2.5 Drying
"Drying" is the phase of the postharvest system during which the beans are rapidly dried until they reach the "safe-moisture" level. After threshing, the moisture content of the beans is sometimes too high for good conservation (13 to 15 percent). The purpose of drying is to lower the moisture content in order to guarantee conditions favourable for storage or for further processing or handling of the product.
Moisture content
Moisture content of a product is a numerical value, expressed in percentage, determined by the relationship between the weight of the water contained in a given sample and the total weight of that sample.
|
Moisture content of the sample ( %)= |
Weight of the sample’s water |
x 100 |
|
Weight of (sample’s dry matter + sample’s water) |
Therefore, to say that soybeans have 15 percent moisture content means within a sample of 100 g of raw product there are 15 g of water and 85 g of dry matter.
Relative humidity
Soybeans, as all grains, are "hydroscopic", meaning that in ambient air they can either give off or absorb water in the form of vapour. At a given temperature, the air cannot absorb unlimited quantities of water vapour. The air is "saturated" when it is unable to absorb water vapour at a given temperature, that is, it has a relative humidity of 100 percent. Air containing a given amount of water vapour tends to become saturated if its temperature is lowered. Contrary, if it is desirable to increase the "drying power" of this air, that is its capacity for absorbing more water vapour, it is necessary to heat it.
The relative humidity (RH) of the air, expressed in percentage, is defined as the relationship between the weight of the water vapour contained in 1 kg of air and the weight of the water vapour contained in 1 kg of saturated air, at a given temperature.
|
Relative humidity of the air (%)= |
Weight of water vapour in 1 kg of air |
x 100 |
|
Weight of water vapour in 1 kg of saturated air |
The term equilibrium relative humidity (ERH) simply means RH in the adjacent air after allowing sufficient time for moisture in the seed to equilibrate with the air and can be determined by analyzing the headspace in a sealed equilibrated container.
Table 16 shows moisture-relative humidity equilibrium values for soybeans at 25° C. The higher the RH of the air, the greater amount of vapour absorbed by the beans. At 35 percent RH, the beans contain 6.5 percent of moisture, whereas at 85 percent RH, the moisture content of the beans is 18.4 percent.
Table 16. Moisture-Relative humidity equilibrium values for soybeans at 25° C.
|
Relative humidity (%) |
Moisture in soybeans (%) |
|
35 |
6.5 |
|
50 |
8.0 |
|
60 |
9.6 |
|
70 |
12.4 |
|
85 |
18.4 |
Source: Barger (1981).
Drying of beans can be done by circulating air at varying degrees of heat through a mass of beans. As the air moves through the beans, it imparts heat to the beans, while absorbing the humidity of the outermost layers. The exchange of heat and humidity between the air and the beans takes place as follows:
The water present in the outer layers of the beans evaporates much faster and easily than that of the internal layers. Thus, it is much harder to lower the moisture content of the beans from 17 to 12 percent than from 22 to 17 percent. Some farmers have the erroneous belief that rapid drying at high temperature overcomes this situation, but these drying conditions create internal tensions and produce tiny cracks that can lead to rupture of the beans during subsequent handling.
There are essentially two methods of drying natural and artificial drying.
Natural drying consists of exposing the threshed beans to the sun, mostly in the field. To obtain the desired moisture content, the beans are spread in thin layers on a drying-floor, where they are exposed to the air (in sun) between 1 to 2 weeks. The beans must be stirred frequently to encourage uniform drying. As a rule of thumb, the relative humidity of the ambient air must not exceed 70 percent for drying to be effective. This is the reason why grains must not be exposed at night. The cold of the night fosters rehumidification of the grains. This method should not be used in humid regions or during the rainy season.
Insufficient or excessively slow drying can bring about severe losses of beans during storage from the self-generated heat of "green" beans. Also, prolong exposure of beans to atmospheric factors (insects, rodents, and birds) and microorganisms (moulds) can cause losses of the product. In China, considerable losses are incurred annually during storage and transportation of grain, as a result of inadequate drying (Ren and Graver, 1996).
Despite these disadvantages, natural drying is recommended in the following situations:
Artificial drying is unavoidable in humid tropical and subtropical regions, given unfavourable weather conditions at harvest time. In these regions, it is often difficult to safeguard the quality of the beans. With the introduction of high-yielding soybean varieties and the agriculture mechanization it is possible to harvest large quantities of soybeans in a relative short time. The need of using the land for cultivating other crops forces farmers to harvest soybeans with high moisture content. Consequently, it is necessary to dry the beans artificially. This method of drying consists of exposing the beans to forced ventilation of air that is heated to certain degree in special equipment called "dryers".
The dryer consists of 3 basic elements (Figure 14):
Figure 14. Grain dryer, capacity 20 tonnes/day; left: body, middle: air generator, right: ventilator.
There are two types of dryers: static or discontinuous and continuous dryers. The dryer shown in Figure 14 is a continuous dryer. The discontinuous dryer is relatively inexpensive and can treat only modest quantities of grain; then it is better adapted to the needs of small- and medium-scale collection centres and processing of products. Small-scale dryers used in South East Asia have a drying capacity between 1 to 6 tonnes per 8 hour operating day (Tumambing, 1996). In Thailand, continuous-flow dryers locally made are used for soybean drying (Bovornusvakool, 1996).
The continuous dryers are high-flow dryers that require a more complex infrastructure, complementary equipment and special planning and organization. They are more appropriate for big centres, silos or warehouses, where very large quantities of product are treated. One disadvantage of artificial drying is that it is fairly costly (purchase of dryers, use of fuel). In Mexico, the cost of drying a tonne of beans or any other grain depends on the original and desired moisture content of the grain. Table 17 shows the charge of grain drying at different original and desired moisture contents. It is estimated that 10 percent of the charge represents the profit for the owner of the drying facility.
Table 17. Average cost of drying grain in Mexico.
|
Original grain moisture (%) |
Desired grain moisture (%) |
Average cost (US$/ tonne) |
|
16.0 |
14.0 |
3.5 |
|
18.0 |
14.0 |
4.0 |
|
20.0 |
14.0 |
4.5 |
|
25.0 |
14.0 |
5.5 – 6.0 |
The amount of water to evaporate per tonne of grain, Qr, with an initial moisture Wi (%) to be set at a final moisture Wf (%), is calculate by the following equation:
|
Qr = |
Wi – Wf |
1 000 (kg/tonne) |
|
100 – Wf |
If 5 tonnes of beans harvested at 28 percent moisture were dehydrated to final moisture of 14 percent, the amount of water eliminated would be:
|
Qr = |
28 – 14 |
1 000 x 5= 162.8 x 5= 813.95 kg |
|
100 – 14 |
It may be interesting to know the amount of water evaporated per tonne of dried grain Qr’ to a final moisture Wf. In this case the result would be:
|
Qr3 = |
Wi – Wf |
1 000 (kg/tonne) |
|
100 – Wf |
For the same moisture content and amount of grain of the previous example, the water eliminated would be:
|
Qr3 = |
28 – 14 |
1 000 x 5= 972.22 kg |
|
100 - 28 |
It would be eliminated a higher amount of water since 5 tonnes of beans correspond approximately to 6 tonnes of humid beans.
Table 18 shows the amount of water to be removed per tonne of grain (humid or dried) as a function of the original moisture and final moisture of 14 percent.
The drying rates of 25 Argentinean varieties of soybeans were determined by Giner et al. (1994). Variability in seed size and diffusion coefficient accounted for 93.8 percent of variation in soybean seed drying rates.
Table 18. Amount of water to be eliminated from the grain to take it from the harvesting moisture to 14 percent.
|
Grain (%) |
Amount of water evaporated per tonne of humid grain Qr (kg/tonne) |
Amount of water evaporated per tonne of dried grain Qr’ (kg/tonne) |
|
28 |
162.79 |
194.44 |
|
27 |
151.16 |
178.08 |
|
26 |
139.53 |
162.16 |
|
25 |
127.91 |
146.67 |
|
24 |
116.28 |
131.58 |
|
23 |
104.65 |
116.88 |
|
22 |
93.02 |
102.56 |
|
21 |
81.40 |
88.61 |
|
20 |
69.77 |
75.00 |
|
19 |
58.14 |
61.73 |
|
18 |
46.51 |
48.78 |
|
17 |
34.8 |
36.14 |
|
16 |
23.25 |
23.81 |
|
15 |
11.63 |
11.76 |
Adapted from De Zanche (1991).