News last update:6 Aug 2012

Extrusion improves soybean meal for fish

High temperature and short time extrusion of soybean meal gives greatest weight gain in rainbow trout, ARS researchers report.

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.

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