Case Hardening and Other Effects of Drying
Case hardening is 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. It is caused by drying too quickly during the initial (constant rate) period and can be prevented by using cooler drying air.
Other changes to foods include color loss, flavor loss and hardening. Experiments with air temperature and speed can be used to select the best conditions for each food. The color 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 4.
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 and only inhibits their growth.
So heavily contaminated fresh foods will become heavily contaminated dried and rehydrated foods. 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
Solar drying is popular with agencies and research stations. However, there are no small-scale solar driers that are yet operating economically. There are a number of reasons for this:
- The amount of food lost in traditional drying is often over estimated (people report the worst case and the average amount).
- The loss of quality is not necessarily reflected in lower prices. People are willing to pay nearly the same amount for discolored or damaged foods and there is therefore no incentive for producers to risk higher amounts of money in a drier when there is not a great return.
- Different quality standards are applied by agencies and rural people. It is not necessary to achieve export quality for sale in rural areas.
- Driers are only needed in villages if the weather is unsuitable for traditional methods. If these conditions are not very common, the drier will not be needed. Even short periods of sunshine are enough to prevent serious crop losses. Some producers wait for sunshine rather than risk the expense of using a drier. The food is then either spoiled or the drier is not big enough to handle the amounts involved.
- Other methods are available to preserve the food if it rains during harvest, for example the harvest can be delayed, food can be stacked in a way which prevents it from getting wet, or small amounts can be dried over a kitchen fire, or mixed with dry crop.
- Some benefits of proper drying (for example absence of mould, and better milling characteristics of grains) cannot be seen and there is therefore no increase in value of the food.
Other disadvantages of both solar and mechanical driers include greater space and labor requirements than traditional methods (for example loading, unloading of trays). These costs are given lower value by agencies than by villagers.
Solar driers operate by raising the temperature of the air to between 10-30ºC above room temperature. This makes the air move through the drier and also reduces its humidity.
There are advantages to solar drying as follows:
The higher temperature, movement of the air and lower humidity, increases the rate of drying. Food is enclosed in the drier and therefore protected from dust, insects, birds and animals. The higher temperature deters insects and the faster drying rate reduces the risk of spoilage by micro-organisms. The higher drying rate also gives a higher throughput of food and hence a smaller drying area. The driers are water proof and the food does not therefore need to be moved when it rains. Driers can be constructed from locally available materials and are relatively low cost.
Designs vary from very simple direct driers (for example a box covered with plastic to trap the sun’s heat) to more complex indirect designs which have separate collectors and drying chambers. The most common type of collector is a bare galvanized iron plate which is painted matt black. These give a temperature increase of 10ºC and increases the air speed to about 5m/s. Other types include burnt rice husks or charcoal.
The collectors are covered with a transparent material to ensure uniform airflow. Glass covers are best but they break easily, are heavy and expensive. Plastic often has poor stability to sunlight and weather, but is about 10% of the weight of glass and does not break. The best types of plastic are polyester and polycarbonate when available. Polythene is cheaper and more widely available but is not as strong and is less resistant to damage by light and weather.
source: practicalaction.org, photo from omick.net