Drying is used to remove water from foods for two reasons: to prevent (or inhibit) micro-organisms and hence preserve the food and to reduce the weight and bulk of food for cheaper transport and storage.
When carried out correctly, the nutritional quality, color, flavor and texture of rehydrated foods are slightly less than fresh food but, for most people, this has only minor nutritional significance as dried foods form one component in the diet.
However, if drying is carried out incorrectly there is a greater loss of nutritional and eating qualities and more seriously, a risk of microbial spoilage and possibly even food poisoning.
This technical brief therefore describes some of the requirements for proper drying and summarizes information on the various drying equipment available.
Drying can be carried out using hot air or, less commonly, hot metal pans. We shall concentrate on drying using hot air.
For effective drying, air should be hot, dry and moving. These factors are inter-related and it is important that each factor is correct (for example, cold moving air or hot, wet moving air is unsatisfactory). The dryness of air is termed ‘humidity’ – the lower the humidity, the drier the air. There are two ways of expressing humidity (or RH) the most useful is a ratio of the water vapor in air to air which is fully saturated with water. So 0% RH is completely dry air and 100% RH is air that is fully saturated with water vapor.
Low RH (or dry) air must be blown over foods so that it has the capacity to pick up water vapor from the food and remove it. If high RH (or wet) air is used it quickly becomes saturated and can not pick up further water vapor from the food.
The temperature of the air affects the humidity (higher temperatures reduce the humidity and allow the air to carry more water vapor). The relationship between temperature and RH is conveniently shown on a psychrometric chart, Figure 1.
Note that there are two types of air temperature:
The temperature of the air, measured by a thermometer bulb, is termed the dry-bulb temperature. If the thermometer bulb is surrounded by a wet cloth, heat is removed by evaporation of the water from the cloth and the temperature falls (to the ‘wet bulb’ temperature). The difference between the two temperatures is used to find the relative humidity of air of the psychrometric chart.
The dew point is the temperature at which air becomes saturated with moisture (100% RH) and any further cooling from this point results in condensation of the water from the air. This is seen at night when air cools and water vapor forms as dew on the ground. Adiabatic cooling lines are the parallel straight lines sloping across the chart, which show how absolute humidity decreases as the air temperature increases.
The psychrometric chart is useful for finding changes to air during drying and hence the efficiency of a drier. The following examples show how it is used.
Figure 1: The psychrometric chart (click to enlarge)
A psychrometric chart presents physical and thermal properties of moist air in a graphical form. It can be very helpful in troubleshooting greenhouse or livestock building environmental problems and in determining solutions. Understanding psychrometric charts helps visualization of environmental control concepts such as why heated air can hold more moisture, and conversely, how allowing moist air to cool will result in condensation. The objective of this fact sheet is to explain characteristics of moist air and how they are used in a psychrometric chart.
Using Figure 1, find:
- the absolute humidity of air which has 50% RH and a dry-bulb temperature of 60°C
- the wet-bulb temperature under these conditions
- the RH of air having a wet-bulb temperature of 45°C and a dry-bulb temperature of 75°C
- the dew point of air cooled adiabatically from a dry-bulb temperature of 55°C and 30% RH
- the change in RH of air with a wet-bulb temperature of 39°C, heated from a dry-bulb temperature of 50°C to a dry-bulb temperature of 86°C
- the change in RH of air with a wet-bulb temperature of 35°C, cooled adiabatically from a dry-bulb temperature of 70°C to 40°C.
source: practicalaction.org, photo from omick.net