How to use the DRYIT semi-continuous tray dryer
Practical Action
Having ensured that the maximum amount of material is on each tray then the temperature
should be adjusted up to the point where the amount of humidity in the exiting air is starting
to decrease.
Other factors which control efficiency are the efficient use of heat used for drying and the
efficient use of labour.
At the end of the day the key questions for the tray dryer user are how many kilos have been
produced today for how many gallons of diesel or kilos of gas and what the quality is like
Equipment
Based on the controlling factors some equipment would be necessary if one were to start
researching the optimum drying conditions for different foodstuffs. Practical Action and many
other organisations have already done a lot of work in this area and are in a position to provide
the drying temperatures, drying times and potential drying capacities for many herbs, fruits
and other foodstuffs. This data will give a good indication of drying conditions. However,
some adjustments may be necessary due to variations in humidity in different places and at
different times of the year.
It would be useful to have (or access to) some scientific equipment to make a number of
measurements especially for the early stages of investigation. Such equipment would not
need to be used all the time once the operating conditions have been established.
The most useful scientific instruments are:
Humidity meter
Thermometer
Anemometer
Drying as science
In this section the four key factors are discussed in relation to the scientific principles of
drying.
Why does drying help to conserve foods?
All forms of life need water in one form or another. Micro-organisms are no different and in
food processing a crucially important objective is to ensure that micro-organisms cannot spoil
food. Therefore reducing the availability of water will reduce the opportunity for the growth of
micro-organisms.
Drying is one way of reducing the availability of water. The key phrase is the availability of
water not dryness. There are many foods, which are not exactly dry but have a low availability
of water for micro-organisms. For example dried figs are not dry in the conventional sense.
They contain about 25% water yet they last a very long time without spoiling. On the other
hand any cereal with that much water would soon spoil.
The difference is in the way in which water in contained within the food and the composition
of the food. In dried dates the sugar helps to contain the water yet in a way that makes it
unavailable for micro-organisms. In cereals only very small quantities of water can be
contained in this way. Hence when this level is exceeded the remaining water can support the
growth of micro-organisms. The moisture level in a food at or above which micro-organisms
can grow is sometimes called the critical moisture content. Therefore in dates the critical
moisture content is about 25%. In cereals the critical moisture content is about 12%.
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