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A new iron–casein complex (ICC) has been developed for iron (Fe) fortification of dairy matrices. The objective was to assess the impact of ascorbic acid (AA) on its in vitro bioavailability in comparison with ferrous sulfate (FeSO4) and ferric pyrophosphate (FePP). A simulated digestion coupled with the Caco-2 cell culture model was used in parallel with solubility and dissociation tests. Under diluted acidic conditions, the ICC was as soluble as FeSO4, but only part of the iron was found to dissociate from the caseins, indicating that the ICC was an iron chelate. The Caco-2 cell results in milk showed that the addition of AA (2:1 molar ratio) enhanced iron uptake from the ICCs and FeSO4 to a similar level (p = 0.582; p = 0.852) and to a significantly higher level than that from FePP (p < 0.01). This translated into a relative in vitro bioavailability to FeSO4 of 36% for FePP and 114 and 104% for the two ICCs. Similar results were obtained from water. Increasing the AA to iron molar ratio (4:1 molar ratio) had no additional effect on the ICCs and FePP. However, ICC absorption remained similar to that from FeSO4 (p = 0.666; p = 0.113), and was still significantly higher than that from FePP (p < 0.003). Therefore, even though iron from ICC does not fully dissociate under gastric digestion, iron uptake suggested that ICCs are absorbed to a similar amount as FeSO4 in the presence of AA and thus provide an excellent source of iron.

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Objectives The two objectives were 1) to evaluate the solubility of two iron casein complexes (ICCs) under a condition mimicking gastric pH, 2) to evaluate the impact of ascorbic acid (AA) on the in vitro iron absorption of ICCs after incorporation in reconstituted whole milk powder. Methods The in vitro solubility was determined over time after addition of diluted HCl (pH 1.7), ultracentrifugation and measurement of iron appearing in the supernatant by ICP-OES (n = 2). The impact of AA on iron uptake from the Fe compounds in reconstituted milk was determined using the in vitro digestion coupled with the Caco-2 cell model and the measurement of ferritin/total protein produced by the cells (n = 3). The molar ratio of AA to iron of 2 to 1 recommended by the WHO for iron absorption optimization has been tested with an iron level corresponding to 3.3 mg Fe/serving of milk. Ferrous sulfate (FeSO4), the reference compound for iron bioavailability and micronized ferric pyrophosphate (FePP), main salt used for milk fortification were used as references. Results The dissolution test showed a rapid solubilization of iron from the ICCs i.e., &gt;75 ± 19.3% at 5 min and &gt;89 ± 0.3% at 90 min. The kinetics of soluble iron from the complexes were like that from FeSO4. The solubility of FePP was only 37.6 ± 4.7% at 90 min. Without AA, the iron uptake from FeSO4 was lower than expected translating into a relative in vitro bioavailability (iRBA) of FePP and of the two ICCs to FeSO4 of 66, 169 and 215%. This might be explained by a rapid conversion of soluble iron from FeSO4 into Fe3+ and insoluble iron hydroxide when the pH increased from 2 to &gt;7 during in vitro digestion. However, with the addition of AA in the milk, iron uptake by the cells was found to be increased to levels of 341.8 ± 8.9, 124 ± 12.2, 403.1 ± 117.8 and 362.9 ± 36.9 ng ferritin/mg protein for FeSO4, FePP and the two ICCs respectively. This translates into iRBAs to FeSO4 of 36% for FePP and of 118 and 106% for the two ICCs. Conclusions The solubility and the demonstrated impact of AA on Fe uptake suggest that ICCs are absorbed to a similar amount as FeSO4 and thus provide an excellent source of Fe. Funding Sources Société des Produits Nestlé, NPTC Konolfingen, Switzerland.