By Tawanda Mandoga
Dried foods can be divided into three broad groups based on the value-added through the drying process.
Group 1- Low value crops are cereals, legumes, and root crops, very little value is added per kilogram processed.
Group 2 – Medium value crops were a little more value per unit mass is added to food by drying; vegetables, fruits, and fish.
Group 3- High value crops such as spices, herbs, medicinal plants, and nuts.
In addition to the prevention of microorganisms, drying offers ease in handling, packaging, shipping, and consumption. Freshly harvested produce can contain up to 95% water and is thus adequately moist to support both enzyme activity and growth of micro-organisms. Drying aims to reduce water activity, thus preserving foods by avoiding microbiological growth and deteriorative chemical reactions. Water activity is the amount of unbound water in a food product. Water that is not bound to the food itself can be used by unwanted microorganisms which could lead to one of the contributing factors for food spoilage. The optimum moisture content for prolonged shelf stability is about 5-15%, depending on the commodity. The effects of heat on microorganisms and the activity of enzymes is also important in the drying of foods. When drying food products, it is important to maximize microorganisms and enzyme inactivation for preventing spoilage and enhancing safety and reduce the components responsible for the deterioration of the dried foods. There exist different methods of drying produce.
Sun drying is a process that is practiced throughout the world where food is exposed to the wind and sun-rays. In this drying process, the direct heat energy that comes from the sun is used to dry food materials. During sun drying the product is spread in a thin layer on the ground and heat is transferred to the food in two ways: one is by convection process and another by the direct solar radiation process. This increases the inner temperature of the foods and results in evaporation of the water from the food. The surface water generally is evaporated by the natural airflow.
Artificial heat drying is characterised by hot air being supplied by an energy source other than the sun. This is a more reliable method of drying since there is a constant supply of dry heat. The cost of this system depends on the nature of fuel or energy used for heating the air. Hot-air drying is for drying different fruits and vegetables, for example, banana, mango, and pineapple, tea leaves, and herbs such as basil, and bay leaves. Drying should be as rapid as possible in order to maintain quality and minimize nutrient loss. The rate of drying depends on:
• the exposure of a large surface area of the produce, which speeds drying; most produce should be cut into strips not more than 5 mm thick.
• the temperature should be high enough (50-70 degrees Celsius) to give rapid moisture removal. Please note that temperatures over 70 degrees cause discolouration of the product.
• the warm air current must be dry; if it is humid it cannot absorb moisture from the drying product.
Special treatments may be given to certain types of produce before drying, for example;
- Fruit and vegetables may be treated with sulphur dioxide before drying to prevent enzymatic browning. It also slows the breakdown of vitamin C and kills some micro-organisms.
- Most vegetables except onions and garlic are blanched by dipping them in hot water for a few minutes before drying. This stops the action of enzymes which may not be killed by the sun-drying process.
- Green vegetables retain their colour better during drying if about 0.25 percent of bicarbonate of soda is added to the blanching water, but this will speed up the loss of vitamin C.
- Dried cassava forms an important part of the staple diet in parts of Africa and Latin America. The dried product may be in the form of chips, granules or flour. In some areas, the grated cassava root is fermented for a short time before being dried by artificial heat or sunlight.
Drying has been proven to mitigate post-harvest losses, and continues to play an integral part in food preservation in various fresh food chain systems.
About the author.
Quality Manager at Albina Snacks in Lund, Sweden
Msc Food Technology and Nutrition
Value addition ; A Zimbabwean farmer, Rob Fletcher who makes a signature chilli sauce on his farm https://bambazonke.co.zw/dr-troubles-chilli-sauce/
Topic introduction to value addition ; https://bambazonke.co.zw/value-addition-to-horticulture-products/
Mechanised Post-Harvest Grain Drying; https://bambazonke.co.zw/mechanised-post-harvest-grain-drying/