Metabisulphite is used to control the pH, the SO2 forms sulphurous acid in solution, in combination with Sulphite it also has a buffering effect.

The Sulphite/Metabisulphite buffer range is between pH 8 and pH 6.5, becoming more acidic as the proportion of Metabisulphite is increased, this is compared to the Borax/Boric Acid buffer range of pH 9.2 to pH 8.

As a preservative Metabisulphite is many times more effective than Sulphite., which is why it's used in the food trade and wine making.

Ian