2.8 Storage

 Figure 19. In bulk storage system, left: storehouse used mainly in developing countries, middle: concrete silos used in storage centres, right: corrugated-steel silos for on-farm storage.

Designs of storage facilities are dictated by needs for aeration of seed and its angle of repose, that is, the minimum angle in degrees at which a pile maintains its slope (Appel, 1973). This sometimes is reflected in the pitch of conical roofs on storage bins. Similarly, downspouts and the conical bottom of bins must have pitches steeper than the angle of repose for soybean or meal to flow smoothly. Higher moisture and oil contents increase the angle of repose. The angle of repose and bulk density of soybean and soybean products are shown in Table 21.

Different on-farm storage structures to those of Figure 19 exist in rural areas of developing countries. In these areas, local available materials are used for their construction. Some examples are the enclosed earthen granaries of the dry zones and the ventilated granaries made of plant fibre and wood that are used in humid zones (de Lucia and Assennato, 1994). Osunade and Lasisi (1995) compared the performance of metal silos with laterized concrete silos in Nigeria. The laterized concrete is concrete in which some, or all, of the fine aggregate is from lateritic soils. Laterized concrete silos maintained a uniform temperature of about 26° C, whereas there were high temperature fluctuations within the metal silos and outside both silos. The results of this study suggest that laterized concrete silos are better than metal silos and that they can be used for grain storage in tropical regions. Oti-Boateng (1995) presented traditional and improved storage methods in developing countries. Construction materials, costs, capacity and appropriate crops and storage time for each storage method was presented.

Whatever grain storage system requires ventilation. Sometimes, unventilated silos are used for storing grain but they are limited to zones with low relative humidity and protected form outside temperature variations (concrete bins). To use this type of silo, the grain must be completely clean, dry and treated with long-lasting insecticides. In addition, the grain temperature needs to be monitored by thermocouples distributed within the bin.

Small weed seeds and broken kernels (fine material) tend to accumulate in spoutlines, pockets and layers whenever soybeans are transferred. The spoutline material in bins of soybeans may consists of 80 percent weed seeds and it is in this material that spoilage first starts in bins and tanks of stored soybeans. The first indication of spoilage is a rise in temperature. When the temperature in the bin or in the storage facility is not monitored spoilage progress to the final stages of heating. There was a report on discolouration of 10 000 bushels of soybeans of the 90 000 bushels stored in a bin for 3 months due to severe heating.

Turning the mass of grain with motorized equipment or by hand is another way of providing ventilation to the grain. It is not an efficient way of cooling grain but it helps to dissipate heat accumulated in certain areas of the mass of grain called "hot spots" and disperse insects capable of growing and/or surviving under the mass of grain.

Soybean storage studies

It has been observed that soybean genotypes differ in their capacity to maintain viability and vigour during storage (Coelho et al., 1978; Fernandez, 1970; Kueneman, 1982; Herrera and Rosales, 1987). Twenty five soybean genotypes were stored under four storage conditions: (1) bunches of plants were hanged up at 2 m in a well-ventilated rustic warehouse and dusted with malathion (4 percent) and captan (3 g/kg of seeds) to avoid weevil attacks, (2) seeds were placed in cloth bags in a well-ventilated warehouse at ambient conditions, (3) seeds were placed in sealed containers (jars) at ambient conditions and (4) seeds were placed in cloth bags kept in a chamber at 15° C and 60 percent relative humidity. The storage period was 8 months, the normal storage period from soybean harvesting to the next planting date in Costa Rica. The genotypes’ capacity to maintain their viability and vigour varied with the type of storage. After 8-month storage at 15° C and 60 percent RH, soybeans showed the best conservation with an average germination of 95 percent and low variability among genotypes (Herrera and Rosales, 1987). Seeds kept in cloth bags and in plant bunches stored in warehouse at ambient conditions showed the lowest germination values.

Three different threshing methods were used with Indian soybean cultivars, PK-327, PK-416 and PK-564. Seeds were threshed by hand beating, machine threshing, or tractor treading and kept under ambient storage conditions and then tested for percentage germination, electrical conductivity and the effect of ageing. Hand beating resulted in higher percentage germination levels and less deterioration of seed than the other two techniques at all stages of storage (Jha et al., 1995). PK-327 maintained the maximum standard percentage germination and lowest electrical conductivity value after 8 months of storage.

The effect of storage period on germination and health of 8 soybean cultivars grown in Brazil was determined (de Resende et al., 1995). Seeds were stored for 3.5, 9 or 14 months under ambient conditions in the laboratory. Average germination percentage decreased from 91.4 percent to 65.6 percent after 3.5 and 9 months, respectively. Average fungal contamination decreased from a peak of 10.4 percent after 9 months to 1.0 percent after 14 months with the dominant fungus, Phomopsis sp. disappearing completely by the end of the storage period.

The effect of ethylene oxide and methyl formate fumigation on seed microflora and germination of stored soybeans in Nigeria was reported (Bankole, 1996). The fumigated seeds were stored in airtight metal bins for a year and the microflora and in vitro germination then determined. While 8 genera and 29 species of fungi were isolated from untreated seeds, only Aspergillus flavus appeared on the fumigated seeds. A. niger and Penicillium citrinum appeared on soybeans fumigated with methyl formate. Seeds treated with fumigants recorded significantly higher in vitro germination than untreated seeds.

In Pakistan, nine fungal species were isolated from 20 soybean seed samples collected from grain markets (Ali et al., 1995). Of the nine species, M. phaseolina was the most pathogenic in germination tests. Thiophanate-methyl (Topsin-M) and benomyl (Benlate) were effective in controlling M. phaseolina. In a separate study, Anwar et al. (1995) reported high incidence of storage fungi Aspergillus, Penicillium and Rhizopus spp., which reduced seed germination potential in-vitro.

In Nigeria, the effect of temperature and relative humidity on the storability of five soybean cultivars was examined (Nkang et al., 1997). Germinability differed significantly after six-month storage among the cultivars. Optimum storage conditions were found to be at temperatures of 25 to 30 ° C and relative humidities of 55 to 65 percent.

Chemical and biological changes during storage

Soybean seeds are living tissues and undergo physicochemical and biological changes. Many of these changes can lead to both nutritional and functional deterioration and ultimately to losses of commercial value. Among the changes found during postharvest handling is the one associated with seed proteins, a major component of soybeans (Yoshino et al., 1977; Saio et al., 1980, 1982; Nakayama et al., 1981; Yanagi et al., 1985; Narayan et al. 1988a,b; Thomas et al., 1989; Lambrecht et al., 1996). Changes in extractable proteins from defatted soy meal and whole beans during six months of storage were reported by Saio et al. (1982). They kept the products at any of four storage temperature and relative humidity conditions (25° C, 50 percent RH; 25° C, 85 percent RH; 35° C, 50 percent RH; and 35° C, 85 percent RH) and found that the percentage of extractable protein from either defatted meal or whole beans decrease with time. The decreasing rate depends on storage conditions. The higher the temperature and the higher the humidity, the higher the decreasing rate in protein extractability. The moisture content (or RH) appeared to have a stronger effect than the storage temperature. In addition, under identical storage conditions, defatted meal showed a more rapid decrease of protein extractability with time than whole beans. When extractability of each protein component was followed, all components decreased with storage time except for the 2S component. Among protein components, the 11S component decreased most rapidly. Later, Yanagi et al. (1985) found that sedimentation pattern of water extract from stored soybeans exhibited a decrease in peak height for the 11S fraction and increases in peak height for both smaller fractions (2S and 7S) and larger fraction (15S). The rapid decrease of 11S found by both researchers could be explained by its degradation and aggregation during storage (Liu, 1997).

Besides storage protein, other components in soybeans also undergo changes during storage. These include increases in non-protein nitrogen, free fatty acids and peroxide value, decreases in sugars, trypsin inhibitor activity, available lysine, pigments and lipoxygenase activity (Narayan et al., 1988a) and decomposition of phospholipids (Nakayama et al., 1981). Increases in total ash, various minerals, sugars and reducing constituents in soaking water of stored beans were reported (Saio et al., 1980).

Important indicators of biological changes during soybean storage are darkening in bean colour, a decrease in water absorption rate, an increase in leakage during soaking and an increase in acid value of extracted crude oil as well as in the acidity of beans (Saio et al., 1980, 1982; Thomas et al., 1989). This last change indicates that the neutral fat in fresh beans had been hydrolyzed to free fatty acids during storage (Liu, 1997).

Several studies (Yoshino et al., 1977;Narayan et al., 1988b; Thomas et al., 1989; Lambrecht et al., 1996) have shown that the reduction in overall production yields and organoleptic properties is a function of storage time and become severe when storage temperature and relative humidity are both high. Apparently, most of the effects can be attributed to decreased protein functionality as a result of storage (Liu, 1997).

The effect of storage on the nutritional values of soybeans has not been addressed. However, studies with other legume species, particularly dry beans (Phaseolus vulgaris), showed that adverse storage leads to decreases in protein efficiency ratio, protein digestibility and availability of sulphur amino acids (Atunes and Sgarbieri, 1979).