Zinc is a key micronutrient required by plants and animals alike. The need for sufficient dietary zinc – a need not met for nearly a third of the human population – has led to efforts to biofortify, or increase levels of the element, in commercial maize and sweetcorn. However, merely increasing the amount of zinc in the tissues of corn kernels is not enough: the zinc must be of a bioavailable species, that is, a form that can be taken up and used by the body. Zinc that occurs as Zn-phytate has a poor bioavailability, while zinc as part of a nitrogen- or sulphur-containing compound is more easily taken up. In order to determine the most efficient biofortification strategies, researchers must understand what species of zinc occur in what proportions in different edible parts of the crop, and how these change with the age of the kernels.
A recent paper published in Annals of Botany by Zhong Xiang Cheah and colleagues aims to give us a clearer picture of zinc accumulation in maize and sweetcorn kernels. The authors analyzed three maize and 12 sweetcorn varieties at 21 days after pollination (DAP), quantifying the concentration and species of the mineral in embryos, endosperm, and whole kernels. One variety of each type was further analyzed at 28 and 56 DAP to investigate changes that took place as the kernel matured.
The analyses determined that whole kernel zinc concentration is 24% higher in sweetcorn than maize at the 21 DAP stage, the stage at which sweetcorn is eaten. However, the larger kernel size in maize resulted in roughly the same amount of zinc by dry weight. Though the embryos of both kernel types had a higher concentration of zinc than did the endosperm, more than 88% of it was in a non-bioavailable form at any given time. Conversely, in the endosperm of 21 DAP kernels, more than 87% of the zinc was bioavailable. In maize, which is consumed at a later stage and without the embryo, the endosperm zinc shifted from bioavailable Zn-histidine toward Zn-phytate over time, making it largely unavailable by the time it would be eaten. Decrease in zinc bioavailability over time was minimal in sweetcorn.
The take-away message of the study is that sweetcorn is the better target for biofortification efforts, both because it is consumed at an earlier developmental stage, and because what little zinc can be gained from the embryo is not lost to processing. “This [result] has major implications for Zn biofortification in maize which is often consumed at the mature kernel stage,” the authors wrote, “as it suggests that minimal dietary benefit can be derived from increasing Zn concentration in maize kernels.”