A digital image analysis method for quantification of sulfite in beverages

Abstract

A novel, simple and low-cost analytical procedure for sulfite determination in beverage samples is presented. The approach proposed consists of image capture from a sulfite colorimetric reaction, using a system built with low-cost materials for luminosity control and digital image decomposition into the primary colors red (R), green (G) and blue (B). The colorimetric reaction is based on the reduction of Fe(III) to Fe(II) in the presence of sulfite and further reaction with o-phenanthroline to form the red complex [Fe(C₁₂H₈N₂)₃]²⁺. Under optimized reaction and system conditions, the analytical curve was linear in a sulfite concentration range of 8.0 to 140 mg L⁻¹, with limits of detection and quantification of 2.6 mg L⁻¹ and 8.0 mg L⁻¹, respectively. The analytical method was applied to sulfite quantification in different beverage samples such as white wine, vinegar, rosé wine, cashew juice and coconut water. The results acquired were in close agreement with those obtained using iodometric titration as a comparative method, with a confidence level of 95%. Moreover, the method can be useful with regard to social and environmental impacts, due to the low generated residues (800 μL per spot-test) and employs easily available instrumentation with the potential for in situ determination during the application of sulfites as additives during beverage production and quality control.