Drying of food
Practical Action
Case hardening
Case hardening is a process that happens to some foods during drying. It is characterised by
the formation of a hard skin on the surface of fruits, fish and some other foods which slows
the rate of drying and may allow mould growth. During drying, moisture from within the food
moves from the centre to the outside of the food, where it is evaporated. If a tough crust
develops on the outside, it will be impossible for the moisture in the centre of the food to
escape. Case hardening is caused by drying too quickly during the initial (constant rate)
period and can be prevented by using cooler drying air at the start of the drying process.
The importance of particle size
The main factor that controls drying rate is the rate that moisture can move from the interior of a
piece of food to the surface. Therefore, the shorter the distance that moisture has to travel, the
faster the drying rate. For this reason, wherever possible, products should be cut into small
pieces prior to drying. Reducing the size also increase the surface area of the food in relation to
the volume of the pieces, which increases the rate at which water can be evaporated from the
food.
Stability and storage of dried foods
To ensure safe storage the final moisture content of the food should be less than 20% for fruits
and meat, less than 10% for vegetables and 10-15% for grains.
The stability of a dried food during storage depends on its moisture content and the ease with
which the food can pick up moisture from the air. Clearly the risk of moisture pick up is greater
in regions of high humidity. However, different foods pick up moisture to different extents
(compare for example the effect of high humidity on salt or sugar with the effect on pepper
powder -salt and sugar pick up moisture, pepper doesn't).
For foods that readily pick up moisture it is necessary to package them in a moisture proof
material.
A low moisture content is only an indication of food stability and not a guarantee. It is the
availability of moisture for microbial growth that is more important and the term water activity
(aw) is used to describe this. Water activity varies from 0-1.00. The lower the value the more
difficult it is for micro-organisms to grow on a food.
Examples of moisture contents and aw values for selected foods and their packaging
requirements are shown in Table 1.
6