Restricted process water limits starch gelatinization, and reduces digestibility of starch, lipid, and energy in extruded rainbow trout (Oncorhynchus mykiss) diets

Abstract

Abstract Five diets were produced with crude protein (CP) from hydrolysed wheat gluten (HWG) replacing graded (0, 12.5, 25, 50, 75%) amounts of CP from LT fish meal. Diets with 0–25% of CP from HWG were extruded with 19–20% water added in the extruder. Water addition was limited to 11% of the extrudate for the 50% HWG diet, and no water was added to the diet with 75% CP from HWG in order to obtain even flow of material in the extruder. Based on the degree of starch gelatinization (DG), estimated by differential scanning calorimetry (DSC), the diets with 19–20% process water were characterized by high gelatinization (HG), medium DG was obtained by limiting water addition to 11% (MG), and low gelatinization (LG) was the result of extruding without water added. Each diet was fed to two groups of rainbow trout in freshwater, and feces were obtained by abdominal stripping. Yttrium oxide (Y 2 O 3 ) was used as inert marker. The average apparent digestibility of starch ranged from 97 to 99% in the HG group, to 76% in the MG group, and 52% in the LG group. The AD of starch increased linearly (R 2 = 0.99) with increasing water addition in the extruder. The AD of lipid was reduced in MG and LG groups, and there was a linear (R 2 = 0.87) relationship between AD of starch and AD of lipid. As a consequence of AD of starch being quantitatively more influenced by water supplementation in the extruder than the AD of lipid, the regression of water supplementation on AD of energy was best explained (R 2 = 0.99) by a 2nd degree polynomial function, in which both the 1st and 2nd degree components were > 0. The findings emphasize the necessity of using sufficient process water during extrusion of salmonid diets both to ensure efficient digestion of starch and lipid. Statement of relevance This paper will increase the awareness in the feed industries of the necessity to secure complete gelatinization in extruded diets, since it is of high importance for digestible energy in the feed.