Comparison of three different methods for the determination of sulphur dioxide in fruit and vegetable products

Sulphite is a food additive used worldwide. Globally, for concentrations above 10 mg.kg-1, sulphite compounds must be labelled as sulphur dioxide (SO2) on the packaging due to their potential health risks. This study compares spectrophotometric (S), titration (T) (modified optimised Monier-Williams, OMW), and reflectoquant (R) methods for measuring sulphur dioxide in twenty fruit and vegetable products. The samples comprise sulphited, unsulphited, and naturally sulphur-containing products (Allium genus such as garlic and onion). The article discusses the strengths and weaknesses of the method used. Allium genus samples yielded false-positive results, especially in fresh garlic samples with average SO2 concentrations of 46, 1 152, and 40 mg.kg-1 obtained by titration, spectrophotometric, and reflectoquant methods, respectively, therefore, none of the methods is suitable for testing this type of vegetables or products containing a low proportion of them. For other types of samples, the methods showed acceptable working characteristics. Recovery tests showed 89.5, 82.0, and 75.2% recovery with 2.8, 3.9, and 13.2% repeatability and the limit of quantification of 1, 10, and 25 mg.kg-1 in the spectrophotometric, titration, and reflectoquant methods. The result highlights the importance of method selection based on sample characteristics and regulatory compliance.