Physicochemical properties and relative bioavailability of five sources of ferric orthophosphate in ready-to-eat cereal

Ferric orthophosphate (FePO4) is a promising iron source for food fortification because of its light color and oxidative stability. However, FePO4 has had limited use in cereal due its variable and often reported low bioavailability. An understanding of the mechanisms underlying this variability may facilitate production of a FePO4 source with consistent bioavailability. The bioavailability of iron sources used for fortification is thought to be largely dependent on their solubility in the dilute HCl in the stomach. Solubility is dependent on a combination of physicochemical properties, such as chemical composition, particle size, surface area and crystal structure. This research determined the relative bioavailability values (RBV) of two (control food) hydrogen-reduced iron sources (Sources 2 and 3) and six commercially available FePO4 sources (Sources 4--9) in ready-to-eat cereal (45% RDA, 0.27--0.30 mg iron/g cereal) using the AOAC Rat Hemoglobin Repletion Bioassay (ferrous sulfate standard, RBV 100%). The solubility of the iron sources in dilute HC1 was measured in vitro over the range of normal physiological HCl concentrations (0.02, 0.05 and 0.1N) using the solubility method of Shah and others (1977) that was modified to decrease assay variability (CV < 1%). Regression analysis found it to be a good predictor of RBV, R2 90%; P = 0.008). Particle size distributions (PSD) and surface areas (SA) were measured by laser light diffraction and nitrogen absorption, respectively.;The RBVs for the FePO4 sources ranged from 51--99 ( P < 0.05) and was higher than that reported in the literature. Most of the PSDs were multimodal and the sources could be grouped into three statistically different distributions (P < 0.05). The first group had a uniform PSD (Median PS 16.5 mum) and the lowest RBVs (51 & 60%); The second group was composed of a bimodal PSD of fine particles (Median PS 2 mum) and intermediate RBVs (78 & 83%), and the remaining Source 4 (RBV 99%) had a broad bimodal PSD (Median PS 9 mum) with one mode composed of larger 50 mum particles and the second mode composed of fine 6 mum particles. Regression analysis of PS and SA were found to partially explain the variable RBV of FePO4.;In an effort to further explain the variability of RBV, the presence of amorphous microstructure was investigated. Amorphous content is known to have important mechanistic properties that affect the moisture absorption, reactivity and stability of many materials. Very little data on the amorphous content of ferric orthophosphate exists in the literature. Dynamic gravimetric vapor sorption was used to estimate the amount of amorphous content (by moisture uptake). A regression model of median particle size and moisture uptake versus solubility in 0.10 N HCl was found to be an excellent predictor of solubility (R2 91%; P = 0.001). In conclusion, two factors, mean particle size and moisture uptake (as determined by amorphous content) were found to influence the solubility of FePO4, which may help explain the variable RBV in the literature.