13 different commercial batches of Selenized yeasts were analyzed. Not only the content in SeMet was variable and often largely below the regulatory and mandatory value of 63%, but an inorganic Se species, elemental Se or Se°, was measured at levels up to 50% thanks to the development of a new analytical method.

Such variability in the organic Se content of these Se-yeasts explain their reduced efficacy compared to pure organic Se sources such as SeMet or OH-SeMet.