Fat-containing bleaching earth as a feed constituent for rainbow trout

Abstract

Fat-containing bleaching earth (FBE) is a waste product from the bleaching of edible oils in the fat industry. The experiments described were carried out to investigate if FBE may be utilized as an ingredient in feed for rainbow trout ( Salmo gairdneri, Richardson). Using bomb calorimetry, a sample of FBE from the bleaching of marine oils was found to contain 3400 kcal/kg. This indicates a fat content of about 36% even though analysis showed an extractable crude fat content of only 23%. The digestibility of the crude fat was estimated at about 80%. Fish growth was very good on all four experimental diets, which contained 0, 10, 20 and 30% FBE. Level of dietary FBE had no measureable effect on fish mortality, condition factor, dressed-out weight, fat content, or fatty acid composition. Fish fed diets containing FBE showed a significantly higher frequency of discoloured livers than fish fed control diets, indicating that the quality of the fat in FBE might be reduced. An organoleptic evaluation conducted on the fish showed differences between fish fed different diets, but no negative effects of FBE.