Effect of additional heating of fish meal on in vitro protein digestibility and growth performance of white trevally (Pseudocaranx dentex) juveniles

Fish meal (FM) used in fish farming is prepared by steaming, squeezing, defatting, drying and crushing raw fish material. However, how drying affects protein digestibility has not been systematically studied. Herein, we evaluate the effect of additional heating of FM on in vitro FM protein digestibility and growth performance of white trevally (Pseudocaranx dentex) juveniles. We heated conventional steam-dried Chilean FM from 50-200 degrees C for 1 h to prepare additionally heated FM (AHFM). In-vitro AHFM protein digestibility started to decrease significantly above 150 degrees C, along with substantial reduction in the protein digestion speed. Five isonitrogenous/isolipidic experimental diets were prepared by substituting different proportions of FM with AHFM produced by heating FM to 180 degrees C. In-vitro protein digestibility and digestion speed of the experimental diets decreased linearly with increasing AHFM content. After a 33-d feeding trial, the final bodyweight, specific growth rate, weight gain and feed efficiency of white trevallies decreased linearly and/or quadratically with increasing dietary AHFM levels. These findings indicate that in vitro protein digestibility of AHFM decreases linearly at temperatures above 150 degrees C, and the growth and feed efficiency of white trevallies are negatively affected by dietary inclusion of AHFM processed at 180 degrees C.