Chapter 22 Enrichment in Selenium and Other Trace Elements

In addition to proteins, lipids, and various vitamins, eggs contain a wide range of minerals, including calcium, phosphorus, zinc, iron, magnesium, copper, and selenium (Se). However, egg consumption by humans accounts for only 1–3% of the dietary mineral intake. Further, manipulation of the egg’s minerals is quite a difficult task since their delivery to the egg is physiologically regulated to ensure that the embryo is not exposed to toxic levels. Therefore, it is practically impossible to increase the level of zinc, copper, or iron in the egg to such an extent that they would be considered as an important source of these minerals in the human diet. Almost all the calcium, phosphorus, iron, magnesium, manganese, and many other trace elements of the yolk are found in the dense granules where they are bound to the protein phosvitin. This protein derives from the proteolytic cleavage of the much larger yolk precursor protein, vitellogenin (Griffin et al. 1984). Phosvitin is a highly unusual protein, with serine accounting for about 55% of its amino acid residues, almost all of the serines being phosphorylated. The negatively charged phosphate groups bind large amounts of positive ions, mainly calcium but with significant amounts of other minerals (White 1991). Since the phosvitin phosphate groups are normally saturated with the bound positive ions, there is little scope to further increase the concentrations of these elements in the yolk. These limitations do not, however, apply to Se since this element is incorporated into yolk proteins as a component of seleno-amino acids. Thus, the concentration of Se in the egg is a reflection of its concentration in the diet of the hen. Current diets for poultry are usually relatively low in Se. Moreover, inorganic forms of Se such as sodium selenite, which is widely used as a supplement in poultry diets, are relatively ineffective with regard to enhancing the Se concentration in the egg. Organic forms of Se are much more effective. Therefore, it is only the development of the technology