Availability of extruded carbohydrate ingredients to rainbow trout Oncorhynchus mykiss and carp Cyprinus carpio

In order to examine whether or not the availability to fish of carbohydrate ingredients as an energy source is improved by the extrusion process, potato starch, corn starch, corn, rye, and wheat flour were gelatinized by an extruder. Dietary values were compared between those containing raw and extruded ingredients using fingerling rainbow trout Oncorhynchus mykiss and carp Cyprinus carpio. In both species, the growth, feed efficiency, and protein efficiency ratio were greater in fish fed diets containing extruded ingredients than those on diets containing raw ingredients except wheat flour which showed no differences in these performances between both the ingredients. Improvement of dietary value of carbohydrates by the extrusion process was found to be mainly due to elevation of gelatinized ratios in the diet. In case of wheat flour there was no big difference in gelatinized ratio between raw and the treated materials. The apparent digestibilities of dietary protein and lipid were not influenced by inclusion of carbohydrate ingredients, but that of starch was effectively improved by the extrusion. Thus, the availability of carbohydrate ingredients as an energy source was found to be improved by the extrusion treatment in terms of elevation of gelatinized ratio.