Extrusion improves soybean meal for fish

Researchers
Frederic Barrows from the USDA/ARS Hagerman Fish Culture Experiment Station and
David Stone and Ronald Hardy from the University of Idaho report on the effects
of extrusion conditions on the nutritional value of soybean meal for rainbow
trout (Oncorhynchus mykiss) in the latest issue of the journal Aquaculture.

Their
study was conducted to determine the effects of extrusion cooking, the primary
method of aqua feed production, on the nutritional value of fish meal and
soybean meal based diets.

Pre-cooking, time in
the extruder barrel (18 or 37 seconds) and extruder temperature (93 or 127 °C)
were the fixed parameters in this trial, with these conditions chosen to achieve
both under and overcooking of the soy protein.

A
twin-screw Buhler, DNDL-44 extruder was used to produce eight experimental diets
and each was fed to triplicate groups of 40 g trout for 84
days.

Trypsin inhibitor levels (TIU), protein dispersibility index (PDI), nitrogen solubility index (NSI), and apparent digestibility coefficients (ADC)
of protein, organic matter, lipid, energy and carbohydrate were measured for
each diet.

Processing decreases TIU
Commercially
purchased, solvent extracted SBM contained 5100 TIU/g, and after pre-cooking
through the extruder with barrel temperatures of 127 °C for 17 seconds,
contained 2300 TIU/g.

The diets contained less
than the detectable limit of 2000 TIU/g, probably due to a combination of
dilution and processing effects.

PDI values of
the diets suggested that pre-cooking solvent extracted soybean meal decreased
protein value, but neither PDI nor NSI values were correlated to weight
gain.

Extrusion decreases feed intake
There
was no significant effect of pre-cooking or extruder temperature on feed intake
or weight gain, although dwell time in the extruder barrel significantly
affected feed intake and weight gain – longer extrusion time significantly
decreased feed intake and weight gain.

Higher
temperature in the extruder barrel significantly improved FCR. Pre-cooking
solvent extracted soybean meal before inclusion in the diet significantly
improved the ADC for organic matter, energy, and
carbohydrates.

The researchers conclude that
these results demonstrate the importance of extruder processing conditions on
fish performance and indicate that high temperature (127 °C) and short time in
the extruder barrel gives the greatest weight gain in rainbow
trout.