How to use the DRYIT semi-continuous tray dryer
Practical Action
remains as strong as possible during drying and during storage. This is done by adding 1
percent of sodium bicarbonate to the water ie 10 grams per litre of water. However use of
sodium bicarbonate will accelerate the loss of vitamin C by about a factor of five.
Blanching by immersion is very straightforward but can cause some nutrients and colours to
leach out from the raw material. This can be overcome by blanching in steam for
approximately 10-15 minutes. The choice of blanching methods depends on the final quality
of the product that is required, cost and availability of equipment.
After blanching the raw material can either go on to a sulphur dioxide step or directly to the
Dryer.
Use of sulphur dioxide as a preservative
Sulphur Dioxide (SO2) is used to prevent growth of yeasts and moulds and to prevent fruits
and vegetables from becoming brown during storage. Nearly all fruits are susceptible to
browning and hence this step is recommended. As a general guide foods that are preserved by
sulphur dioxide are not blanched.
There are two basic ways of adding SO2: one is by immersion in a sulphite solution
(sulphiting); the other is to burn sulphur and allow the fumes to penetrate the raw material
(sulphuring).
Sulphiting is more commonly used for vegetables whereas sulphuring is more commonly used
for fruits.
Sulphuring has the advantage of not getting the raw material any wetter than what it is
already and also gives a more even distribution of the sulphur dioxide preservative.
Furthermore it is often the case that rock sulphur (burnt to give fumes of sulphur dioxide) is
more widely available than sodium metabisulphite.
Too much sulphur dioxide will give the food a very strong, offensive flavour and if this can be
detected by you or the consumer then the legal limit has probably been exceeded.
Method of sulphuring
A sulphuring cabinet needs to be built. See diagram. Approximately 5g of rock sulphur (1
level teaspoon) should be used for each kg of raw material to be sulphured. The correct
quantity of sulphur should be placed on a shallow ceramic container and then ignited. The
sulphuring procedure should be allowed to proceed for about 2 - 3 hours.
The fumes from burning sulphur are dangerous. They should not be inhaled. After sulphuring
the product should be immediately dried.
Method of sulphiting
Sodium metabisulphite is a powder and a highly concentrated source of sulphur dioxide,
which is the actual preserving substance. Therefore only very small quantities of this powder
are needed. Sulphur dioxide is toxic so care must be taken when using sodium
metabisulphite.
Add no more than a ½-teaspoon of sodium metabisulphite to each two litres of water. This
gives a concentration of 0.1% sulphur dioxide.
The prepared raw material is then dipped in this solution for 1 minute only.
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