Extrusion a processing tool for a variety of feed

Extrusion cooking is a very flexible processing tool that can produce a wide range of animal feed – aquatic, bird and livestock – when you use the proper raw materials, hardware and processing techniques. Dr Mian Riaz of the Food Protein R&D Centre at Texas A&M University in the US explains
1. Raw materials
These and their characteristics – selection, specification, variability, storage/processing and records – are the most important extrusion variable. There are four main types of raw materials used to produce animal feeds:
a. Protein is available from plant or animal sources. Plant sources – such as soy, legumes, wheat/corn glutens and cereal grains – are low in cost, have good functional properties, but may have compromised amino acid profiles. Animal sources of protein – meat, fish, poultry, blood or gelatine – on the other hand, are higher in cost, have a good amino acid profile, but poor functional properties unless they are fresh or spray dried.
b. Starch acts as an energy source, assists expansion, and improves binding and pellet durability, and is normally used at levels of 10-16% in aquatic feed.
c. Lipids are the main source of energy in aquatic feed – levels can exceed 40% in some diets. They usually comprise <20% in extrusion with the remaining fat sprayed after extrusion. Floating feed can absorb about 30% fat after extrusion, and sinking feed 15% fat. The effect of fat levels on feed quality is as follows:
– 0-12%: little or no effect
– 12-17%: for each 1% of fat above 12%, the bulk density of the final product will increase by 16g/litre
– 17-22%: product will have little or no expansion, but will remain durable
– >22%: final product durability will be poor
d. Minor ingredients include minerals, vitamins, pigments, flavours and processing aids. Vitamin, pigment and
flavour potency is affected by energy – in the form of heat, moisture and retention time in the process.
2. System configuration
This hardware consists of dry recipe delivery, preconditioning and extruding.
a. Dry recipe delivery: using a volumetric or gravimetric delivery system to deliver a uniform flow rate of dry raw materials to the system. The first delivers the material by volume and the second by weighing. This system is mounted on a weighing scale, which weighs the exact amount of material for delivery. .
b. Preconditioning: hydrates and heats raw material particles, and mixes raw materials added in separate streams.
c. Extruding: twin screw extrusion is the process of choice when you are producing ultra-high fat foods (>17%), using ultrahigh levels of fresh meat (>35%), creating portioned feeds (uniform shape and size) of ultra-small sizes (10%); allows you to achieve higher levels of “cook” (>40%) and fat (>10%); high fibre; pasteurisation;
a range of cubes (molasses/wholegrain); and to incorporate byproduct streams, such as wet distiller’s
grains, milk by-products, offal/viscera/old laying-hens, and fruit/sugar processing waste.
The potential benefits for extruded feed include resistance to mechanical durability, moisture content as high as
55%, the production of different shapes and sizes of feed, and the ability to make floating and sinking feed. Floating feed is suitable for catfish and carp, slow-sinking feed for salmon and trout, and fast sinking feed for shrimp.
3. Processing techniques
According to Riaz, you can improve feed processing and conversion through employing flexibility in least-cost
formulation, uniform sizes (0.6-30mm), correct pellet density, good pellet durability (minimum fines and breakage),rapid hydration and good water stability. He adds that you can maintain consistent product density when using least-cost formulation by:
– Varying extrusion moisture
– Varying extruder screw speeds (variable frequency drive)
– Using the back pressure valve to maintain density
– Varying the open area of the die
– Using a vented extruder barrel and midbarrel valve Die design guidelines
– 150-200mm²/ton for floating pellets
– 350-400mm²/ton for sinking pellets (>3mm). Use lower end of open area range for >125mg/g fat coating
– 450-500mm²/ton for sinking pellets <3mm
– 1:1 land length for sinking pellets <6mm
– 2:1 land length for sinking pellets >6mm
– Teflon die for sinking pellets >12mm

What causes non-uniform pellet sizes?
1. Missing or broken knife blades
2. Improper die flow
• Too much open area
• More steam and water required to improve flow
• Die holes not symmetrically arranged
• Not using back pressure valve or venture
• Fat and other liquids not blended properly
3. Extruder temperature too high – not enough cooling on jackets of extruder barrel

To reduce bulk density

– Increase thermal energy inputs
– Increase steam
– Hot water
– Increase preconditioner retention time
– Increase mechanical energy inputs
– Increase extruder speed
– More aggressive extruder configuration: decrease rate, decrease die open area, and increase die restriction
Common causes of surging
– Wear
– Non-uniform feed delivery
– Barrel temperature too high
– Moisture level too low – Incorrect open die area
– Too much steam injection into thebarrel
To eliminate multi-coloured products
– Increase steam or water to the differential diameter conditioner, preconditioner or extruder
– Improve mixing in the extruder configuration
Causes of rough surface
– Too hot
– Mechanical energy too high
– Too dry
– Die land length too long
– Grind too course
– Harsh handling
Causes of poor cut
– Knife blade dull
– Knife blade not set close to die surface
– Insert not flush with face of die
– Worn die opening
– Die face not parallel with knife blades
Wedged product
– Too dry
– Too hot
– Knife speed too slow
– Knife blade too wide Poor die flow properties
To reduce breakage and improve product durability
1. Decrease internal fat levels, and increase starch and functional protein levels in the recipe
2. Increase “cook” of product by more special mechanical energy input and more steam/water
3. Make sure die flow problems are not contributing
To reduce waste and ensure quick start-up, check the:
1. Retention time control in the differential diameter conditioner or preconditioner
2. Screw element and liner designs give positive conveyance
3. High extruder speeds and variablespeed drives
4. Warm-restart control systems compress start-up times to less than one minute
5. Operator experience and training
6. Recipe characteristics
7. Wet waste recycling systems
Riaz: Tel 979-845-2774; fax 979-845-2744; mnriaz@tamu.edu; website: http://foodprotein.tamu.edu/extrusion