FEED TECHNOLOGY UPDATE - AquaFeed.com

DENSITY CONTROL
Product Wet Density (grams / liter)
380
360
340
320
300
280
260
240
220
200
Figure 6 shows the impact of screw profile
on product density.
The single flight screws resulted in heavier
densities than the double flight screws.
The double fight
results in additional 600
retention time and
therefore can generate
additional
500
cook.
As shown in Figure 400
7, there is a strong
linear correlation
300
between SME input
and the final bulk
density of the product.
200
As the energy increases
the density
100
decreases.
The final process
0
variable that is
used to control
product density is
extrusion moisture
content. Extrusion
at relatively low
moisture contents
tends to produce a
dense, unexpanded
product.
As the extrusion
moisture content
increases, the density
declines and
the product becomes
lighter.
The higher moisture
level contributes to
cooking the available
starch and allows
the product to
expand. However,
the extrusion moisture
level can
increase to a level
that actually causes the density to increase.
High moisture levels tend to reduce the viscosity
of the material in the extruder barrel
and make it more difficult to expand the
single flight screw
double flight screw
0 1000 2000 3000 4000 5000
Extruder Feed Rate (kg/hr)
Figure 6 Effect of Screw Profile on Density
Extrudate Bulk Density (g/l)
0 20 40 60 80 100 120
Specific Mechanical Energy (kJ/kg)
Figure 7 Effect SME on bulk density
• FEED TECHNOLOGY UPDATE — June/July 2006 •