Direct automatic determination of the methanol content in red wines based on the temperature effect of the KMnO₄/K₂S₂O₅/fuchsin sodium sulfite reaction system

The standard method for methanol assay in wine is based on a methanol/KMnO₄/H₂C₂O₄/fuchsin sodium sulfite (FSS) reaction system. However, it is difficult to control the degree of colour and the temperature of the reaction product in this assay, and its repeatability is also poor due to the generation of CO and CO₂ in the reaction. Therefore, to solve these problems, potassium metabisulfite was selected to replace H₂C₂O₄, and an automatic analysis method was developed which can realize rapid and accurate determination of methanol and can be used to make an online analyzer. It was discovered that the reactions of methanol/KMnO₄ and acetaldehyde/FSS are exothermic, while the reactions of methanol/KMnO₄ and formaldehyde/FSS are endothermic. Consequently, based on the temperature effect, not only was the interference of ethanol eliminated in detecting methanol in wines, the purpose of the research was achieved to directly and accurately determine methanol without sample pretreatment. By optimizing the system, the obtained conditions for determining methanol in wines were as follows: 20 g L⁻¹ concentration for KMnO₄; 3 g L⁻¹ concentration for FSS; 40 cm length for the first reaction coil (RC₁); 100 cm length for RC₂; 700 cm (I.D.: 0.8 mm) length for RC₃; 50 °C for RC₃; about 20 °C for RC₁ and RC₂; 330 μL for the sample volume. The method showed a linear response in the range 25–1000 mg L⁻¹, with a 0.6% RSD, 8.8 mg L⁻¹ detection limit and 25 samples per h, and was successfully used for testing representative wine samples. It also obtained better accuracy than previous methods. Due to its superiority in automated operation, reproducibility, analysis speed and test cost, this method and system can serve as a supplementary standard for methanol assay, and for the quality control of the winemaking process and the final wine-product, as well as for low-alcohol drinks.