The effects of extrusion conditions on the nutritional value of soybean meal for rainbow trout ( Oncorhynchus mykiss)

Abstract

Feed ingredients and complete feeds are exposed to varying degrees of heating during manufacturing and processing. Heat treatment of soybeans is necessary to de-nature endogenous trypsin inhibitors and maximize nutritional value, but overheating can damage protein and reduce nutritional value. This study was conducted to determine the effects of extrusion cooking, the primary method of aquafeed production, on the nutritional value of fish meal and soybean meal based diets. A 2 × 2 × 2 factorial treatment design was used with pre-cooking (+ or −), time in extruder barrel (18 or 37 s), and extruder temperature (93 or 127 °C) as the fixed factors. These conditions were selected to achieve both under and overcooking of the soy protein, given the limitations of the equipment. A twin-screw cooking extruder (Buhler, DNDL-44) was used to produce the 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. Commercially purchased, solvent extracted SBM contained 5100 TIU/g, and after pre-cooking through the extruder with barrel temperatures of 127 °C for 17 s, 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 suggest that pre-cooking SE-SBM decreased protein value, however, 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 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 SE-SBM before inclusion in the diet significantly improved the ADC for organic matter, energy, and carbohydrates. These results demonstrate the importance of extruder processing conditions on fish performance, and indicate high temperature (127 °C) and short time in the extruder barrel results in the greatest weight gain of rainbow trout.