moisture content of these products make them hard to handle and a hygiene risk to the feed mill. Storing them in bins and silos can cause unwanted situations due to the high growth rates of mold and bacteria in these favorable conditions. Off-spec materials from the mash conditioner and extruder used to be blended with other off-spec products from coating and cooling processes and added as raw materials. They were often named as dry rework. Off-specs are also mixed with protein raw materials such as corn gluten meal and poultry meal. Drying these materials was tested but it was not a successful solution due to the high cost of drying, the risk of over-drying producing burnt material or an incomplete dry that causes microbial proliferation. Wet rework, mainly washing water, is normally discharged as waste. Some innovations were introduced in recent years based on a simple principle, dissolve all off-specs making a sludge that can be

put back in the extrusion process, pumping it into the mash conditioner.

Rework materials features The three types of rework materials have different features. The meal from the mash conditioner is wet and easily dissolves in water. The extrudate is often a mix of high and low-density pellets. The highdensity pellets sink in the water but the low-density ones easily float requiring strong mixing for them to be more quickly and easily disolved. The washing sludge is composed of water and solids. Hardness and solubility of these solids are key to their handling requirements. New plant-based protein sources create products that are hard to dissolve in water and hard cakes from the die area can be very large. The larger the extruder, the larger the build up of material behind the die. Adding oil in the extruder at the last minute before shut down can make a big difference to the hardness of the extrudate and how it dissolves in a wet rework system. All of these parameters create demands on the equipment.

Aquafeed: Advances in Processing & Formulation Vol 12 Issue 1 2020