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Abstract:

Wheat whole grains are a good source of dietary fiber and an array of bio-actives including minerals such as iron (Fe) and zinc (Zn). However, only 5 to 10% of Fe and Zn ions are bio-accessible as these minerals are both physically and chemically entrapped. The physical barrier is set by the rigid cell walls which withstand conventional milling and human digestion enzymes. Phytic acid, the main storage form of phosphorus (P), chelates divalent cations like Fe and Zn. During germination hydrolytic enzymes are activated and de novo synthesized to fuel seedling growth. Endoxylanases alter the cell wall by hydrolyzing and solubilizing arabinoxylan into water extractable arabinoxylan (WEAX). At the same time, the increase in phytase activity makes minerals available. Both the opening of aleurone cells and breakdown of phytate by germination may result in a greater mineral accessibility in the human gastro-intestinal tract. To explore the potential of steeping and germination of wheat optimal conditions were selected from a multifactor experiment based on phytate and WEAX content. Steeping for 36 h at 15 °C and germinating for 120 h at 26 °C decreased phytate content from 0.96% to 0.64% of initial dry matter and increased WEAX content from 0.48% to 1.34% of initial dry matter. While the Fe and Zn bio-accessibility in the grains was 5% and 3%, respectively, that in the germinated grains was 6% and 8% when the cellular matrix was preserved, and 22% and 21% when it was mechanically disrupted by milling. This revealed that controlled germination of wheat improves mineral bio-accessibility when the cell walls are substantially broken down. Moreover, high-definition µ-X-ray fluorescence microscopy was here for the first time used to map changes in distribution of Fe and Zn between and within wheat tissues during germination. While Zn and Fe are clearly confined to the aleurone cells in regular wheat, co-localizing with P globoids, Zn was translocated to the pericarp, coleoptile and radicle during germination. This proves that it is more available in the seedling than in the aleurone, where it is used for different biological functions, such as protein synthesis.