Drying of food
Practical Action
Food
Moisture
content %
Water
activity
Degree of protection required
Fresh meat
70
0.985
Package to prevent moisture
Bread
40
0.96
loss
Marmalade 35 0.86
Rice
Wheat flour
Raisins
Macaroni
Marzipan
Oats
Nuts
15-17
14.5
27
10
15-17
10
18
0.80
0.72
0.60
0.45
0.75
0.65
0.65
Minimum protection or no
packaging required
Toffee
Boiled sweets
Biscuits
Milk
Potato crisps
Spices
Dried vegetables
Breakfast cereal
8
3.0
5.0
3.5
1.5
5-8
5
5
0.60
0.30
0.20
0.11
0.08
0.50
0.20
0.20
Packaged to prevent
moisture dried uptake
Table 1: Food type characteristics and packaging requirements
Changes to food during drying
Case hardening is one of the
changes that can take place while
food is drying. Other changes
include the loss of colour, flavour
and nutrient content. Experiments
with air temperature and speed can
be used to select the best conditions
for each food. The colour of many
fruits can be preserved by dipping in
a solution of 0.2-0.5% sodium
metabisulphite or by exposing to
sulphur dioxide in a sulphuring
cabinet (Figure 5).
Figure 5: Sulphuring cabinet
Vitamin losses are often greater
during peeling/slicing etc than during drying. Loss of fat soluble vitamins can be reduced by
shade drying and loss of water soluble vitamins by careful slicing using sharp knives. Blanching
of vegetables is necessary before drying and water soluble vitamins are also lost in this stage. It
should be noted that drying does not destroy micro-organisms, it only inhibits their growth.
Therefore, heavily contaminated fresh foods will become heavily contaminated dried and re-
hydrated foods. It is essential to make sure that foods are not contaminated with micro-
organisms before drying. Blanching is one method of reducing the levels of initial
contamination. Thorough washing of fresh foods should be done routinely before drying.
Summary of small-scale drying equipment available
7