How to use the DRYIT semi-continuous tray dryer
Practical Action
Case hardening generally affects sugary and starchy foods such as bananas, potatoes and
whole fruits. Fruit in slices, herbs and most vegetables are not susceptible to case hardening.
So how does case hardening happen?
During drying, water is evaporating at the surface of the food. Hence the surface of the food is
effectively dryer than the inside of the food. This is an imbalance which nature will attempt to
correct by causing water inside the food to move towards the surface. In some foods however,
the water which moves to the surface carries with it a very large amount of other components
such as sugars, starches, and minerals. When the water evaporates these components will be
left on the surface of the food. They gradually build up and form a barrier through which
water cannot pass. When no more water is reaching the surface, the surface dries out,
becomes hard and further drying stops.
In those foodstuffs that are susceptible to case hardening the greater the rate of movement of
water to the surface the greater the chance of case hardening. It can be seen then that case
hardening is dependant on the type of food to be dried and rate of drying. The simplest ways
to make drying proceed more slowly are to use a lower drying temperature or reduce the
surface area. But the surface area should be large for those foodstuffs which easily become
case hardened because a large surface will mean that the sugars, starches etc which move to
the surface will be distributed more thinly across a greater area. Hence the most effective way
to reduce the rate of drying is to lower the drying temperature. For foodstuffs that are
susceptible to case hardening temperatures the humidity of the air used for drying should not
be too low. The temperature will control this.
Summary
The objective is the most dried product of the highest quality in the shortest drying time at
the highest quality.
The four controllable factors in drying are:
-temperature
-humidity
- quantity of air
- surface area
The higher the temperature, the lower the humidity and hence the greater the capacity of the
air to pick up moisture from the material to be dried. Air is made even more useful by
increasing the volume that passes through the drying chamber. Increasing the surface area of
the material will also increase the rate of drying.
Care is needed to avoid case hardening in some foodstuffs. Normally the airspeed can be
decreased although not increased.
With a careful balance of these four factors the quality of the product will be high ie no case
hardening, no potential problems of spoilage, no serious losses of colour and flavour and
production costs will be lower.
Maximising efficiency
The most efficient drying system is one in which the air which is exiting from the dryer is as
'useless' as possible. In other words it has little or no capacity to absorb more water vapour.
The first step though in getting close to this situation is to load the dryer with as much
material as possible. Here care needs to be taken to ensure that the air can pass over all the
surfaces of the material. With herbaceous materials this is not too difficult to achieve.
However with sliced materials: apples, potatoes etc only one layer should be on each tray.
Here a large number of very shallow trays would be required.
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