Changes in mineral contents of hens’ egg yolk fractions due to biological and technological influences Veränderungen des Mineralstoffgehaltes von Hühnereigelbfraktionen durch biologische und technologische Einflüsse

Calcium, magnesium, copper, iron and zinc are stored in hens’ egg yolk and, except for copper, the majority of these minerals are located in egg yolk granules (Table 1). Presumably, up to 90% calcium, magnesium, iron and zinc are firmly bound in the phosvitin-lipovitellin complex of granules (GREENGARD et al., 1964; TABORSKY, 1980; TUPPER et al., 1954). As was discussed by Richards, copper ions are bound by lipovitellin and phosvitin, but it is not known to which extent (RICHARDS, 1997). The mentioned minerals are necessary for the development of the chick embryo and there are two possible mechanisms for the intake of minerals by the yolk sac membrane. In electron micrographs it was shown in 1958 by Bellairs that granules can be found in yolk sac endodermal cells (BELLAIRS, 1958). Endocytosis of the granules by the yolk sac membrane was later discussed by various authors (JUURLINK and GIBSON, 1973; LAMBSON, 1970; MOBBS and MCMILLAN, 1979). Therefore, it is possible that minerals bound in the granules are absorbed as well and released in the endodermal cells due to digestion of the granules. However, there was no report found which actually confirms this assumption. Besides this intracellular digestion, another extracellular digestion is discussed (FREEMAN and VINCE, 1974). This digestion of granules could also free minerals which would probably be noticeable by a higher mineral content in the liquid plasma fraction of the egg yolk. The absorption of free minerals by the yolk sac membrane was only indirectly proven by DEWAR et al. (1974), who inoculated the yolk sac membrane with certain minerals and measured subsequently higher mineral content in avian embryos. However, we found no study in which the mineral content of egg yolk fractions of embryonated eggs was determined, so it is not known if there are more mineral ions in plasma of embryonated egg yolk which could be absorbed by the yolk sac membrane. We therefore determined mineral content in egg yolk plasma and granules of native and embryonated eggs to find out if there are significant differences in mineral content. If minerals are released during