Evaluation of the influence of drying process on the nutritional value of lupin protein concentrates when fed to rainbow trout (Oncorhynchus mykiss)

A series of studies were undertaken to examine the effect of drying processes on the composition, digestibility and utilisation efficiencies of different types of lupin (L. angustifolius cv. Myallie) protein concentrate when fed to rainbow trout. Three different LPC drying methods (freeze-drying: FD, spray-drying: SD, and heat-drying: HD) were studied. Significant effects of drying process were observed on the composition of the LPC; most notable was the relative increase in the level of crude fibre and decrease in crude protein with the heat-dried product. The digestibilities of each of the LPC were assessed using the diet-substitution method. Each of the digestibility diets were fed to fish of 442 ± 58 g (mean ± S.D.), with faecal collection undertaken using stripping techniques. No significant differences in the digestibilities of protein or energy, or total digestible protein and energy concentrations were observed among the LPC. To assess the utilisation of protein and energy, fish were fed diets with a 300 g/kg inclusion level of either the spray-dried or heat-dried LPC. A third fishmeal based reference diet was also used. The diets were formulated to equivalent digestible protein and energy specifications based on predetermined digestibility values for each of the ingredients used. Each of the diets was fed at one of three ration levels and an additional starved treatment was also included. In a 28-day growth study, fish of 96.4 ± 1.7 g (mean ± S.D.) kept in freshwater at 13.9 ± 0.2 °C grew in accordance with their ration level, but with some significant differences observed among the diets. The comparison of the three diets in this experiment shows that the dietary inclusion of the heat-dried LPC significantly reduced the efficiency of energy gain. Utilisation of digestible protein at lower digestible protein intake levels did not appear less efficient with the heat-dried LPC, but at higher protein intake levels it was not as efficiently used as spray-dried LPC or fishmeal protein. A greater proportion of the nitrogen excretion from the fish fed the heat-dried LPC diet was observed as urea, supporting that this ingredient was not metabolised as efficiently as the other diets. This study demonstrates that the drying regime used on a processed grain product may not affect the ability of fish to digest the protein and energy from that grain product, but may affect the ability of the fish to utilise the dietary digestible protein and energy of the ingredient.