Down-stream processing of baker's yeast (Saccharomyces cerevisiae) – Effect on nutrient digestibility and immune response in Atlantic salmon (Salmo salar)

The increased demand for high-quality feed ingredients for the aquaculture industry has led to an increased focus on microbial ingredients as nutrient sources. Limited information exists, however, on the effect of down-stream processing of yeast on nutrient digestibility and health in Atlantic salmon (Salmo salar). In the present study, three laboratory-scale down-stream processing methods were used: direct inactivation by spray-drying, autolysis, or cell crushing. All yeast was inactivated with sray-drying post treatment. The yeast products were evaluated for their nutritional value and health effects in diets for Atlantic salmon. The experimental diets consisted of a high-quality fishmeal-based reference diet (FM) and six experimental diets were 300 g kg⁻¹ of each of the processed yeast candidates were added to the FM control mixture. Fish were fed the diets for 21 days, and at the end of the experiment, feces were collected, and spleen, head kidney, distal intestine (DI) and plasma were sampled for assessment of immune response. Processing of yeast resulted in increased level of soluble protein in the yeast cream, with the highest level for yeast disrupted by cell crushing (20 K PSI). This was confirmed with scanning electron microscopy, where disrupted and cracked cells could be observed. Significant lower protein digestibility of the directly inactivated yeast coincided with the increased detection of whole yeast cells by flow cytometry in the DI content of fish fed the directly inactivated yeast. Autolysis for 16 h resulted in the highest protein digestibility, which was similar to the FM control. Interestingly, 16 h autolyzed yeast induced the secretion of IL-8, while cell crushed yeast induced the secretion of TNFα in the DI as analyzed by ELISA. To conclude, different down-stream processing of S. cerevisiae led to increased protein and β-glucan solubility that further increased protein digestibility in Atlantic salmon. Furthermore, different processing of the yeast triggered different immune stimulatory effects in Atlantic salmon.