Use of Sulphur dioxide (SO2)
SO2 has been widely used in fruit and vegetable products to control enzymatic colour changes such
as the darkening of a freshly cut apple or potato. It also acts as a preservative, controlling the growth
of moulds and yeasts. SO2 is produced by either burning a small piece of sulphur or by dissolving
sodium metabisulphite in water. The second method is more controllable.
The levels of SO2 used are measured in parts per million or ppm. Concentrations of 400 to
1000ppm are used for dips to control colour changes and retard the growth of moulds and yeasts. A
400ppm bath, for example, is made by dissolving 6g of sodium metabisulphite in 10 litres of water.
NB SO2 gas is harmful if breathed in, it should only be used in a well ventilated room
In recent years, the use of SO2 has been increasingly controlled and it has been banned in many
foods in the USA. Similar changes to food laws are is likely in Europe. In such situations browning
can be controlled by the addition of citric acid but this is far less efficient than sulphur dioxide.
The most convenient production plan for very small producers is to use fruits that are in season at any
given time. This does, however, have disadvantages that include:
one particular flavour of fruit leather may be much more popular than others
it will only be possible to produce small quantities of product in a short season
It is, however, possible to produce all year by preserving prepared fruit (or fruit puree) in sealed
drums with added SO2 at a level of 600ppm. Fruit may be stored for many months in this way.
Intermediate preservation also allows fruits to be purchased at the peak of the harvest when prices
are at their lowest. While most of the SO2 absorbed during intermediate preservation will be lost
during drying it is recommended that purees made from preserved fruits should be briefly boiled prior
to drying to reduce the level of residual SO2
Preparation of puree
At the simplest level fruit may be pulped to a puree by hand using a food mill, or Mouli Legume as
shown in figure 1, in which the food is pushed through a mesh by a rotating paddle. If electric power
is available a food liquidiser, followed by sieving will greatly increase production outputs. At larger
scale, powered high-speed blender wands are recommended.