Maintaining Corn Quality for Wet Milling
AE-1119, September 1996
Dr. Kenneth J. Hellevang, PE,
Extension Agricultural Engineer, North Dakota State University
Dr. William F. Wilcke,
Extension Agricultural Engineer, University of Minnesota
"Corn producers have some control over corn quality through variety
selection, timing and care used in harvesting, selection and operation of dryers and
conveyors, and storage management."
Desired Quality for Wet Milling
Harvesting
Holding Wet Corn
Selecting and Managing Dryers
Cooling Corn from High-Temperature Dryers
Combination Drying
Natural-Air and Low-Temperature Drying
Storage Management
Measuring Moisture Content
Conveying and Handling
Grain Sampling
Summary
Other Drying and Storage Information Available
Desired Quality for Wet Milling
Desired corn characteristics for wet milling include uniform kernel size, high kernel
integrity (no scratches, holes, cracks or breaks), high test weight, good starch quality,
uniform moisture, soft endosperm, and no mold. The milling industry estimates that poor
corn quality causes about a 5-10% reduction in milling capacity. Corn producers have some
control over corn quality through variety selection, timing and care used in harvesting,
selection and operation of dryers and conveyors, and storage management.
Obtaining quality corn for wet milling starts with selecting corn varieties that will
reach maturity prior to frost. Frost-damaged corn has small kernels, a low test weight,
poor starch quality, and is often susceptible to cracking and breaking.
Test weight is a bulk density measurement; it is the number of pounds of grain in a
volume bushel (1.244 cubic feet). In addition to being affected by maturity, test weight
is affected by corn variety, growing season, and corn moisture content. Test weight is
also affected by drying temperature and drying rate. Generally, lower drying temperatures
and slower drying rates result in higher test weights. Slower cooling rates also typically
give higher test weights. Test weight usually increases during drying in a high
temperature dryer by about 0.25 lb/bu per percentage point of moisture reduction.
Cracks in the kernel affect the steep time, cause non-uniform steeping, and cause more
starch to be lost in steepwater during processing. Mechanical cracking and breakage occur
during harvesting and rough handling. Stress cracks are caused by fast drying with high
drying temperatures followed by fast cooling. Stress cracks and over drying increase
kernel breakage during handling. Since some corn varieties are more susceptible to
cracking and breaking, this characteristic needs to be considered in selecting the variety
to plant.
Take care at harvest to limit the amount of kernel damage. The optimum moisture content
for limiting mechanical damage during harvest is about 22%. Increased damage occurs both
below and above this moisture content. Combine cylinder speed and cylinder-concave
clearance are also important factors determining mechanical damage at harvest. Damage
increases with increasing cylinder speed.
Follow the recommendations in the operators manual for combine settings and recommended
harvesting procedures. Know why you must make an adjustment before doing it. Make only one
adjustment at a time, checking the results before making another adjustment. Check for
grain losses and damage frequently, particularly as harvest conditions change.
To avoid mold damage while holding wet corn, it is necessary to keep the corn cool. An
aeration system delivering a uniform airflow of about 0.5 cfm/bu with cool outdoor
temperatures is needed to carry away heat generated by corn and mold respiration. The
approximate allowable storage time for corn is shown in Table 1. The allowable storage
time is approximately doubled by reducing the corn temperature by 10�F.
Table 1. Approximate allowable storage time for shelled corn based on 0.5% maximum dry
matter loss. (Transactions ASAE 333-337, 1972)
------------------------------------------------------------
Grain - - - - - - - - - Corn Moisture (%) - - - - - - - -
Temp. 18 20 22 24 26 28 30
------------------------------------------------------------
(F) - - - - - - - - - - - - Days - - - - - - - - - - -
30 648 321 190 127 94 74 61
40 288 142 84 56 41 32 27
50 128 63 37 25 18 14 12
60 56 28 17 11 8 7 5
70 31 16 9 6 5 4 3
80 17 9 5 4 3 2 2
------------------------------------------------------------
Selecting and Managing Dryers
The kernel temperature should not exceed about 140�F during drying to prevent damage
to milling quality. At temperatures exceeding about 140�F, milling efficiency is reduced
because of starch gelatinization, protein denaturation, and proteolytic denaturation.
Also, the germ of the kernel is damaged causing a loss of viability. Some wet millers use
a germination test to assess corn quality.
The kernel temperature reached during drying depends on the initial moisture content,
drying air temperature, the type of dryer and dryer management. In some dryers, the kernel
temperature of part of the corn reaches the drying air temperature. Corn harvested at
higher moisture contents will be in a dryer longer, so there is more potential for some of
the kernels next to the inside of the drying column to become excessively hot in some
types of dryers.
Dryers should be selected and operated to produce as uniform a final moisture as is
possible. Uniform corn moisture facilitates management of the steeping process.
Column Dryers
A cross-flow dryer, shown in Figure 1, is the most common type of dryer used. It is
referred to as a cross-flow dryer because the heated air moves across the grain column
perpendicular to the flow of the grain. The grain moisture content and temperature varies
across the column of a cross-flow batch dryer as shown in Figure 2. Grain near the inside
of the column is at a lower moisture content and a higher temperature than grain near the
outside of the column. Therefore, even though the average moisture content may be at 14%,
the moisture content of grain near the inside of the column may be 6 to 10%, and grain
near the outside of the column may be at 16 to 28% moisture. Also, even though the average
temperature may be at 140�F, grain near the inside of the column may be at 200�F, and
grain near the outside of the column may be at 80�F.
Figure 1. Cross-flow dryer with forced-air drying
and cooling. (9KB b&w diagram)
Figure 2. Example of different temperature and
moisture zones in a cross-flow dryer. (7KB b&w diagram)
The temperature of corn in a dryer increases as the corn dries. At higher moisture
contents, the kernel is kept cool by the cooling effect of moisture evaporation. In many
dryers, grain temperature is used to determine when the grain is dry.
A cross-flow dryer used for drying corn for wet milling should be operated at moderate
temperatures or include features to minimize variation across the column. A recirculating
batch dryer is one way to mix the grain to create more uniform drying (Figure 3). Some
continuous cross-flow dryers use a grain turner that moves corn from the inside of the
column to the outside, and corn from the outside of the column to the inside (Figure 4).
This minimizes the amount of time that the corn is adjacent to the inside of the column.
Figure 3. Recirculating batch dryer. (26KB b&w diagram)
Figure 4. Grain turner used to mix grain in
column-type dryers. (40KB b&w diagram)
Another feature that can minimize variation across the drying column is having the
drying columns tapered, so the grain on the plenum side of the column moves faster than
grain near the outside of the column. In multi-stage, continuous-flow, cross-flow dryers,
the top stage, which contains the wettest corn, can be operated at higher temperatures,
and the bottom stages, which contain drier corn, can be operated at lower temperatures.
Some cross-flow dryers use tempering sections to limit the exposure to high drying-air
temperatures. Tempering sections installed between the drying section and cooling section
of a continuous-flow dryer reduce the potential for stress crack formation during cooling.
Although lower drying temperatures give better corn quality, they unfortunately
decrease drying capacity and energy efficiency. The energy required to remove a pound of
water from corn at various airflow rates and drying temperatures is shown in Figure 5. The
airflow rate selected is a compromise between energy efficiency, dryer capacity, average
grain temperature, and moisture variation across the column. Using a higher airflow rate
results in a higher drying rate and less variation in temperature and moisture across the
column, but it increases the energy requirement. Higher airflow rates also increase the
average grain temperature, as shown in Figure 6. Some cross-flow dryers utilize air
recirculation to increase energy efficiency (Figure 7).
Figure 5. Energy requirements of a conventional
crossflow dryer as a function of drying air temperature and airflow rate. (University of
Nebraska) (5KB b&w diagram)
Figure 6. Final grain temperature in the three basic
continuous-flow drying methods. (8KB b&w diagram)
Figure 7. Exhaust air recirculation to improve
energy efficiency. (17KB b&w diagram)
In concurrent-flow dryers, airflow enters the wet grain and travels in the same
direction as the grain. This results in a much lower grain temperature (see Figure 6). A
two-stage concurrent-flow dryer with cooling is shown in Figure 8.
Figure 8. Two-stage concurrent-flow grain dryer. (19KB b&w diagram)
In the mixed-flow or rack type dryer, the grain flows over alternating rows of air
supply ducts and air exhaust ducts (Figure 9). This action provides mixing of the grain
and alternate exposure to hot drying air and air which has been cooled by previous contact
with the grain. This promotes moisture uniformity and limits grain temperature.
Figure 9. Mixed-flow grain dryer (14KB b&w diagram)
Bin Dryers
High-temperature batch-in-bin drying involves using a bin as a batch dryer. A 3 to 4
feet deep layer of corn is placed in the bin and the fan and heater are started. Typical
drying air temperatures are 120 to 160�F with airflow rates of 8 to 15 cubic feet of air
per minute per bushel of grain (cfm/bu.) Drying begins at the floor and progresses upward.
Grain at the floor of the bin becomes excessively dry while the top layer of the batch
remains fairly wet. As the grain is moved from the bin, the grain is mixed so the average
moisture content going into storage is acceptable. There are variations, however, in grain
temperature and moisture content in the dried corn.
A stirring device can be added to batch-in-bin dryers to provide a more uniform
moisture content and corn temperature. Stirring will also increase the airflow, which
increases the drying speed. Stirring allows depths of up to 6 to 8 feet for corn. Grain
stirrers tend to sift fine materials to the bin floor, so it is important to clean the bin
floor between batches.
A recirculating bin dryer incorporates a tapered sweep auger which removes grain from
the bottom of the bin and places it on the top of the corn in the bin (Figure 10). This
creates more uniform drying.
Figure 10. Grain recirculators convert a bin dryer
to a high speed recirculating batch or continuous flow dryer. (7KB
b&w diagram)
A continuous-flow bin dryer also incorporates a sweep auger which removes the corn on
the bottom of the bin when it is dry. Cooling occurs in a separate bin. Counter-flow
drying occurs in these dryers (Figure 10). The airflow moves from the driest grain on the
bottom to the wettest grain on the top. Because all kernels approach the drying air
temperature in this type of dryer, the drying temperature needs to be reduced to prevent
damaging the grain.
Be aware that increasing the grain depth reduces the airflow rate (cfm) and therefore
the drying rate of in-bin dryers.
Cooling Corn from High-Temperature Dryers
Dryeration involves tempering, then cooling the grain slowly in a bin rather than in
the dryer to achieve a large reduction in breakage susceptibility. Other advantages of
dryeration include about 2 percentage points of moisture removal, a 20 to 40% energy
savings, and a 50 to 75% increase in dryer capacity. The dryer capacity increases because
the corn is only dried to about 17.5% moisture and cooling time is eliminated. The amount
of moisture removal is related to the amount of cooling that occurs. About 0.25 percentage
point of moisture is removed for each 10�F of cooling. For example, about 2 percentage
points of moisture removal would be expected when cooling corn from 130 to 50�F; [(130 -
50) / 10 x 0.25 =2.0].
With dryeration (Figure 11), the corn is moved directly to a cooling bin, is allowed to
temper without airflow for 4 to 6 hours, and then is cooled over a 12 to 24-hour period.
An airflow rate of 0.5 to 1.0 cfm/bu is normally needed to cool the corn within 24 hours.
Condensation forms along the bin wall, which rewets some corn during the tempering period.
The corn must be moved from the cooling bin to prevent this wet grain from spoiling. The
wet corn is mixed with dry corn as it is moved into storage.
Figure 11. Schematic of dryeration system. Grain is
dried in a high speed dryer to 1-2% of safe storage moisture content. The hot grain is
moved to the dryeration bin where it sits without airflow for 4 to 6 hours. After cooling,
the grain is moved to storage. (11KB b&w diagram)
In-storage cooling is different than dryeration. The grain is moved directly to the
storage bin without cooling in the dryer, but unlike dryeration, the grain is cooled
without delay. Size the fan to provide an airflow rate of 12 cfm per bushel per hour of
dryer capacity or fill rate. For example, if hot corn is being added to the bin at the
rate of 500 bushels per hour, size the fan to provide 6,000 cfm of airflow. The air should
flow upward through the corn, so additional hot corn can be added to the top to be cooled
without reheating the corn below.
Advantages of in-storage cooling over cooling in the dryer include increased dryer
capacity of 20 to 40% in a batch dryer, and in a continuous flow dryer if the cooling
chamber is used as a drying chamber. There is, however, some potential for condensation
along the bin wall and on the underside of the bin roof, which can lead to grain spoilage.
The potential for condensation increases as the cooling rate decreases, or if cooling is
not started immediately. There is not as much moisture removal or as large a reduction in
cracking potential with in-storage cooling as with dryeration, but you do not need to move
the corn after cooling.
Combination drying combines some high-temperature drying and some low-temperature or
natural-air drying. Advantages of combination drying include increased high-temperature
dryer capacity, improved corn quality, and improved energy efficiency. Also, higher
moisture contents can be dried than are allowable with natural-air and low-temperature
drying alone. With combination drying, corn may be harvested at moisture contents in
excess of 20%, dried in the high temperature dryer to about 20%, then, without any
cooling, moved to a bin equipped for natural-air or low-temperature drying to finish
drying.
Natural-Air and Low-Temperature Drying
Natural-air and low-temperature (NA/LT) crop drying (Figure 12) maintain the high
quality of the corn, do not require constant supervision, are energy efficient, and do not
limit harvest capacity. A drying fan is required for each bin, and the initial moisture
content that can be dried in a full bin is limited to about 21%. An airflow rate of 1.25
cfm/bu will dry 21% moisture content corn to about 15% in about 36 days under average
Upper Midwest October conditions.
Figure 12. A typical bin dryer utilizing natural
air/low temperature drying. (13KB b&w diagram)
NA/LT drying works very well during October but is not efficient with typical mid to
late November weather conditions. It will take about 70 days to natural air dry 21%
moisture content corn to 18% under November conditions with an airflow rate of 1.25
cfm/bu. The corn only dries to 18% moisture content, because that is the equilibrium
moisture content (EMC) of corn for November conditions. Heating the November air by 5�F
reduces the EMC to 14.6% moisture and reduces the drying time to 52 days. However, since
November has only 30 days, 43% of the corn still is not dried after running the fan and
heater the entire month of November. Adding more heat will overdry the corn without
substantially increasing the drying speed. A stirring device in the bin is required if
more than a 5�F temperature rise is used.
The drying fan will warm the air 3 to 5�F depending on fan type and operating
conditions. This needs to be considered in designing a system.
The following equation can be used to size a heater:
Btu/hr = cfm x 1.1 x Temperature Increase (F)
For example, to heat 10,000 cfm of airflow by 5�F requires a 55,000 Btu/hr heater.
To convert to kilowatts (kW) for an electric heater, divide the Btu/hr by 3,413. A 16
kW electric heater is equivalent to the 55,000 Btu/hr.
Corn can be held over winter and dried in the spring. Based on average April conditions,
21% moisture corn can be dried to about 15.3% in 41 days using an airflow rate of 1.25
cfm/bu. Based on average May conditions, the corn can be dried to about 13.5% moisture in
35 days using the same airflow rate. This is a good option instead of drying during late
November, if the corn does not need to be delivered before spring. Cool the corn to about
25�F for winter storage.
Higher airflow rates and larger amounts of heat increase the cost of drying (Table 2).
With an electrical cost of $0.06/kWh, the cost per bushel to dry 23% moisture corn with a
3�F temperature rise is $0.11/bu with an airflow rate of 1 cfm/bu, $0.16/bu with an
airflow rate of 2 cfm/bu, and $0.22/bu with an airflow rate of 3 cfm/bu. Drying speed is
related to the airflow rate. Since doubling the airflow rate requires increasing the fan
horsepower by about five times, the cost of drying increases with higher airflow rates.
Costs vary depending on climatic and operating conditions, so these values should be
considered estimates.
Table 2. Estimated energy required to dry corn.
------------------------------------------------------
Temperature Final Airflow Rate (cfm/bu)
Increase (F) Moisture 1 2 3
------------------------------------------------------
(F) (%) - - - - kWh/bu - - -
3 14.9 1.8 2.7 3.7
10 11.9 2.5 4.5 6.0
20 10.0 5.0 7.5 10.4
30 8.0 5.3 7.9 11.1
------------------------------------------------------
Initial corn moisture content = 23%
Outside air conditions = 45 F, 80% relative humidity
Resistance to airflow, static pressure, and fan horsepower requirements may become
excessive at higher airflow rates. For example, for a 21-foot diameter bin filled 18 feet
deep with corn, at 1 cfm/bu the static pressure is about 3.2 inches and the fan horsepower
needed is about 5 hp. At 2 cfm/bu, the static pressure is about 8.7 inches and the fan
horsepower needed is about 27 hp. At 3 cfm/bu, the static pressure is about 16 inches and
the fan horsepower needed is about 76 hp.
Corn wetter than 21% can be dried by filling the bin only part full, which results in a
higher airflow rate. This can be done by adding batches of 4 to 6 feet, with progressively
drier corn added when the corn in the bin is dry (Figure 13).
Figure 13. Example of layer drying. The higher
airflow rates on a per bushel basis early in the filling permit a higher initial moisture
content to be loaded. (7KB b&w diagram)
A fully perforated drying floor is recommended for in-bin drying. The corn should be
cleaned before it is placed in the bin to eliminate fines and "bees wings" which
reduce airflow. The top surface needs to be leveled, so uniform airflow and drying is
achieved. If the corn is peaked, air will tend to flow near the edges and corn in the peak
will dry much slower. A grain spreader helps level the grain surface.
Corn needs to be dried to a safe storage moisture and then cooled by aeration during
storage to prevent mold growth and limit insect activity. Molds consume corn dry matter,
produce odors, and sometimes produce toxins. Corn should be dried to 15.5% for short-term
storage over winter, to about 14% for one-year storage, and to 13.5% for long-term
storage.
Grain stores best if kept cool and dry. Optimum temperatures for insects and mold are
between 70 and 90�F. At grain temperatures below 40�F, insect and mold activity is
limited. Corn should be cooled, using aeration, to about 25�F for winter storage to
minimize moisture migration and to enhance storability.
Stored grain should be checked at least monthly. Check the corn temperature and
moisture content at several locations and record the information.
Cover fans and air ducts when not in use to prevent rodents and moisture from entering
and to prevent excessive warming in the spring.
A representative sample must be used to determine the moisture content of a load of
grain. Also, the moisture content should be uniform in the kernel. Most meters are
affected by the moisture content of the outside surface of the kernel, so if the outside
is drier than the inside of the kernel, such as when corn comes directly from a dryer, the
meter will give an erroneously low reading. A temperature adjustment must be used if the
sample is not at the standard temperature, which is usually about 75�F. A moisture meter
should be checked against a reference, such as where the grain is marketed or other
meters, periodically to assure that accurate readings are being provided.
It is difficult to accurately measure the moisture content of hot grain. It is best to
cool the samples slowly in a sealed moisture-proof container before checking the moisture
content. By comparing the difference between the moisture content of a cooled sample and a
sample immediately out of the dryer, an adjustment factor can be developed and used as an
estimate for managing the dryer. It is only an estimate, since the adjustment factor will
vary depending on initial moisture content, drying rate, and other factors. Remember to
add or subtract the temperature correction factor for your moisture meter, if your meter
doesn't have automatic temperature compensation.
Select conveyors that are gentle on the grain and operate them in manner to reduce
damage. Augers are not a primary source of grain damage if operated properly. Reducing
auger speed and operating the auger at full capacity greatly reduces the risk of kernel
damage. Damage can occur when grain is pinched between the auger flighting and casing, so
select and maintain the clearance either greater or less than the corn kernel thickness.
Drop height should be minimized during grain transfer. Consider installing grain
decelerators for heights exceeding about 40 feet.
Connections between pipe sections need to be smooth in a pneumatic system, and corners
in piping should be made with a large radius turn to minimize kernel damage. Grain damage
is related to conveying velocity. Research indicates that air velocity should not exceed
about 5,000 feet per minute to minimize grain damage. Maintain the manufacturer's
recommended air-to-grain volume ratio to minimize kernel damage. Also, use a grain
decelerator at the discharge.
Corn breakage susceptibility during handling increases as moisture contents decrease
below about 14%. Breakage susceptibility is also higher for colder grain. Handling corn
with kernel temperatures below about 0�F will increase the breakage potential.
Accurate grain sampling is important because information from the sample is used to
establish the quality characteristics and the value of the grain. Grain stream sampling
done properly at the farm will permit knowing the characteristics of the corn in storage.
Tests have shown that the following requirements must be met if an endgate sample of grain
is to be representative:
- Sampling device must collect grain from the entire grain stream without overflowing.
- Samples must not be taken from the first or the last portions of a load.
- The entire grain stream must be cut (sampled) with a side-to-side sweep of the sampler,
cutting the full thickness of the stream – front to back.
- Take at least two samples from the load at random intervals.
To produce corn having the quality needed for profitable wet milling:
- Select corn varieties that have low breakage susceptibility and are mature with low
moisture contents before frost.
- Harvest at corn moisture contents and with combine settings that result in minimum corn
kernel damage.
- Select and manage dryers to keep kernel temperature below 140�F and achieve uniform
corn moisture.
- Dry corn to 15.5% moisture for winter storage, 14% for storage through summer, and to
13.5% for long-term storage.
- Cool corn to about 25�F for winter storage.
Other Drying and Storage Information Available
Available from Distribution Center, North Dakota State University (701) 231-7882.
AE701 Grain Drying
AE791 Crop Storage Management
AE808 Crop Dryeration and In-Storage Cooling
AE850 Pneumatic Grain Conveyors
AE905 Grain Moisture Content Effects and Management
AE923 Calculating Grain Drying Cost
AE945 Equivalent Weights of Grain and Oilseeds
AE1044 Grain Stream Sampling and Sampler Construction
EB-35 Natural Air and Low Temperature Crop Drying
EB-45 Insect Pest Management for Farm Stored Grain
MWPS-13 Grain Drying, Handling and Storage Handbook
NCH-14 Energy Conservation and Alternative Energy Sources for Corn Drying
Available from University of Minnesota Distribution Center, (612) 625-8173.
MN BU-6577-E Natural-Air Corn Drying in the Upper Midwest
MN AG-FO-1324 Dryeration and In-Storage Cooling for Corn Drying
MN AG-FO-1327 Management of Stored Grain with Aeration
MN AG-FO-5716 Selecting Fans and Determining Airflow for Crop Drying, Cooling, and Storage
Available from Biosystems and Agricultural Engineering Department, University of
Minnesota, (612) 625-9733
FANSA Fan Selection Computer Program
AE-1119, September 1996
|
