Â Sodium Metabisulphite, SMS, Na2S2O5
This is another white crystalline powder. It is used in very small quantities to chemically modify the gluten quality in semisweet biscuit doughs. It behaves as a reducing agent through the liberation of sulphur dioxide gas (unpleasant smell) when in solution.
Sodium metabisulphite has a dramatic effect on dough quality. It causes the gluten to become more extensible and less elastic and therefore reduces the shrinkage of dough pieces as they are baked. It is a very useful processing aid to compensate for variations in gluten quality that are encountered from month to month and year to year. It has only limited usefulness with flours that have basically too strong a gluten to make semisweet doughs.
As only very small amounts are used in a dough, it is normal to use the salt as a solution in water. A 10% solution is commonly prepared. Solutions should be used within a day.
The normal level at which SMS is used in doughs is up to 50g per 100 kg of flour.