Submitted to: Aquaculture, Elsevier Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/16/2007
Publication Date: 6/10/2007
Citation: Barrows, F., Stone, D.A., Hardy, R.W. 2007. The effects of extrusion conditions on the nutritional value of soybean meal for rainbow trout (Oncorhynchus mykiss. Aquaculture, Elsevier Science. v 265:244-252 Interpretive Summary: In order to replace marine fish meal in trout diets, plant proteins are increasingly being incorporated. Soybean meal has been the dominant plant protein in aquafeeds due to its abundance and price. Like other plant proteins, however, soybean meal contains anti-nutrients that need to be deactivated by heat before the fish consume the feed. Too much heating reduces the nutritional value of the feed, and fish growth and efficiency are subsequently reduced. 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. Three main effects were evaluated; pre-cooking soybean meal before diet processing, time that the feed was held in the extruder barrel, and temperature of the extruder barrel. The first objective of this study was to evaluate the additive effect of heat processing on SE-SBM throughout ingredient production and diet processing. Even with additional heating provided by the pre-cooking treatment, fish performance was not positively or negatively affected. The second objective was to determine if various chemical tests could be used to predict performance of rainbow trout fed diets with high levels of SE-SBM. Since there were no apparent effect of extruder conditions on the chemical tests use (protein dispersibility index, nitrogen solubility index), but there was an effect of extruder conditions on weight gain, FCR and feed intake, we can conclude that under the conditions of this study these tests are not suitable as indicators of the effect of processing conditions on fish performance. The third objective was to determine if extrusion processing conditions would affect performance of rainbow trout. Processing a diet with high temperature (127°C) and short time in the extruder (18 seconds) resulted in the highest weight gains and lowest feed conversion ratios. Optimizing extruder conditions will allow for more efficient use of feed, and shorter times for trout to reach market size.
Technical 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 x 2 x 2 factorial treatment design was used with pre-cooking (+ or -), time in extruder barrel (18 or 37 seconds), 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. 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 5,100 TIU/g, and after pre-cooking through the extruder with barrel temperatures of 127°C for 17 sec., contained 2,300 TIU/g. The diets contained less than the detectable limit of 2,000 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 (127oC) and short time in the extruder barrel results in the greatest weight gain of rainbow trout.