Pearl millet : influence of mineral biofortification and simple processing technologies on minerals and antinutrients

Abstract

Pearl millet is an important staple food in rural Africa. However, the mineral bioavailability of pearl millet is low due to its high content of antinutrients, particularly phytate. This research investigated the effects of mineral biofortification, steeping/lactic acid fermentation and parboiling alone and in combination with abrasive decortication of pearl millet grain on its mineral and antinutrient contents. Six normal varieties and two mineral biofortified hybrids were investigated. There was considerable variability in mineral content among the varieties. Iron content ranged from 3.0 to 9.6 mg /100 g and zinc from 3.0 to 4.8 mg /100 g. The mineral biofortified hybrids Dhanashakti and ICMH 1201 had substantially higher iron (21-68%) and zinc (15-39%) contents compared to the normal varieties. Phytate content differed substantially, with levels from 830 to 1360 mg /100 g. There was no definite trend between the phytate content of normal and mineral biofortified types. Decortication did not cause significant losses in zinc, but resulted considerable iron losses (mean 31%) across the varieties. There were minimal effects of steeping/lactic acid fermentation and parboiling on iron and zinc contents. Mineral biofortified hybrids were associated with high iron and zinc content after all processing treatments. Decortication of raw grain substantially reduced phytate (mean 24%) and by a further 12 percentage points when applied after steeping/lactic acid fermentation. Parboiling plus decortication was less effective in reducing phytate content. The critical phytate: iron molar ratio of <1, above which iron absorption is seriously impaired, was not achieved with any of the processes. However, steeping/lactic acid fermentation plus decortication and parboiling plus decortication reduced the phytate: zinc molar ratio to below the critical level of <15 in some varieties. Generally, the mineral biofortified hybrids had improved phytate: mineral molar ratios than the normal varieties for both raw and processed grains. Decortication greatly reduced total phenolic content (mean 24%) across the varieties and by an additional 14 percentage points after steeping/lactic fermentation and parboiling. Abrasive decortication in combination with steeping/lactic acid fermentation is an effective way of reducing phytate content in pearl millet grain, and hence somewhat improving estimated iron and zinc availability. It is recommended that the process is utilised to a greater extent in pearl millet food processing. Parboiling is also effective in phytate reduction, and can be adopted. Because mineral biofortified pearl millet hybrids have much higher iron and zinc contents, their breeding and cultivation should be promoted in rural Africa.