Evaluation of protein hydrolysis in raw sources by digestive proteases of Senegalese sole (Solea senegalensis, Kaup 1858) using a combination of an in vitro assay and sodium dodecyl sulphate polyacrylamide gel electrophoresis analysis of products

The hydrolysis of protein in different animal and plant sources by the intestinal proteases of Senegalese sole (Solea senegalensis) was studied using a combination of an in vitro digestibility assay and the evaluation of the protein fractionation by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS‐PAGE). In vitro hydrolysis was performed for 90 min in a closed reactor maintained at constant pH and temperature. Samples of the reaction mixture at different time intervals were developed in SDS‐PAGE gels to evaluate the progressive hydrolysis of the different protein fractions in each protein source. A numerical value [coefficient of protein degradation (CPD)], which integrates the information obtained after image analysis of the gels, is proposed for comparison among proteins, according to the intensity of the hydrolysis produced by sole proteases. Additionally, the total free amino acid released from proteins was measured during the in vitro assay. Casein, squid meal and soybean concentrate (SBC) proteins showed very similar degradation patterns, with a quick and almost complete proteolysis within the first minutes of the enzymatic reaction. Fish and krill meals were hydrolysed more progressively. On the contrary, pea meal (PM) and corn gluten meal (CGM) showed scarce changes in their protein profile after 90 min of reaction. For animal protein sources, the final CPD values ranged from 77.6% to 87.0%, showing not significant differences. By contrast, PM (30.5%) and CGM (32.3%) presented significantly lower CPD values (P<0.05) as compared with SBC (90.6%). In general, a linear fitting was found between CPD and the release of free amino acids during in vitro protein hydrolysis. The present study provides detailed information, which, combined with the conventional in vitro digestibility studies, may help in the evaluation of different raw sources according to their protein degradation patterns. This information can be applied directly to estimate the protein nutritional quality of ingredients for Senegalese sole feeds.