**4. Drying technologies**

Scientists from all over the world continuously search for new and effective means and use of renewable sources of energy as a result of the continuous increase in the price of fossil fuels and increased levels of greenhouse gas emissions. The world's energy intake is doubled every 20 years and this increase in energy consumption, has resulted in fossil fuels causing many environmental problems and pollution [15]. Drying is a processing technique used for food product preservation and reducing food spoilage. About 3.62% of the world's energy is used for the drying of agricultural products [16].

Presently, the requirement for new drying technology that promotes the higher quality product and efficient drying in shorter periods is the current need. And as a result, hybrid drying systems have emerged as an excellent technique for their versatile drying outcomes, with lower energy requirements and minimum environmental impact. Lately, various hybrid solar dryers which are more efficient in conjunction with other sources for heating the air, hence reducing drying cost and energy consumption have been developed [17, 18].

Grain legumes are usually dried after harvesting before storage in storage facilities [17]. Drying grain legumes to a recommended safe moisture level is fundamental in achieving safe storage of grain legumes. However, too rapid drying of nuts can lead to hardening of the grain core with poor interior while very slow drying may result in microbial growth which will lead to quality deterioration. Recirculation of the solar drying air is thus employed to make efficient use of the heated air by giving a drying rate that provides acceptable product quality.

Drying of pulses is essential because they contain high moisture content of about 18–25% at the time of harvest and, for safe storage, the optimum moisture content need to be in the range of 9–12% to avoid mycotoxin production. It is essential that the grain is dried to a safe moisture level as quickly as possible to avoid deterioration regardless of the drying system employed. There are several techniques of non-natural open-sun drying of grains with hot air. Some of these forms of drying include spouted-bed drying, fixed bed drying, moving bed drying, fluidized-bed drying, and thin-layer drying [19]. Apart from some of these specialized dryers used for grain drying, all-purpose grain drying systems can as well be used in the drying of grain legumes. Generally, as

#### *Postharvest Technology - Recent Advances, New Perspectives and Applications*

documented by [20], dryers or drying systems are categorized depending on the following:


Regardless of the type of dryer used in drying grains, the concurrent heat energy transfer and moisture loss principle/process is the same for the drying of grain legumes and equally for other grains [19]. The process of drying grains involves the loss of free moisture which involves the drying of the grain until its equilibrium moisture content is attained. The equilibrium moisture content of the grain implies the final moisture content attained by the grain at a pre-determined relative humidity and temperature. The cardinal factors that influence the drying rate of grain legumes are temperature, grain moisture content, relative humidity, and air velocity [19].

The use of solar dryers is also another medium for drying legumes. A lot of solar drying systems exist for grain drying such as direct, non-direct, and solar. Solar dryers have the problem of the dehydration process being stopped as a result of an absence of solar radiation and absence of radiation at night or low insulation, which decreases the quality of the grains. So far, there have been efforts to proffer solutions to the problems of solar systems, some of which include – the addition of thermal storage materials, phase change materials, and adding a variety of heating modes either direct or indirect [21]. This has led to the evolution of several types of solar dryers. Thermal storage materials have the ability to store thermal energy when there is solar radiation and then make use of this thermal energy when the sun is not available. Three main forms of solar dryers exist with varying sizes, designs, and magnitude [22].
