Drying of Foods Part 2 – Answers


  1. 0.068kg per kilogram of dry air (find the intersection of the 60°C and 50% RH lines, and then follow the chart horizontally right to read off the absolute humidity)
  2. 246.5°C (from the intersection of the 60°C and 50% RH lines, move left parallel to the wet-bulb lines to read off the wet-bulb temperature)
  3. 20% (find the intersection of the 45°C and 75°C lines and follow the sloping RH line upwards to read off the % RH)
  4. 36°C (find the intersection of the 55°C and 30% RH lines and follow the wet-bulb line left until the RH reaches 100%)
  5. 50-10% (find the intersection of the 39°C wet-bulb and the 50°C dry-bulb temperatures, and follow the horizontal line to the intersection with the 86°C dry-bulb line; read the sloping RH line at each intersection (this represents the changes that take place when air is heated prior to being blown over food))
  6. 10-70% (find the intersection of the 35°C wet-bulb and 70°C dry-bulb temperature, and follow the wet-bulb line left until the intersection with the 40°C dry-bulb line; read sloping RH line at each intersection (this represents the changes taking place as the air is used to dry food; the air is cooled and becomes more humid as it picks up moisture from the food). If a new type of drier is to be used, or if a different type of food is to be dried, it is necessary to do some experiments to find the rate of drying.

The information can then be used to find the time that the food should spend in the drier before the moisture content is low enough to prevent spoilage by micro-organisms. The rate of drying also has an important effect on the quality of the dried foods and (in artificial driers) the fuel consumption. To find the rate of drying you will need a clock/watch and a set of scales. Food is weighed, placed in the drier and left for 5 – 10 minutes. It is then removed, reweighed and replaced.

This is continued until the weight of the food no longer changes. The interval between weighings can be increased when the changes in weight start to become less. You should also make a note of the wet and dry bulb temperatures of the air inside the drier and the air outside. The results are plotted on a graph, Figure 2 and show two distinct phases of drying – the “constant” and “falling” rate periods. In the constant rate the surface of the food remains wet and it can therefore be spoiled by moulds and bacteria. In the falling rate the surface is dry and the risk of spoilage is much smaller. The food should therefore be dried to a weight that corresponds to the end of the constant rate period as quickly as possible (however see ‘case hardening’ below).

The information from an experiment can be more usefully shown as in Figure 3, by calculating drying rate for each 10 minute period as follows:

Drying rate = initial weight – final weight / ime interval (eg 10 minutes)

The moisture content of both the fresh food and the final dried food can be found by weighing the food, heating at 100°C in an oven for 24 hours and reweighing. The moisture content is found as follows:

Moisture content (%) = initial weight – final weight x 100 / initial weight

Other values of moisture content during the drying period can be found by relating these two results to the weights of food recorded during the drying experiment and applying similar factors to intermediate weights. Figure 3 gives two important pieces of information:

  1. The actual drying rate during the constant rate period which shows how efficient the drier is.
  2. The final moisture content of the dried food which shows whether it will be stable during storage.

Typically, a drying rate of 0.25kg/hr would be expected for solar driers depending on the design and climate, and 10-15kg/hr for artificial driers. 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.

If the drying rate is lower than this, the air temperature or speed is too low and/or the RH is too high. This can be checked by the temperature measurements made during the experiment and by using the psychrometric chart. Normally the air in the drier should be 10-15ºC above room temperature in solar driers and 60-70ºC in artificial driers. The RH of air entering the drier will vary according to local conditions, but should ideally be below about 60% RH.

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 and 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 (click to enlarge).

source: practicalaction.org, photo from omick.net


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