The impact of sprouting or hydrothermal processing of wheat on iron and zinc bioavailability

Promovendus/a: Marie Huyskens

Promotor(en): Prof. dr. ir. Erik Smolders, Prof. dr. ir. Jan Delcour, Prof. dr. Kristin Verbeke

While wheat is a good source of iron (Fe) and zinc (Zn), their bioaccessibility (i.e. the extent to which minerals are released from their matrix during human digestion) is limited due to their binding with phytic acid. This interaction leads to the formation of the insoluble salt known as phytate. Unit operations that target phytate breakdown through the action of the enzyme phytase can be effective at increasing mineral bioaccessibility. One such approach is steeping and sprouting of wheat, where the enzyme is activated and synthesized. Another relevant approach is hydrothermal processing, in which wheat grains are steeped under conditions that activate the phytase in the grain. Phytase breaks down phytate, resulting in the release of minerals previously bound to it and thereby increasing their mineral bioaccessibility.

However, prior to this work, it was unclear whether the minerals released by these processes were actually absorbed by the human body, and therefore, truly bioavailable. To address this knowledge gap, the overall objective of this work was to investigate the effect of sprouting or hydrothermal processing of wheat on Fe and Zn bioavailability using both in vitro (Caco-2 cells) and in vivo (human study) approaches.

Steeping and sprouting of wheat effectively increases Fe and Zn bioaccessibility but this did not consistently translate into higher bioavailability of these minerals, estimated using Caco-2 cells. Only wheat sprouted for 120 h showed a slight increase in Caco-2 bioavailability of Zn. A human study revealed that consuming breakfast flakes made from steeped and sprouted wheat did not lead to more Zn absorbed compared to regular flakes. The fact that Fe and Zn bioavailability was not increased by sprouting of wheat may be partly due to the formation of stable mineral complexes that the body cannot absorb.

Hydrothermal processing (pH 4.0, 60 °C) of wheat in citrate buffer greatly improves Fe and Zn bioaccessibility and Caco-2 bioavailability, likely because soluble mineral-citrate chelates are formed. Consuming breakfast flakes made from these processed wheat grains led to slightly higher Zn absorption relative to the Zn content of the flakes, but the absolute amount of Zn absorbed was not greater than that from regular flakes. The modest increase in Zn absorption was likely due to the formation of stable, non-bioavailable Zn complexes and Zn homeostasis in the human body.

Overall, phytate content is a major, but not the only determinant of mineral bioaccessibility and bioavailability in wheat. Even when phytate contents are substantially reduced, other factors may still limit mineral bioavailability. This work provides in-depth insights into how unit operations impact Fe and Zn bioavailability, contributing to the production and consumption of foods with improved nutritional quality, ultimately benefiting public health.