May 1, 2007

US study finds high temperature, shorter extrusion improve fish feed

High temperature and short time extrusion of soymeal gives the greatest weight gain in rainbow trout, USDA's Agricultural Research Service said.

The findings, by researchers Frederic Barrows from the USDA/ARS and David Stonnee and Ronald Hardy from the University of Idaho was reported in the latest issue of the journal Aquaculture.

The researchers aimed to determine the effects of extrusion cooking, the main method of aqua feed production, on the nutritional value of fish meal and soymeal based diets.

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.

Commercially purchased, solvent extracted soymeal contained 5100 TIU/g, and after pre-cooking through the extruder with barrel temperatures of 127  deg 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.

There results indicated that neither PDI nor NSI values were correlated to weight gain.

There was no significant effect of pre-cooking or extruder temperature on feed intake or weight gain, but a 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 also improved the ADC for organic matter, energy, and carbohydrates.

The researchers thus concluded that the results demonstrate that high temperature (127 ^ºC) and a shorter time in the extruder barrel gives the greatest weight gain in rainbow trout.