Introduction

Food products such as pickles, chillies, amlas, fish, fruits, and spices tend to have a longer life after they have been dried. Drying is a process that removes moisture content from food products and prevents the growth of microorganisms such as bacteria, yeast, and other moisture-related reactions in the product. Other benefits of drying include a reduction in the weight and volume of the food products, which will minimize time and cost associated with its (i) packaging, (ii) storage, and (iii) transportation. Drying also makes food products more sustainable under ambient conditions.

Methods of Drying:

Natural drying process is usually done using direct sunlight, ie., the sun’s rays hit the product and remove its moisture through natural air circulation. This method is a slow process and has certain disadvantages such as dust contamination, insect infestation, and spoilage due to unexpected rain. These problems can be eliminated by using a mechanised method of drying.

 

 

Mechanised drying:

In mechanised drying methods, the most commonly used dryers are either (i) biomass-fired or (ii) electrically operated.  

(i) A biomass-fired dryer consists of a burner and a drying chamber. Food products to be dried are placed in the drying chamber and coconut shells or briquettes which are used as fuel are placed in the burner. Once the fuel is ignited, the drying chamber is maintained at a temperature of 80-degree Celsius. The burner manages to retain heat for 6 hours which ensures effective drying of the food products.

(ii) Electrically operated dryers contain the following parts: (a) an evaporator, (b) a condenser, (c) a blower, and (d) a drying chamber.

Air enters the dryer through the evaporator and gets heated when it flows over the condenser.  It further gets supplied to the drying chamber as regulated by the blower. Once the food products in the drying chamber have dried completely, humid air leaves the dryer, through the holes provided on top of the chamber.

 

Solar Drying

Unlike mechanised methods of drying, solar drying is environment-friendly and is done using solar dryers. Solar dryers help provide more heat than the atmospheric temperature. In a solar dryer, air enters the drying chamber through the process of natural convection or through an external source like fan, pump, suction device, etc. Air gets heated as it passes through the chamber and then partially cools as it absorbs moisture from the food product placed in the chamber. Then, the humid air is removed by an exhaust fan or chimney.

Advantages of solar drying

  • The rate of drying increases with higher temperature and movement of air in the chamber.

  • Food is enclosed in the dryer and therefore protected from dust, insects, birds, and animals.

  • Higher temperature prevents insect infestation and the faster drying rate reduces the risk of spoilage by microorganisms.

  • The dryers are waterproof, therefore, the food does not need to be moved during the rainy season.

  • A dryer can be constructed from locally available materials at a relatively low cost.

  • Solar dryers last longer. A typical dryer can last 15-20 years and will need minimum maintenance.

Limitations with solar drying

  • Can be only used during day time when an adequate amount of solar energy is present.

  • Lack of skilled personnel for operation and maintenance.

  • Takes longer as compared to the modern type of electric dryers.

  • A backup heating system is necessary for products requiring continuous drying.

Different types of solar dryers

  1. Integrated Solar Dryers: In this type, the solar energy collection and drying takes place in a single unit. Some of the examples for this category includes step type dryers, cabinet dryers, rack dryers, tunnel dryers, greenhouse dryers, and multi-rack dryers.

  2. Distributed Solar Dryers: In this type, the solar energy collection and drying takes place in two different units namely, a flat plate air-heater and a drying chamber. The flat plate heater can be placed on the roof of a building or on the ground or the place where the Sun's intensity is high. Air is heated in the flat plate heater and with the help of a blower, the heated air gets circulated in the drying chamber.   

  3. Mixed Mode dryer: In this type, the solar energy collection takes place at both the flat plate air heater as well as a drying chamber and the drying takes place only at the drying chamber. The outer part of the dryer will also get solar energy; this helps to remove the moisture quickly.

Among the many types of solar dryers, examples of the integrated type of solar dryers such as step type and tunnel dryer are explained below. While step type solar dryers are commonly used for domestic purposes, solar tunnel dryers are widely used in industrial sectors.

Step type solar dryer:

In step type solar dryer, the dryer area is covered with two layers of 3 mm thick plain glass with an air gap of 2.5 cm. The absorber is made up of galvanised iron sheet with black paint on the top. Dryer racks are placed over it. For the entry of fresh air, holes are provided below the first step of the rack.

Solar radiation hits the plain glass surface and heats the fresh air passing inside. Thus the heated air takes up the moisture content from the product and escapes through the chimney placed at the other end of the dryer.

The cost of this unit is Rs. 15,000/- approx., and it can be used for drying of fruits, vegetables and other agricultural commodities. (Link)

Solar tunnel dryer:

As its name suggests, the tunnel dryer is a semi-cylindrical tunnel-shaped dryer. The frame of the structure is made of galvanized iron and covered with ultraviolet (UV) stabilized polythene sheet of 200-micron size, which maintains heat inside the dryer. While the inlets at the back end of the dryer allow fresh air to enter the dryer, the exhaust fan, fitted at its front side removes the moist air from the product. This type of dryer is used for drying sago, coconut, chillies, amla, onions, and other agricultural products.

If you consider sago, the drying time in the solar tunnel dryer is 5 hours, whereas, in open sunlight, the drying time is 11 hours. Thus, around 55 % of drying time is saved in a solar tunnel dryer. The cost of this dryer is Rs. 1,20,000 approx., for standard size of 3.75 m * 18.0 m.(Link)