Absorption-based detection of urea concentration in hydroalcoholic solutions and white wine with a compact optical setup

Contactless detection of urea concentration in solution is very important in industrial, chemical, biological and agri-food applications. Urea, naturally contained in wine, when reacting with ethanol especially at high temperature may become the precursor of ethyl carbamate (urethane) which is carcinogen. Close monitoring of urea concentration in wine is necessary to prevent risks and diseases related to high urethane intake. In this work, we propose an optoelectronic measuring system for absorption-based detection of urea in water, in hydroalcoholic solutions with ethanol concentration of 12%, and in white wine with a 12% nominal concentration of ethanol. The urea concentration of the tested samples, contained in a 1-mm-deep rectangular glass capillary, was in the range from 0 up to 40 mg/mL. Two low-cost, low-power LEDs with emission spectra in the near infrared were employed as readout sources. The emission peaks of the selected LEDs were at λ = 1450 nm, where water absorbs more than urea, and at λ = 2350 nm, where urea absorption is higher than that of water. An amplified photodiode allowed to measure the transmitted optical power through the sample. To highlight the specificity of the measures, we also compared results of the optical measurements performed on urea-water solutions and on NaCl-water solutions. We demonstrate that measuring light intensity transmitted through each fluid mixture at the selected wavelengths allows not only to identify different urea concentrations, but also to distinguish samples of water-ethanol solutions from samples of white wine with the same urea concentration. This work represents the Proof of Principle of an optical method for specific urea detection in wine. Future work will be devoted to improving the system operation to enhance the limit of detection and to achieve recognition of urea concentration of the order of 0.001 - 0.003 mg/mL, typical values found in winery products.