Fortification of milk with iron-ascorbate promotes lysine glycation and tryptophan oxidation

Infant formulas are supplemented with iron and ascorbate for nutritional purposes, but activation of the advanced Maillard reaction by iron and production of hydroxyl radicals by the iron-ascorbate system may damage the proteins with possible nutritional loss. In this study, we evaluate the deleterious actions of iron and ascorbate, namely lysine damage and tryptophan oxidation on whey-lactose samples incubated at 60 °C. Iron-catalysed ascorbate oxidation enhanced lysine blockage mainly by ascorbylation, whereas lactosylation was little influenced. A three-fold increase in the accumulation rate of fluorescent advanced Maillard products (AMP) was observed in the presence of iron/ascorbate and AMP fluorescence was well correlated to carboxymethyllysine concentration in the samples. The tryptophan degradation rate was 2.5-fold higher in the presence of iron-ascorbate and the loss of this amino-acid was exponentially correlated with AMP fluorescence, similarly whatever iron-ascorbate was present or not. Application of milder heat treatments to infant formulas would allow to decrease the extent of these undesirable reactions.