## 1 Introduction  
Selenium is all an essential mineral for the proper functioning of cellular and body metabolism [1]. Selenium is a component of many enzymes, and also some proteins. Therefore, it also has an enzymatic and stabilizing function. Selenium prevents vascular diseases, stimulates the immune system and exhibits antiproliferative activity [2; 3; 4]. Selenium plays a role in optimizing thyroid function [5]. Selenium is an element found in many enzymes, especially those with antioxidant activity: glutathione peroxidase (GPx), iodothyronine deiodinase (DIO) and thioredoxin reductase (TRoxR). These enzymes prevent the formation of hydrogen peroxide and the peroxidation of phospholipid cell membranes [6; 7].  
A great advantage of enriching yeast cell biomass with selenium is its high absorption from the gastrointestinal tract in humans and animals [8]. Traces of selenium are essential for animals and humans; however, the tradeoff between the necessary and harmful doses should be investigated [9]. The World Health Organization recommends a daily dose of selenium in an amount of 40 \(\upmu\)g for an adult, emphasizing that a dose of 400 \(\upmu\)g/d of selenium is safe. The increasing incidence of selenium deficiency in humans and animals has drawn attention to the yeasts whose cell biomass can be enriched with selenium so that it can be a natural carrier of this element [1; 10].  
Due to the ability of yeasts to permanently incorporate macronutrients and trace elements into the structure of their cells, they have become a source of not only protein but also of microelements for humans and animals. In addition, the body absorbs these elements more efficiently in organic form than in mineral preparations [2; 11]. Yeasts also exhibit decreased toxicity and increased efficacy in the treatment of deficiency of this element. Nowadays, when developing selenium-enriched products attention is being paid to the ability of yeast cells to bioaccumulate trace elements.  
Most often, selenium occurs in combination with the amino acids cysteine and methionine (selenocysteine, selenomethionine). The most favorable form of selenium for humans is selenomethionine [12; 13; 14]. SeMet has antioxidant properties, improves the body's resistance and stimulates the action of DNA repair enzymes. It should be emphasized that organic selenium compounds are absorbed at around 90%-95%, while inorganic compounds at a lower level [1; 15]. Selenomethionine has the ability to be assimilated into proteins in place of methionine. The degree of utilization by the human organism of selenium originating from selenium yeast is estimated to be between 75% and 90% [16]. Few publications [1; 17; 18] indicate that yeasts are able to bind large amounts of selenium (up to 5000 ug/g dry weight).  
Due to their ability to accumulate different elements in their cellular structures, yeasts are a convenient experimental model. They are characterized by high protein content and rapid growth, which is reflected in an intense increase in cell biomass. Collection of selenium in yeast occurs with the use of extracellular ligands of membrane assembly and intracellular accumulation resulting from the transport of this element across the cytoplasmic membrane into the cell interior [19; 20]. Selenium accumulated by yeasts may occur in the organic form, e.g., selenomethionine, or in the form of red elemental selenium.  
There are very few studies in the available literature on the accumulation of selenium by cell biomass of fodder yeasts, hence it is possible to use _Candida utilis_ biomass as a source of this element in the production of food supplements or fodder additives. The objective of this study was to evaluate the possibility of intracellular binding of selenium by _Candida utilis_ ATCC 9950 yeast in media supplemented with selenium in the range of 4-100 mg/L, and to determine the content of protein in yeast biomass.