Evaluation of content and digestibility of disulfide bonds and free thiols in unextruded and extruded diets containing fish meal and soybean protein sources

The study was designed to investigate disulfide bond (SS) digestibility, the effect of extrusion on dietary SS content, and the relationship between dietary SS content and nitrogen (N) and SS digestibility. Three high-quality fish meal-based diets were produced; one control diet (FM) and two diets with 40% of amino acids from fish meal replaced by defatted, toasted soybean meal (SBM) or untoasted soybean meal (white flakes; WF). The diets were given both extruded (EX) and unextruded (UEX) to mink (Mustela vison).Dietary SS content ranged between 23.3 and 32.7 nmol mg−1 crude protein, and was not affected by extrusion at 120 °C and 28% moisture in the FM diet, while it was moderately increased by extrusion of the SBM- and WF-diets. Coefficients of SS digestibility, being about 0.80, were higher than coefficients of free thiol (SH) digestibility in all diets. Coefficients of N and thiol digestibility in the WF UEX diet were much lower than in the other diets, probably due to high activity of protease inhibitors. No effect of dietary SS content on N or SS digestibility could be seen. The results indicate that other factors, such as lumen environment, feed passage rate and SS accessibility, are more important deciding factors than dietary SS content for SS and N digestibility.