Steeping and germination of wheat (Triticum aestivum L.). II. Changes in spatial distribution and speciation of iron and zinc elements using pearling, synchrotron X-ray fluorescence microscopy mapping and X-ray absorption near-edge structure imaging

Abstract

Iron (Fe) and zinc (Zn) in wheat are mainly present as insoluble phytates in the aleurone cells. Phytate breakdown during steeping and germination of wheat enhances their bio-accessibility. However, the accompanying redistribution within the grain and changes in speciation are still unclear. We unravelled these changes using a combination of wheat fractionation by pearling, X-ray fluorescence microscopy mapping and X-ray absorption near-edge structure imaging. A significant fraction of Zn and Fe migrates to radicles and coleoptiles during germination. The remainder has low water extractability (≤8%). The high water extractability (26–27%) of Zn in the radicle and coleoptile indicates its mobility in the seedling. Zn migrates from the aleurone to the pericarp and embryonic tissues, while Fe is dominantly present in the scutellum tissue attached to the seedling. The co-localisation of Zn and sulfur (S) in the developing embryo suggests that translocated Zn is bound to S-containing peptides. In aleurone cells Fe remains phytate bound and although Zn is phytate and S-bound, slight changes in its speciation occur during germination. This study partially explains the impact of steeping and germination on mineral bio-accessibility and opens possibilities for enhancing nutritional quality during food processing.