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Asked on 22.01.2023.

Sulfite Calculation

Hello everyone,

I need some help in understanding how to calculate sulfite residues in order to check that the amount of Sodium Metabisulfite added to our products (biscuits and crackers) is compliant to local regulations.

Based on the Sodium Metabisulfite spec, the SO2 assay is 67%.  Does this mean that 67% of the Sodium Metabisulfite is SO2 and that this will be the final value through lab testing? Does the baking process cause SO2 deterioration?

Any help would be very much appreciated.

Thanks in advanced,


Accepted answer
Answered on 23.01.2023.

Hello Erel,

According to stoichiometry of reaction Na2S2O5 + H2O -> 2 SO2 + 2 NaOH, the average percentage of SO2 should be around 67.4%.

It is also known that, at high temperatures (>150ºC), Sodium Metabisulphite may decompose to oxide fumes of sulfur and sodium. Thus, the final amount of SO2 and will probably be much smaller than that, depending on the baking profile (and possibly other factors too). I sometimes get analytical values ​​up to 10 times smaller than calculated predictions.

Since is not so easy to accurately calculate the final amount of S02 under these conditions, my recommendation is that you send samples of different batches of your product to a certified lab, to get more realistic average values through analysis.



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