*4.1.2 Indirect mode solar dryer*

When it comes to moisture removal and heat transfer, indirect sun dryers differ from direct solar dryers. This style of drier is utilized for quick drying. The atmospheric air is heated in a solar air collector in this dryer, and then this hot air moves towards the drying cabin, where products are kept to dry, and the hot air absorbs some moisture from the drying products before exiting through the chimney.

### *4.1.3 Mixed mode solar dryer*

The term "mixed mode solar dryer" refers to a solar dryer that uses both direct and indirect heating methods. The inlet air is heated at the solar air collector before entering the drying chamber in a mixed mode solar dryer. Some of the drying chamber's sides are composed of glass, which adds to the drying chamber's overall warmth. The product is dried using a combination of hot air and direct sunlight in this procedure. In comparison to direct and indirect solar dryers, mixed mode solar dryers require less drying time. Biomass has been used in hybrid sun dryers as an auxiliary heat source to keep drying going all night. Cashews, for example, have been dried in these dryers [23].

### *4.1.3.1 Greenhouse solar dryer*

Tent dryers are similar to greenhouse sun dryers. They have vent sizes that control airflow. Board glazing is used on all sides of this type of drying system. The greenhouse drying system provides a higher degree of control when used in conjunction with the appropriate settings. The main benefit of a greenhouse solar dryer is that it can provide alternate heating with charcoal or briquette burners during inclement weather and can also be used at night.

Greenhouse solar dryers are a type of solar dryer that was developed to address some of the issues that open solar dryers face. The greenhouse solar dryer might be created out of polycarbonate sheets in parabolic shapes, with direct current blowers to help with airflow in the dryer, which has a floor area made out of concrete [24]. Solar radiation intensity was observed between 390 to 820 W/m<sup>2</sup> .

Greenhouse drying is one of the world's oldest methods of crop preservation. It entails the phenomena of heat and mass transmission. The product's thermal energy is used in two stages. The temperature of the product rises in the first step due to sensible heat, and the moisture in the product vaporizes in the second step due to the provision of latent heat of vaporization [25]. The greenhouse dryer provides a regulated environment in terms of relative humidity and temperature, which is better for crop drying and hence reduces drying time. The essential processes in the construction of a greenhouse system include vaporization. The greenhouse drier provides a regulated environment in terms of relative humidity and temperature, making crop drying more efficient [25].


#### *4.1.3.1.1 Natural convection greenhouse dryer*

Incident sun energy is passed through the canopy and used to heat the crops in a natural convection greenhouse dryer. The temperature of the crop rises as a result of solar radiation absorption. The thermosyphic effect is used to operate the natural convection greenhouse drier. Humid air is vented through the dryer's chimney or evacuated through an outlet on the top, while warm air is pumped through the crop by buoyancy forces. Natural convection mode refers to this type of airflow within the drying chamber, and a natural convection greenhouse drier is one that works in this manner [25].

#### *4.1.3.1.2 Forced convection greenhouse dryer*

The forced convection greenhouse dryer was born out of a desire for increased air circulation and drying rates. To adjust temperature and moisture evaporation

*Postharvest Preservation Technology of Cereals and Legumes DOI: http://dx.doi.org/10.5772/intechopen.102739*

according to the weather conditions, an optimal airflow should be given in the greenhouse drier during the drying process. An exhaust fan on the upper half of the west wall is used to evacuate humid air. Forced convection greenhouse dryers employ a fan or blower to control airflow [25].

The mixed mode solar dryer outperforms other types of solar dryers in terms of drying efficiency, drying time, and thermal efficiency. It has been discovered that a mixed mode solar dryer with a Phase-Change-Material is the best for drying grains with higher efficiency and shorter drying times, as well as being smaller, having fewer moving parts, and requiring less maintenance [19]. In a mixed mode solar dryers with 1.5 m/s air velocity, beans with up to 60% moisture content can be reduced down to 6% within six hours of drying [19]. The time required for drying depends on factors like solar radiation, ambient condition, and relative humidity while the solar collector efficiencies can be as high as 61.82% [21].

### **5. Storage of grain legumes**

Cleaning of grains to remove extraneous materials and contaminants is very fundamental in achieving good and safe storage. It established that cleaning before storage of grains influences the quality of the grain [26]. Cleaning involves the removal of unwanted extraneous material (straws, sand, stone, etc.) from the grain. The storage of grain legumes is a very cardinal stage in the postharvest handling of legumes. Its importance is based on the fact that if the optimal conditions for their safe storage are not maintained a high level of postharvest losses could be incurred. Different microorganisms and pests have the ability to destroy grain legumes after their harvest, during storage, or transportation to various locations of interest. Depending on the prevailing intrinsic and external factors, postharvest losses of grain legumes are estimated to be about 9% for USA and 40–50% for many developing countries [27].

The rapid decline in color, oil quality and ability to germinate, and many other changes in the quality characteristics of grain legumes can be caused by increase in temperature and moisture. High moisture content and elevated temperature of grains can lead to the development of molds in the category of *Aspergillus species, Fusarium species, and Penicillium species,* and the production of some mycotoxins such as aflatoxins, ocharatoxin A, and patulin produced by molds. High moisture content and temperature above optimal levels also aid the infestation of different varieties of insects (granari weevil, grain borer, grain moth, grain beetle, etc.) which feed directly on the grains with a resultant effect of the decline in grain quality and quantity. Infestation of grains by fungi results in reduced nutritional quality, reduction in the quality of proteins that synthesize gluten, and the ability of grains to germinate. Other effects include free fatty acid elevation, lowered starch content, increase in total soluble solids, the decline of non-reducing disaccharides and oligosaccharides. The grains can also be charred due to hot spot development and the formation of mycotoxins may occur as a result of fungal contamination creating very big public health issues [7, 17]. Globally produced grains of about 25% are contaminated by toxins from molds – mycotoxins [28]. The aflatoxins with the greatest intoxicating effect, genotoxic and carcinogenic characteristics of greatest concern are B1, B2, G1, G2, and M1 aflatoxins (**Table 1**) [31].

During storage, grain legume pests are capable of destroying up to 33–50% of global produces [27]. This gives an insight on the seriousness of pest infestation and attack on grains if proper control measures are not put in place. The quality degradation which results in loss of the quantity of leguminous grains globally during storage can get up to 60% in some instances [27]. These losses are primarily


#### **Table 1.**

*Postharvest preservation technologies.*

as a result of insect infestation, rodents attack, micro-organisms like mold as well as the breakdown in the normal physiology of grains. It's a well-known fact that pathogenic micro-organisms, insects, rodents, and unwanted contaminants are capable of posing health hazards in grains when consumed. In storing grains from leguminous crops, the usage of suitable packaging and packaging materials is very crucial in achieving good results in postharvest management of leguminous grains. Packaging also serves a very key role during distribution and marketing (to maintain quality) [27].

In village areas of developing and even developed nations, grains including pulses are still kept in traditional storage facilities which are fabricated with natural materials or woven threads. Typical examples of some of the traditional storage structures used include underground pits, thatched roof storage, plastic containers, and basket silos. Though these local structures have a low construction and maintenance cost, they are not very durable, easily invaded by insects and pests resulting in grain legume quality deterioration. Developing nations are currently adopting warehouse storage structures for storing their grains in very large quantities [17].

The materials used for the construction of storage facilities and structures have a direct influence on the moisture content and temperatures in the storage structures [17]. Wooden sticks, concrete blocks, cement, bamboo, and metals (aluminum or steel) are some of the very common materials used for the fabrication of storage structures for grains.

#### **5.1 Silo**

Silos are currently very common storage facilities for storing grain in many countries and constitute about 79% of all on-farm grain storage facilities in Australia. Silos are very ideal storage alternative for grain legumes (pulses) especially the cone-based variant which makes for very easy grain unloading/discharge with very low seed damage possibilities [15]. For long-term storage of above three

#### *Postharvest Preservation Technology of Cereals and Legumes DOI: http://dx.doi.org/10.5772/intechopen.102739*

months duration, there is a need for the incorporation of aeration cooling systems and the use of gas-tight sealable storage which are recommended for efficient and effective fumigation regimes in managing and achieving best quality control. Metal silos are fabricated by incorporating augers and ventilators for grain aeration in order to reduce the formation of hot spots. Metal silos with ventilators and augers are considered advanced grain storage systems as they have the ability to extend the shelf-life of grain legumes through controlled respiration and the development of unfavorable conditions for all sorts of grain legume pests [7, 17].

It is advisable to always fill and empty silos from apertures provided at the center of the silos. This is especially important with grains as most grains have a high bulk density and loading or unloading outside the central opening at the center will put an uneven load on the structure which may cause the silo to collapse [14].

Metal silos of different sorts, fabricated with galvanized iron or recycled oil drums have been developed as an economic, effective, and efficient containersstorage option. These silos are suitable for a long duration of storage of cereals and grain legumes in a water-resistant and hermetically controlled environment. Grains stored in metal silos provide protection from rodents, insects, and water, and are thus very good storage systems for pulses [32]. However, there is a need to protect or shield silos from direct sun rays and other heating sources capable of increasing the temperature of the grains contained therein to avoid condensation. As an alternative, silos can be situated in well-ventilated areas with shade to avoid elevating the temperature of the silos [32]. It is worthy of note that metal silos are very efficient and effective for grain storage but they are also expensive [33].

If there is direct exposure of silos to sunlight or the external air is lower than that in the silos which contain the grains, there may be a formation of currents of convectional flow. As a result of the convectional air currents generated, the moist air is being blown pass through the grains. As the moist air travels and meets cooler surfaces like the silo walls, condensation of the moisture will take place and the grain within that area will get dampened. This dampening occurrence is a cardinal problem associated with grains stored in silos made of steel and particularly utilized for storage in hot areas with daily clear sky [28]. High day temperatures and cool night temperatures are a result of a clear sky. The problem of elevated temperatures can be mitigated in small silos by providing a shield in form of a roof or a hat, to prevent direct contact of sun rays with the surface of the silos. Solutions for larger silos may involve grain silo ventilation or transferring of the grains from the silo with a high temperature to another one that has a cool condition. Grain movement during the transfer of grains to another cool silo has the tendency to provide grains with more homogenous moisture content. In a case where the moisture content is too high, then there will be a need to dry the grains again [28].

#### **5.2 Hermetic bags/cocoon**

It's still possible for foreign pests like *Callosobruchus maculatus* and *Callosobruchus chinensis* to be located in grain legumes storage systems during storage if appropriate pest management regimes are not strictly adhered to. Grain legumes storage in hermetic bags/Cocoons has to a large extent aided farmers in many countries in storing and extending the shelf life of their produces as they await periods with better produce value and pricing. This has resulted in better financial gains for farmers that make use of Hermetic bags/cocoons storage in extending the shelf life of their produces with the target of a better sales period [33]. The technique of using hermetically sealed polyethylene silo bags is an effective alternative for the protection of stored grain legumes in commercial storage systems and is presently gaining more prominence for both on-farm sites and off-farm sites [34].
