How to use the DRYIT semi-continuous tray dryer
Practical Action
Drying needs to be controlled because food is valuable and food is susceptible to spoilage.
Slow drying may allow the growth of undesirable micro-organisms, which cause food
poisoning or at the very least render the food unsaleable. If the food cannot be sold then
money and profit has been lost. However drying too fast can also cause a loss of quality
through both physical and chemical changes such as case hardening (see below) and loss of
colour, flavour and nutritional value. Hence drying is a balance between obtaining the most
highest quality product at the lowest cost to the business.
The four factors
From the four factors it can be seen that three of them relate to the air and one to the food to
be dried. We are now going to examine the factor that relates to the food to be dried:
1. The surface area
Drying involves the removal of water from a raw material. The most common way of drying
foodstuffs is to allow the air that surrounds the food to take up this water in the form of water
vapour. If there is water vapour then there has been evaporation. In drying, the evaporation of
water into water vapour occurs at the surface of the material, not inside the material.
Therefore the more surface, the more evaporation.
If there is more evaporation then there is more loss of water from the material. In other words
the rate of drying is controlled by the surface area.
The composition of foodstuffs affects the movement of water inside the food to the surface.
For example sugary and starchy foods can be expected to dry more slowly than open textured
or 'low density' foods such as onions and herbs.
In practical terms how can the surface area be increased?
Food preparation techniques such as slicing, dicing and chopping all increase the surface
area. Example: The surface area of a 4cmx4cmx4cm cube is (4x4)x6 = 96cm2.
If that cube is chopped into 2 equal cubes the surface area becomes:
((4 x 4) x 4) + ((2 x 4) x 8) = 128 cm2
Chopping those two cubes each in half gives a surface area of:
(4 x 2) x 16 + (2 x 2) x 8 = 192 cm2
Sometimes of course it is not desirable to increase the surface area of a foodstuff prior to
drying because of the requirements of the market. An example here is spices such as cloves
and cardamoms. The market requires, generally whole cloves and whole cardamoms. Hence
the other controllable factors have to be considered to produce a good quality product at a
competitive price.
2. The temperature of the air
Drying needs dry (low humidity) air and temperature is, for the purposes of drying, the most
common way of reducing the humidity of the air. Dry air has more capacity to take up water
vapour from the surface of the food to be dried. Hence drying goes faster with dry air.
The temperatures used in drying (40 - 60°C) cannot really be considered to destroy micro-
organisms. For this reason good standards of cleaning and washing foods must be used prior
12