“Grain cooling is a comprehensive solution for postharvest management and conservation of a grain bulk. It brings benefits which keep quantity, quality and the processing of a grain in the most economic condition. Its implementation in the warm and humid climates will lead to essential improvement of grain handling, loss reduction and good management practice, which has been proven in moderate climate already for more than 50 years.”
Grain is the essential base of human nutrition and feeding in animal husbandry. It needs to be available throughout the processing and can’t be replaced. Therefore it is mandatory to store it during the harvest time when quality and price are at their best. Most mills have invested in bulk storage facilities like silos to simplify the handling for the processing and to maintain the quality of the grain during the storage. However the management of the storage faces new challenges mostly. They are caused by the nature of grain, the storage environment and the postharvest treatment. Main issues are the high and uneven moisture content, high ambient temperatures and relative humidity, heat rise during storage, weevil and mould infestation. All of those cause extra efforts, additional cost or reduction in processing efficiency which increase by longer storage time. Grain cooling eases the trouble and simplifies the management to one treatment solution for all constrains of grain storage.
THE WAY OF GRAIN COOLING
The grain cooler is connected to the grain storage by a flexible hose. The treated air is blown into the grain bulk. The air flow passes the grain and removes the heat of the grain. The air becomes humid and warm and exits the storage bin through vents at the top. The grain cooling continues until the entire bulk is cooled to the desired temperature, usually in a range of 10 to 18°C depending on the storage time. Afterwards the grain cooler is turned off and the air inlet and vent openings are closed. The cooled grain remains in the silo until it is removed or cooled again if the temperature will increase after several months of storage. Figures of the application at a vertical silo and warehouse are shown in Figure 1 and 2.
THE ACHIEVEMENTS OF GRAIN COOLING FOR GRAIN STORAGE
Prevention of Respiration Loss
Grain continues to respire after being harvested. Losses in freshly harvested grain are primarily caused by its cellular respiration and its heating. The rate of the activity is dependent on the grain’s moisture content and temperature as shown in Figure 3 (Jouin, 1964). Respiration becomes exponentially intensive as the temperature and moisture content of grain increase.
Fig 1: Application of grain cooler at vertical silo (FrigorTec, 2012)
The consequence of heating by respiration is loss of grain weight. Respiration increases as heat increases. Water content rises as result of the oxidation of grain carbohydrates or fats. The water content will decrease the storage life of the grain by favor the infestation of bacteria, mites, insects and fungi. A grain cooler reduces the chances of grains being damaged during storage.
Prevention of weevil development
Weevils and other insects can damage stored grain and their activity and development is influenced by the temperature of the environment. Navarro (1975) shows that at temperature above 20 to 32 °C the development of the insects is in optimum. However temperature less than 10 to 15°C reduce the activity. Therefore the grain is protected when grain is chilled. The Figure 4 shows the details.
Fig 2: Application of grain cooler at warehouse (FrigorTec, 2008)
Prevention of fungi
Microorganisms such as fungi and bacteria adhere to the surface of the grain kernel (Mühlbauer, 2009). The development of fungi depends on the temperature, humidity and the grain’s moisture content (Lacey, 1980). Their growth is prevented in the storage facility by drying and grain cooling. Therefore, it has to be considered that grain is biological active and there is a change over the time. This is certainly true when the respiration activity of the grain is high and water and heat arise. Then moisture content of the grain, its storage temperature and the relative humidity will lead to fungus growth. Thereby the danger of fungal contamination is not only the deterioration of the grain but the increase in mycotoxin which have toxic effects on humans and animals. Most mycotoxins are heat-stable can persist through grain processing. For this reason, the formation of toxins must be prevented by stopping harmful fungi (Rodemann, 1999). Grain cooling can decrease damage from fungi and their mycotoxins as fungal growth doesn’t take place at low temperature even if grain’s moisture content is above 14%.
Fig 3: Heat generation during grain storage modified according to Jouin (1964)
Extension of storage time
The safe storage time of grains depends on the moisture content as well as on the storage temperature of the grain (Agena, 1961; Wimberly, 1983). The storage period timer in Figure 5 shows the estimated safe storage time for grain according to its temperature and moisture content (Sontag, 2014). The safe storage time for any particular condition of grain can be read by matching the grain’s moisture content against its actual temperature. The section of the line on the vertical axis of the storage period gives the possible safe storage time of the grain. It is obvious what grain cooling does for the extension of safe storage life of grains particularly in tropical environments. The example shows that for a moisture content of 14.5% the safe storage life at 31°C is around 18 days while at 10°C it is increased to around 300 days.
BENEFITS OF GRAIN COOLING FOR GRAIN DRYING PROCESS
During the process of grain cooling the heat of the grain is removed and relative humidity of the air is lowered according to the psychromatic chart. Thus, the air can remove moisture from the grain in arrange of 0.5 to 2% moisture content depending on the moisture content of the grain (Boser, 1980). Even if the moisture reduction is limited it contributes to a necessary drying process of moist grain or homogenizes varying moisture contents of a grain bulk.
Fig 4: Weevil developments according to the ambient temperature (FrigorTec, 2008)
Improving of Postharvest Management
Storage management is responsible for maintaining the quality of the grain before processing and for keeping any losses to a minimum. Common techniques include aeration, fumigation and moving the grain to avoid any heat, insect or fungus damage. When aerating the grain, temperature and relative humidity of the ambient air must be observed to avoid or minimize damage of the grain. This is difficult to achieve at harvest time when it is warm or humid. Grain cooling conservation, being independent of the weather, represents an easier option and does not lead to a degradation of the grain quality.
Fig 5: Storage timer of grain (Sontag, 2014)
Taking a look on fumigation leads to several constraints for an easy postharvest management. Adequate protection of human health and of the environment is a must if fumigation is applied. A sealed storage facility is required or regular re-gassing is unavoidable but could lead to developing a resistance to a number of fumigants at low effectiveness. Grain cooling is safer and easier than using fumigants.
Moving grain is sometimes used to deal with the issue of hot spots in grain caused by insects and fungal infestation. This requires investment in equipment for additional storage capacity and transport equipment, incurring labor and energy costs. The management must also be able to react quickly and effectively. This technique is applied for many grains but leads to as grinding losses of up to 0.06 % caused by the conveyor equipment (Boac, 2010) for each turn. When grain cooling is applied no moving of the grain is required at any time which spares cost and losses.
Improving of grain processing by grain cooling
The effect of grain cooling should not be limited to the storage when it is applied but needs to be observed into grain processing. The milling of grain is done by grinding the kernel to flour. The process is done in a highly efficient way but depends on the grain quality and its homogeneity. Therefore grain is graded or sorted before processing. The more homogeneous the grain is the higher the efficiency. When grain is cooled it remains as harvested and there is no change over the storage time. The result is that adjusting of milling equipment is reduced by processing chilled grain. The spared time leads directly into reduced processing time, which increases the efficiency of the process.
Grain cooling is a comprehensive solution for postharvest management and conservation of a grain bulk. It brings benefits which keep quantity, quality and the processing of a grain in the most economic condition. Its implementation in the warm and humid climates will lead to essential improvement of grain handling, loss reduction and good management practice, which has been proven in moderate climate already for more than 50 years (Kolb, 2013).
Agena, M.U., 1961, Untersuchungen über die Kälteeinwirkung auf lagernde Getreidefrüchte mit verschiedenen Wassergehalten, Dissertation Universität Bonn, Germany
Boser F., 1980, Heizöl sparen mit Getreidekühlung, Fachzeitschrift Schweizer Handels-Börse.
Boac, J. R., 2010, Quality changes, dust generation, and commingling during grain elevator handling PH D. Thesis, Kansas State University, Manhattan, Kansas, USA.
FrigorTec, 2008, Treatment of grains through conservation cooling with the GRANIFRIGOR ™
FrigorTec, 2012, Quality assurance of paddy & rice with GRANIFRIGOR™ cooling conservation, Amtzell, Germany
Jouin, C., 1964. Grundlegende Kalkulationen für die Belüftung des Getreides, Getreide und Mehl, Band 14, Heft 6, Beilage der Zeitschrift „Die Mühle“, publisher Moritz Schäfer, Detmold, Germany.
Kolb, R.E., 2013, Getreidekonservierung mit „Granifrigor“-Kühlgeräten seit 50 Jahren, in: Mühle + Mischfutter 150. Jahrgang, Heft 10, Verlag Moritz Schäfer, Detmold, Germany.
Lacey, J., Hill, S.T., Edwards, M.A., 1980, “Microorganisms in stored grains; their enumeration and significance”, in: Tropical stored product information, 39.
Mühlbauer, W.,(2009: Handbuch der Getreidetrocknung, Agrimedia, Clenze, Germany
Navarro, S., 1975, Aeration of grain as a non-chemical method for the control of insects in the grain bulk, Proceedings of the 1st International Working Conference on Stored-Product Entomology, 7-11 October 1974, Savannah, Georgia, USA. Savannah, Georgia, USA
Rodemann, B., 1999, Mykotoxine im Getreide, Rep. 2–99, Biologische Bundesanstalt für Land- und Forstwirtschaft, Braunschweig, Germany.
Sontag, J., 2014. Rice Processing, publisher Erling, Clenze, Germany.
Wimberly, J. R., 1983, Technical Handbook for Paddy Rice Postharvest Industry in Developing Countries, International Rice Research Institute, Los Banos, Laguna, Philippines.