Colloidal stability and chemical reactivity of complex colloids containing Fe3+

The reactivity of iron contained within insoluble colloidal metal-pyrophosphate salts was determined and compared to the reactivity of a soluble iron salt (FeCl3). As a model system for the reactivity of iron in food products, the formation of an iron-polyphenol complex was followed with spectrophotometry. Three types of systems were prepared and their colloidal stability and reactivity studied: Fe3+ pyrophosphate, protein-coated Fe3+ pyrophosphate and mixed-metal pyrophosphates containing Fe3+ and a second cation M. The additional cation used was either monovalent (sodium) or divalent (M2+). It was found that: (i) incorporating iron in a colloidal salt reduced its reactivity compared to free Fe3+ ions; (ii) coating the particles with a layer of hydrophobic protein (zein) increased stability and further decreased the reactivity. Finally, the most surprising result was that (iii) a mixed system containing more Fe3+ than M actually increased the reactivity of the contained iron, while the reverse, a system containing excess M, inhibited the reactivity completely.