Improved iron and zinc availability in sorghum by phytate reduction through genetic modification, fermentation and phytase addition
Iron and zinc deficiencies are highly prevalent in the developing world. In worst cases, the cause of iron and zinc deficiency may be actual insufficient dietary intake, but inhibitors of absorption, mostly found in plant foods, contribute substantially to these deficiencies. Sorghum is an important staple crop in Africa, often in populations with severe iron and zinc deficiencies. Sorghum is commonly consumed as whole grain. While the bran of sorghum contains the most iron, it also contains phytate and sometimes also tannins, depending on the cultivar, which further reduce the already low bioavailability of non-haem iron and zinc. The aim of this research was to evaluate the effect of reducing sorghum and maize phytate content through genetic modification (GM) on in vitro iron and zinc availability in porridges and raw cereal lager brewer’s wort. The effect of phytate reduction through GM on iron and zinc availability was also compared with the effect of phytate reduction through natural lactic acid fermentation and the addition of exogenous phytase. Iron and zinc availabilities were assessed using a dialysability assay (bioaccessibility), Caco-2 cell (uptake) study and an in vivo suckling rat pup model (absorption). A mineral solubility assay was used to analyse the soluble minerals in the raw cereal wort. GM low phytate (approx. 30-40% reduction) non-tannin and tannin sorghums and their respective null controls (NC) were processed into thick unfermented and fermented porridges. The inhibitory effect of the tannins on mineral availability seemed to prevent any increase in in vitro iron and zinc bioaccessibilities regardless of the level of phytate reduction. However, the additive effect of GM in combination with fermentation in reducing the porridge phytate content, caused a substantial increase in in vitro iron bioaccessibility in the non-tannin line. The percentage bioaccessible iron in the GM fermented porridge was approximately 30%, compared to the GM unfermented porridge (approx. 10%) or the NC fermented porridge (approx. 15%). At this level of phytate reduction, the dialysability assay could not detect any effect on zinc bioaccessibility. A larger phytate reduction through GM (approx. 80-90%) in non-tannin sorghum significantly (p<0.05) increased zinc uptake and absorption and iron bioaccessibility. Principal component analysis (PCA) showed an indirect correlation between phytate content and zinc uptake and absorption and iron bioaccessibility and absorption. The dialysability assay used in this research proved ineffective in estimating zinc absorption in GM sorghums. However, the dialysability assay can be used to estimate in vivo iron absorption from sorghum. The Caco-2 cell uptake study used in this research proved ineffective in estimating iron absorption in GM sorghums, as the iron uptake was possibly affected by the varying mineral (Ca, Fe, Zn, P) contents of the sorghums. More research is needed to determine the effect of naturally occurring variations in mineral contents of sorghum on the iron uptake by Caco-2 cells. With regard to raw cereal brewing a phytate reduction through GM (approx. 30- 40%) reduced the spent grain mineral (Fe, Zn, Mg, P, Ca) contents of sorghum by approximately 11-38%. While phytase addition during brewing reduced sorghum spent grain phytate content by 88% and mineral content by 17-59%, it did not, however, affect the maize phytate and mineral contents significantly (p≥0.05). This may be due to the fact that the phytate in maize is more soluble than in sorghum. The reduction in mineral content in the spent grain is an indication of the amount of minerals which would be solubilised during brewing. Compared to addition of exogenous phytase, GM has greater potential for increasing the overall nutritive quality of sorghum wort, as it also increased hot water extract and wort free amino nitrogen substantially. This research indicated that reducing sorghum phytate content through GM, fermentation and phytase addition increases iron and zinc availability from sorghum. However, none of the assays applied gave an unequivocal indication of the magnitude of increase in iron and zinc bioavailability. Also, factors like human health, food processing and other food components have a major influence on iron and zinc bioavailability and absorption in humans. For low phytate sorghum to increase iron and zinc status in subsistence growers of sorghum in semi-arid regions of sub-Saharan Africa where the prevalence of poverty and iron and zinc deficiency are high, it needs to be implemented together with nutrition education and dietary diversification, while simultaneously taking measures to address poverty and morbidity.
UCTD, Sorghum, Raw cereal lager brewer’s wort, Iron and zink deficiencies
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