Targeting iron speciation in wines

During the winemaking process, wine is frequently exposed to oxygen which can affect the organoleptic characteristics and therefore the quality of the finished wine. Most phenolic molecules in wine serve as oxidation substrates affecting colour and flavour. Transition metal ions namely copper and iron act as electron pumps; in fact, Fe(III) is reduced to Fe(II) as a consequence of the oxidation of phenols to quinones and, in the presence of oxygen, Fe(II) is oxidised back to Fe(III). Consequently, speciation analysis by the oxidation state is critical and even more relevant than the total ion concentration when one wants to evaluate the risk for wine oxidation. Interestingly the different reactivity of Fe(III) and Fe(II) makes spectrophotometric methods, namely colorimetric reactions, ideally suited for speciation analysis. In this work, a microfluidic paper-based analytical device (µPAD) was designed for the iron speciation in wine samples. The colorimetric reaction between bathophenanthroline and iron(II) was chosen and the oxidation of Fe(III) to Fe(II), using hydroxylamine, was carried out within the same device. Aiming for application to different types of wine, the developed device also enabled to account for the potential sample colour (subtraction of interfering signal). The designed µPAD enabled the iron speciation in both white and red wines.