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Grain Storage: Considerations To Maintain Quality

Introduction
The primary tools used to manage grain in bulk storage are Grain Temperature Detection Systems, and Aeration. A temperature detection system is an electronic reading device that is attached to special cables, suspended from the roof of the grain storage. A storage structure may have one, or many cables, depending on the structure diameter. The cables contain temperature sensors at approximately two-meter vertical intervals. Grain temperatures change relatively slowly, and are normally read and recorded once per week. Variations in grain temperature across the grain mass, or localized rising temperatures, can be indications of grain spoilage and the need for managers to take action. But since grain temperature detection is another complete presentation, we will not focus on it today. More importantly, we will focus on Aeration, the process of making changes within a grain mass.

Grain aeration basic principles are divided into four subjects: 1) What is aeration; 2) Why do you aerate grain; 3) When do you aerate grain; and 4) How do you aerate grain. An understanding of these four principles will significantly improve your ability to successfully store grain for an extended period of time.

What Is Aeration?
Aeration is the process of passing large volumes of ambient air through a grain mass, using a fan designed for that purpose, and an air delivery system. There are many designs on the air delivery systems. Most are designed into the concrete foundation of a grain storage structure, using a tunnel and a steel cover perforated with many small holes. Air is forced into the tunnel, through the perforated steel floor, and into the grain. The shape of an aeration system can take many forms, some of which are illustrated here (R)C a double ¡ã¡ÞH¡ã¡À, a ¡ã¡Þfour ¡ãAEF¡ãO system, or two rectangular pads. Aeration fans o are connected to the aeration system inside. The type of system is not as important as the skill and experience of the designer, and the reputation of the manufacturer. Most are designed by the provider of the grain storage system, and meet critical design standards. Each system must be designed to provide adequate volumes and pressures of air for the quantity and depth of the stored grain. The size and type of aeration fan is also critical to successful performance of the system. Why Do You Aerate Grain?

Grain has enemies, including insects, mold and fungi, heat and moisture. Insects eat and breed in grain, creating damaged grain that will spoil and decay. Mold and fungi create heat and decay. Heat and decay nourish more mold and fungi. Moisture creates spoilage and decay, which creates more heat, spoilage and decay.

The purpose of aeration is to condition dry grain. It is not for the purpose of drying grain, as air volumes are too low to dry. Aerated, conditioned grain will decrease mold and fungi growth, decrease insect activity, equalize moisture deposits, and equalize heat distribution within the grain mass. Properly managed, aeration will improve your chances of successful storage.

Grain quality and condition can never be improved (R)C it can only be maintained. If spoiled grain i placed in storage, it cannot be retrieved or removed from storage in better condition. It is critically important that only grain in good condition be placed in storage. With good management, good quality grain can be retrieved from storage processing and used.

Consider the food in your home. Many fruits will spoil slowly if left out on the table. If placed in refrigeration, the decay is slowed. If cut open, the decay is accelerated. Grain is the same. In nature, grain has some natural protection. If broken, it decays faster. If kept cool and dry, it decays slower. Why aerate grain? Consider that grain is alive. If it is planted in the ground, it will grow. Like all living organisms, live grain breathes air and releases moisture. When contained in bag or bulk storage, the moisture can accumulate. Changes in temperature can create condensation. Moisture on the surface of grain will start decay and spoilage, which in turn will create heat and accelerate decay. The process of aeration a) equalizes moisture within the grain mass; and b) equalizes heat within the grain mass. Mold and fungi are present in the air and on surfaces wherever you are right now (R)C inside or outsid of a building. Mold, fungi, heat, and moisture create spoilage and decay in grain. The process of aeration will equalize heat and moisture, and restrict the opportunity for growth of mold and fungi. The process of aeration will also limit the life cycle activity of insects that require moisture and comfortable temperatures.

Moisture Temperature Storage Capability
On the left side of this chart (Figure 1) is temperature, and at the bottom is moisture (as a percent of content in whole grain). By reducing the available moisture in grain, the growth opportunity for mold and fungi decreases dramatically. Reducing temperature also contributes to the reduction of mold and fungi growth.

Reducing moisture has less effect on insects, but grain temperatures below 22oC decrease insect life activity (without consideration of moisture content). Temperatures below 18oC stop most insect life cycle activity. In tropical climates this can be achieved only with mechanical refrigeration, which has proven very effective.

Safe Storage Period (Days Corn/Maize)
Moisture content and temperature within a stored grain mass have a direct impact upon the number of days that grain can be stored. The worst-case example on this table, grain with a temperature of 27oC and 30% moisture content can only be stored for 2.6 days before the grain is spoiled. By reducing the moisture content to15%, the total number of possible storage days increases to 87 at 27oC. At the same time if the temperature were reduced to16oC. the available storage days will rise to 250.

The day count begins at the time of harvest (R)C not the day that the grain is placed in storage. Los available storage days can never be recovered. If half of the storage life of grain is lost while the grain is at high temperature or high moisture, those days cannot be restored. Therefore it is important to reduce grain moisture content and temperature as quickly as possible after harvesting.

Safe Storage Period (Days) Temperature & Moisture (Corn)
By examining the safe storage day information in a graph, it becomes apparent how critical the reduction of moisture and temperature become. Notice that grain stored at 13% moisture can be stored in excess of 4000 days at a temperature of 4oC. On the next slide, we will examine in greater detail the Safe Storage Days in the more common range of 16! to 27!C. and moisture content of 13% to 20%.

By reducing the moisture content to 15%, Safe Storage Days increases to between 100 and 250 days depending upon the temperature of the grain. Managers must determine how dry the grain should be, by deciding how long the grain will be stored before use. Remember that the number of days the grain has been stored before you purchase it will affect the total Safe Storage Days.

When To Aerate Grain?
Because moisture and heat can accumulate in hot spots within the grain mass, aeration should be initiated to distribute or remove concentrations of heat and moisture. Accumulations of moisture cannot be detected, but the rise of temperature associated with moisture and spoilage can be detected with temperature detection systems.

Aeration should be initiated on a regular schedule of preventative maintenance in order to equalize the moisture and temperature within the grain mass. The temperature of the grain mass should be maintained equal to the temperature of the outside air. Because temperature differences greater than 9oC between the air and the grain mass can create condensation and moisture in the grain, aeration should only begin when the difference between the grain and the outside temperature is less than 9oC. A regular schedule of aeration maintenance will keep the grain close to the average outside temperature.

The aeration objective is to maintain the grain at a uniform temperature of +/- 2 degrees of the average 30-day daily temperature.

Aeration Rates
How much air is needed to aerate grain in storage? The most common measurement of air is in cubic-feet-per-minute. Typical ratios of air to grain range between 0.05 to 0.20 cubic-feet-per-minute per bushel of grain. This would be the equivalent of 0.054 to 0.222 cubic meters of air per minute for each metric ton of grain. It is most common to design for 0.10 to 0.14 CFM/Bu (0.114 to 0.162 m3/min/ton). If you receive a proposal for an aeration system designed to Imperial measurements, look for air volumes close to 0.10 (1/10) CFM/BU or higher. Each system manufacturer will try to determine the most efficient system, taking into consideration available equipment, the volume and depth of the grain, and the most cost efficient system to purchase and operate.

How To Aerate Grain
The amount of time required to aeration a grain mass will depend upon the volume of air and the volume of grain. Given 0.10 (1/10) CFM/Bu of grain, the time required to aerate the grain mass is 100 hours. In this time, the temperature of the grain mass will rise or fall to the same temperature as the outside air.

If the volume of air is reduced by one-half to 0-.05 (1/20) CFM/Bu., the time required will double to 200 hours. If the ratio of air to grain remains constant, the size of the grain mass is not important (R) the amount of time is relatively fixed.

Aeration Front
Understanding how the air moves through a grain mass is important to understanding how aeration works. Typically, air is pushed into the bottom of the grain mass, and exits the top of the grain mass. As the air moves, it creates a ¡ã¡Þfront¡ã¡À. Grain temperature below the front is equal to the outside ai The front collects temperature and moisture for transport through the grain mass. When the front completely passes through the grain mass, aeration is complete.

Aeration Problems And Solutions
1. Aeration Problems - A common problem that can occur during aeration is a front that stops. If someone stops the aeration fan midway through the aeration process, the moisture and heat- laden front will stall. When the front stalls, it despots the carried moisture and heat in the grain. When aeration is started the second time, the front will again begin at the bottom of the bin, while the heat and moisture deposited above begin to spoil the grain. If the new front does not reach the old front in time spoilage in the old front will create a crust that prevents the movement of air, preventing any additional aeration. The grain must be removed from the storage structure.

Aeration Solution - After beginning an aeration cycle, do not stop the aeration fan(s) initial the aeration cycle is completed.

2. Aeration Problem - Center Fines or Foreign Material (FM). FM is anything that is not whole grain. FM can be broken grain, other organic parts of the grain plant, or weed seeds. Generally, FM has less weight than whole grain. While filling the grain storage structure, the FM will fall into the center of the structure and the heavy good grain will roll or slide to the outside of the structure. The FM in the center of the storage structure can form a column through which air will not pass. This column will spoil very quickly, spreading to the good grain. When the storage structure is unloaded through the bottom, the column will prevent grain from flowing out.

? Aeration Solution #1 (R)C During and after filling the storage structure, you should remove portion of the grain. This will extract the center core. The extracted grain can be processed by cleaning, or by grinding immediately for feed.

? Aeration Solution #2 (R)C During filling, you can use a mechanical spreader to distribute the F in a wider area, preventing the buildup of a center core. The FM will remain in the grain, and can continue to contribute to premature spoilage of the grain.

? Aeration Solution #3 (R)C Clean the grain before filling the storage structure. This requires mechanical cleaner or mechanical screener that will separate good, whole grain from any form of FM. This is the best solution for long-term storage. Whole grain stores much better than FM, and permits easier aeration of better quality.

3. Aeration Problem (R)C An obstructed aeration air delivery system will prevent good aeration. Eve with the fan running, air cannot pass a dirty obstructed tunnel or perforated floor. Vents that are plugged will prevent air from exiting the storage structure, stopping all aeration.

Aeration Solution (R)C Before filling the storage structure, inspect in the tunnels under the aeratio floor for an accumulation of broken, spoiled grains that obstruct air movement in the tunnels. If necessary, clean and remove all FM. This will also remove insects, insect eggs, and their habitat. Also inspect roof vents for any obstructions, and clean if necessary.

4. Aeration Problem (R)C Over filling the storage structure, and grain too close to the roof or touchin the roof. When too much grain is put into a storage structure, two problems are created. 1) Not enough space remains for the movement of air to the exhaust vents during aeration; 2) Grain comes in contact with the roof, where condensation can occur. Condensed moisture will collect at the edge of the roof, and saturate the outside of the grain mass. Spoilage will occur in this area.

Aeration Solution (R)C Do not overfill the storage structure

5. Aeration Problems (R)C Examples of aeration problems. This storage structure has been over filled Grain has blown out of the vent onto the roof of the silo and spoiled; grain remains stuck in the vent and is obstructing airflow; grain has spoiled and stuck to the walls of the storage structure. The spoiled grain will be difficult to remove, and can affect the new grain when the storage structure is filled again.

Aeration Conclusions

a.Grain should be aerated to

i.Maintain uniform temperature in the grain mass.
ii.+/- 2oC Average Daily Ambient Temperature (day and night).
iii.Regularly (R)C every 30 days recommended
iv.When the difference between the grain temperature and the outside are do not exceed 9o'bcC.

b.When you start an aeration cycle, do not stop the aeration fans until the aeration front passes entirely through the air mass.

i.You may use the temperature detection system to determine when the aeration cycle is complete.
ii.Only if necessary, you can aerate by time if you are confidant that you know the aeration rate (0.10 CFM or 1/10 CFM per bushel) and that your aeration system is performing efficiently.

Tags · Grain Storage · Quality · Storage · Handling · Feed Ingredients

17.02.2008. 13:09