Sulphitation:

Sulphitation is the practice of adding sulphur dioxide ($SO_2$) or derivatives to process streams in a sugar factory.

This is done for one of three reasons :

1. pH control - the $SO_2$ in aqueous solution forms sulphurous acid $H_2SO_3(aq)$ which reduces the pH of the process stream. An example of this would the control of diffusion water pH in a beet factory, where keeping the pH below 5.5 reduces the extraction of pectin from the beet cell walls which helps pulp pressing.

2. Biocide - used in sufficient quantities the SO2 inhibits the life cycle of bacteria, reducing the quantity of sugar lost by bacterial degradation to lactic acid. This is similar to the use of sodium metabisulphite for sterilising home brewing equipment. The efficiency of SO2 as a biocide is sometimes challenged in the literature.

3. Colour blocking - $SO_2$ reacts with the carboxyl groups of invert sugars (glucose and fructose) to inhibit their participation in the colour forming Maillard reaction with amino compounds. By adding $SO_2$ to juice before evaporation the increase in colour through the evaporators is kept to a minimum, protecting the juice from excessive colour formation at high temperature in the evaporators.

$SO_2$ can be purchased as a liquefied gas. The containers are pressure vessels usually capable of holding 10 bar g pressure, which covers the vapour pressure of $SO_2$ up to about $50^0 C$.

All liquefied gas systems need pressure relief facilities, however the consequences of a discharge of toxic $SO_2$ to the atmosphere can be serious in a populated area. For this reason special transport regulations apply to the movement of $SO_2$ containers in most countries, and fixed storage installations are covered by Major Hazard legislation usually requiring evacuation plans etc. Liquid $SO_2$ can also be quite expensive, perhaps 4 times the price of the equivalent amount of solid sulphur.

Solid sulphur is the most common material used for sulphitation. Solid sulphur comes from a variety of sources including sulphur mines and oil or gas desulphurisation plants. Sulphur is burned to form $SO_2$.

The basic reaction is $S + O_2 = SO_2$ where the oxygen is supplied by atmospheric air.

The undesirable side reaction $SO_2 + \frac{1}{2} O_2 = SO_3$ is minimised by the design of the burning equipment, although it would not be a problem where $SO_2$ is used purely for acidification. For biocide action and colour blocking it is $SO_2$ that is required, $SO_3$ will form sulphuric acid on dissolving in water which will leave the sugar process in molasses, increasing the sugar loss.