Sulphite analysis of food ingredients: false positive responses with butter flavourings in the optimized Monier-Williams method.

The analysis of commercial butter flavouring materials for sulphite by the optimized Monier-Williams method revealed detectable levels of apparent sulphur dioxide (1810 ppm for butter oil, 5760 ppm for butter flavour, and 14.5 ppm for butter derivative) even though the manufacturers claimed that no sulphites were used in processing these ingredients. The presence of volatile fatty acids in these ingredients may explain the anomalous results. Volatile fatty acids were found to interfere with the optimized Monier-Williams method for sulphite determination, detectable levels of titratable acid being found in the hydrogen peroxide trap with amounts of volatile fatty acids as low as 0.3 g. The levels of volatile fatty acids in foods would not be predicted to cause significant interference in the analysis of sulphite residues by the optimized Monier-Williams method. However, this analytical procedure is not recommended for the analysis of sulphite residues in food ingredients containing substantial amounts of volatile fatty acids. Alternative methods for the detection of residual sulphite, such as the sulphite oxidase assay and the colorimetric para-rosaniline method, are not subject to interference by volatile fatty acids.