Electrochemical Removal of Dissolved Oxygen for the on-Site Sensing of Free Sulfites in Wines

Sulfites are a commonly used anti-oxydants, food preservatives and anti-browning agents in the food industry (dried fruits and vegetables, pickles and beverages). Sulfites play also an important role in the preservation of wines as they are used at various stages of the winemaking process to protect the wine from oxidation and help the preservation of its flavors, aromas, and color. Sulfites are present in wines as ‘free’ or they can be bound to carbohydrates, lipids, or proteins. Only the free form is active in the wine protection. Winemakers must check, the ‘free’ and total sulfite content of their wine to make sure that the concentration of sulfite is sufficiently high to protect the wine and that the total sulfite concentration is not above the maximum authorized concentration. Samples are thus sent regularly to specialized laboratories for analysis. We report here the development of an electrochemical sensor for the on-site detection of free sulfites in wine. The electrochemical determination of free sulfite can be achieved by either oxidation or reduction. The presence of many interferents (ascorbates, polyphenols) complexifies the determination based on its oxidation, while determination by reduction is simpler as dissolved oxygen is the sole interferent. However, the electrochemical signal of sulfite reduction may be hidden by the reduction of dissolved oxygen. In laboratory conditions, the interference of dissolved oxygen is usually eliminated by displacement through the bubbling of an inert gas (e.g. Ar or N2). However, as sulfite is a gas under acidic conditions, nitrogen bubbling may also remove it from the samples. We report here the principle of the electrochemical removal of dissolved oxygen in the vicinity of a sensing electrode, making on-site detection possible [1]. The local elimination of dissolved oxygen was achieved at a platinum mesh electrode distant from the sensing electrode by 250 µm. The reduction potential was carefully optimized to allow the diffusion of sulfite through the mesh and its detection at the sensor electrode. With such an experimental set-up, the amperometric response for free sulfite in wines was linear in the 7.5 – 200 mg L-1 concentration range and with a limit of detection of 7.5 mg L-1. The samples were analyzed in less than 5 minutes without the need for solution purging with an inert gas, confirming the advantage of local elimination of dissolved oxygen for on-site analysis. A series of 27 wines (14 reds and 13 whites) from various regions of France was analyzed and the results were validated by conventional iodometric titration. Our amperometric sensor demonstrated that there were no interferences other than the one from dissolved oxygen, which is removed. Even ascorbic acid, which is a major interferent for the iodometric method, did not alter the amperometric response [2]. The electrochemical system was tested in an independent winery (Dahlenheim, France) to confirm the ability to perform on-site analysis in a representative working environment. The on-site results were validated by both amperometric and iodometric analysis in the lab. The ease of use and versatility of the sensor open possibilities to use this method for the detection of other food additives (e.g. nitrite), which determination is hindered by the presence of dissolved oxygen, in other types of foodstuff. [1] M. Etienne, T.X.H. Le, T. Nasir, G. Herzog, Electrochemical Filter To Remove Oxygen Interference Locally, Rapidly, and Temporarily for Sensing Applications, Anal. Chem. 92 (2020) 7425–7429. https://doi.org/10.1021/acs.analchem.0c00395. [2] J. Gajdár, G. Herzog, M. Etienne, Amperometric Sensor for Selective On-Site Analysis of Free Sulfite in Wines, ACS Sensors. 7 (2022) 2209–2217. https://doi.org/10.1021/acssensors.2c00611.