Grain phytate concentration is strongly associated with total phosphorus in a collection of Australian bread wheat (Triticum aestivum cv. Scepter)

Christopher N Reed

Phytate, the primary storage form of phosphorus (P) in grain, is a potent anti-nutritional factor in human diets. Australia is the largest wheat (Triticum aestivum) grain producer per capita. Yet, there is little information about phytate levels in Australian wheat and how phytate levels can affect the bioavailability of essential micronutrients, such as iron (Fe) and zinc (Zn). The aims of this research were to determine phytate concentration in a collection of Australian bread wheat (cv. Scepter) and investigate the relationships between phytate and other grain characteristics; to estimate Fe and Zn bioavailability in the collection; and to investigate the effect of growing region, agronomic factors, and environmental factors on phytate concentration, grain characteristics, and Fe and Zn bioavailability. A commercially available colorimetric method from Megazyme (K-PHYT assay) was used to determine phytate in Australian bread wheat (cv. Scepter) grown at National Variety Trial (NVT) sites in 2021. The K-PHYT assay was scaled down to analyse a large grain collection (n = 58). Environmental and agronomical data from NVT sites, available online, were used to investigate the effect of phytate on mineral bioavailability. All data were analysed using RStudio version 4.2.3 (R Core Team, 2023). All statistical analyses had a confidence interval of 95% (P < 0.05). The mean value of phytate in whole grain flour was 5.6 ± 1.4 g/kg SD, which is in line with previous reports. Grain concentrations of Fe (35.1 ± 5.4 mg/kg SD) and Zn (18.4 ± 4.8 mg/kg SD) were determined by inductively coupled plasma optical emission spectrometry (ICP-OES). The phytate: mineral molar ratio model revealed a substantial limitation on the bioavailability of Fe and Zn in wheat grain. The phytate: Fe molar ratio at 13.6 ± 3.2 SD was above the phytate: Fe threshold value (>1), and the phytate: Zn molar ratio at 31.2 ± 8.2 SD was above the phytate: Zn threshold value (>15). K-PHYT assay analysis of phytate-P (1.6 ± 0.4 g/kg SD) and ICP-OES analysis of total P (2.8 ± 0.6 g/kg SD) grain concentrations indicated a phytate-P: total P ratio of 56.8% ± 6.3, which is lower than previous reports. This result should be interpreted with caution as the K-PHYT assay extracted 62.9 ± 7.6 % SD of total P. Statistical analysis detected no significant differences between agroecological zones of Australia regarding phytate concentrations in wheat grain. A linear model detected a positive linear regression between phytate and P (P ≤ 0.001; R2 = 0.84) but no significant correlations between phytate, P, Fe, or Zn and environmental/agronomic data. Overall, findings suggest that Australian wheat is low in Fe and Zn concentrations and that Fe and Zn bioavailability are also low due to the high grain phytate: Fe and phytate: Zn molar ratios.

Grain phytate concentration is strongly associated with total phosphorus in a collection of Australian bread wheat (Triticum aestivum cv. Scepter)

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