Production of organically bound selenium yeast by continuous fermentation

This paper describes a protocol for incorporation of sodium selenite or sodium selenate into Saccharomyces cerevisiae biomass by continuous fermentation in a medium with minimal sulfur and methionine concentrations. Selenium incorporation was followed by atomic absorption analysis and methylene blue reduction time (MBRT). Continuous fermentation at 0.2 h^-1 dilution rate and sodium selenite addition gradient up to 0.69 g/L of Na²SeO³ yielded 1.89 g/L of biomass with 1904 μg of selenium/g of dry biomass. However, MBRT was 0.1 min, which indicated that the majority of selenium was in the inorganic form. On the other hand, continuous fermentation at 0.2 h^-1 dilution rate and sodium selenate gradient up to 0.28 g/L of Na₂SeO₄ yielded 0.76 g/L of dry biomass with 687 μg of selenium/g of dry biomass, and MBRT was 26 min, which indicated a high concentration of organically bound selenium. Overall, the results indicate a Se/S ratio of 3.9:1 and a dry biomass/Se ratio of 5.5:1 as optimal for continuous production of organically bound selenium.