Characterisation of Iron Content and Speciation in Australian Eggs

The quantity and bioavailability of iron (Fe) in commercial chicken eggs have been subject to ongoing debate. Understanding the chemical form of Fe in eggs, and how different laying conditions or cooking environments may alter chemical form is important to guide future studies of Fe bioavailability. To address these unanswered questions, this study aimed to accurately quantify and characterise Fe speciation (including haem and non-haem Fe) in egg yolk, albumen and whole eggs (mixed yolk and albumen), in both raw and cooked eggs. Eggs were obtained from four different hen housing systems: free-range, cage, barn and organic. Total Fe was measured using microwave plasma atomic emission spectrometry, and X-ray absorption near-edge structure spectroscopy was used to quantify the relative proportions of different chemical forms of Fe (Fe speciation). These analyses were conducted on raw albumen, yolk, and whole egg samples (combined yolk and albumen) from eggs produced across all housing systems, as well as on baked and boiled albumen, yolk, and whole egg samples from free-range eggs. Haem Fe was not detected by the analytical methods used, confirming that eggs are not a nutritionally relevant source of haem Fe. Mixing yolk and albumen alters Fe speciation, decreasing relative phosphate coordination of Fe and increasing Fe associated with protein carboxylate and chloride groups. Subsequent baking causes a significant reduction in carboxylate- and chloride-bound Fe, accompanied by an increase in sulfur-bound Fe. Boiling eggs was found to have minimal effects on Fe speciation. Despite contributing little Fe, albumen plays an important role in modulating Fe speciation, which may subsequently impact bioavailability. Cooking eggs changes the Fe speciation, particularly increasing the amount of Fe-S coordination, which should be taken into consideration for future study design when assessing Fe bioavailability.