IV U LB i tDY.
[:L- i 9 l98o
IF.A.S.- Univ. of Florida
DRYING AND STORAGE
. . ..
The sixty thousand acre Florida peanut crop has a very high
per acre value. It is important that care be taken to harvest
the largest amount of peanuts possible and to market a product of
the best possible quality.
Even though the in-field cost of production is continually
increasing, the cost of harvesting, drying, storing, transporting
and selling the crop remains a major component of the total cost
of production. Reduction of harvest and postharvest (drying and
storage) losses is just as important as increasing yields through
improved production practices. This is especially apparent when
considering that all the cost inputs and energy inputs for seed,
fertilizer, lime, herbicides, insecticides, fungicides, etc., are
wasted for that portion of the crop which is lost during harvest
and post harvest operations.
The harvesting and drying operations have more to do with
peanut quality than most other phases of production. Improper
dryer management either by the farmer or commercial dryer can be
responsible for ruining the quality of the crop after it is dug.
Proper harvesting, drying and storage management procedures
for improved quality and increased peanut production are outlined
in this bulletin.
Harvesting is one of the most critical operations in peanut
production. Harvesting is a broad term used to describe all
procedures for gathering peanuts from the soil prior to
processing the peanuts for storage. Recommended mechanized
peanut harvesting procedures include field preparation, vine
clipping, digging, shaking, re-shaking, windrowing and combining.
A key to proper harvesting is having the right equipment in good
operating condition at the time needed.
Areas for peanut production should be managed and main-
tained for efficient machinery operation. Smooth, well-drained
sands or loamy sands are easily managed and prepared for optimum
Field preparation for efficient peanut harvesting begins
before planting. Good crop rotations and deep turning for deep
burial of trash during land preparation will reduce disease and
pest problems and enable the farmer to select a digging date
rather than being forced to dig because of a disease or pest
Preshaped beds greatly contribute to harvest efficiency and
can reduce plant disease by preventing water from standing around
the base of the plants. Bedding provides a firm wheel middle and
soft, unpacked soil media for optimum peanut production and
recovery at harvest.
Maintaining good leaf spot control, good weed control, and
otherwise producing a healthy crop of peanuts are important to
Additional field improvements may be obtained by keeping
field borders free of weeds and grass during the growing season.
This may be accomplished either mechanically or chemically.
Border areas should be soft and litter free at harvest time to
allow complete digger blade penetration before blades reach the
rows. A clean, sharp blade cuts peanut tap roots, while a dull
blade drags and pulls pods off under the soil.
When To Dig
Determining when to dig is important since peanut maturity
greatly influences yield, quality, sound mature kernel content
and subsequent dollar value per ton.
The most commonly used method of determining peanut maturi-
ty is the shell-out method. This involves the removal and shel-
ling of all the pods from several plants randomly selected from
each field. The average days to maturity (135 days for Florunner
and 120-125 days for Early Bunch) are used as a guide to deter-
mine the development stage of the nuts. After shelling, the
percentage of hulls, that are turning dark on the inside, is
determined. When 80% of the pods are dark, digging should begin.
The hull-scrape method is a more recent method which
involves removal and scraping the outside hull of all the pods
(white, soft, watery pods are removed but not scraped) from
several plants randomly selected from each field. All the pods
are separated into maturity groups on the basis of the color of
the pod determined by scraping and the pod physical characteris-
tics. The number of pods in each maturity group is used to
calculate the percentage of black pods and black plus brown pods,
which in turn are used with a nomograph maturity guide chart to
determine the approximate number of days until harvest.
For detailed information concerning either method mentioned
above, or other assistance in determining maturity, a county
cooperative extension agent should be contacted.
The actual number of days to maturity for a particular
variety will vary with the date of planting, leaf spot control,
light and heat intensity, soil type, rotation, and the amount and
distribution of rainfall. The time to ideal maturity will often
be longer than the averages if leaf spot is controlled or if
adequate moisture for pegging is delayed.
It is common for peanut yields to increase more than 10
percent during the last 10 to 14 days prior to optimum maturity
time. Digging too early should be avoided, since yield and grade
will be sacrificed. Naturally, it may be necessary to dig the
peanuts earlier than this optimum maturity time if there is a
weakening of the pegs, excess pod rot, white mold or other
disease problems or if the weather dictates.
Vines may need to be clipped in those fields that have an
excessive amount of weed and vine growth. This can be
accomplished by a rotary or flail type cutter with sharp blades
used to clip the top 1/3 or 1/2 of the vines, 3 or 4 days before
digging which will allow time for leaves and stems to wither and
fall to the ground.
Excessive clipping should be avoided because the efficiency
of inverting-type shakers would be reduced. Generally, vine
clipping is not necessary with present-day equipment. However,
in instances where there is excessive weed or vine growth,
harvesting efficiency can be increased by removing excessive
foliage prior to digging. Removal of this excessive foliage will
also allow the peanuts to dry more quickly in the field.
Digging losses represent a very high percentage (as much as
80%) of total harvest losses. Consequently, reducing digging
losses is a major consideration toward increased harvest effi-
ciency. Plant diseases can have a tremendous effect on digging
losses. White mold weakens the stems and causes nuts to pull off
in the ground. Leaf spot may also weaken stems, increasing
Both soil texture and moisture are critical factors in dig-
ging efficiency. High moisture can cause excessive digging
losses, especially in soils that are relatively heavy. Moisture
makes the soil stick together and requires more force to pull the
nuts out. This causes the stems to break and the nuts to remain
in the ground. Peanuts should be dug when the soil moisture is
relatively low so that the soil crumbles above the digging
Diggers should be inspected, adjusted and the blade
sharpened before going on the field. Sharp, rigid blades with a
very slight pitch provide optimum cutting action when they are
set to cut the taproot just below the pods. The digger-shaker
speed is critical and will vary widely, becoming a key factor for
the inverting shakers. The travel speed should be sufficient to
produce a flow of the soil and vines over the blades and through
the shaker. Excessive speeds may strip pods from the vines while
slow speeds may not allow soil to flow properly over the blades.
Digger-shakers should provide adequate agitation to remove
soil from the pods and roots. The speed of the shaker conveyor
chain should be slightly faster than the forward speed of the
digger-shaker. This prevents a pileup of vines ahead of the
pickup point. Correct chain speed should lift the peanut gently
from the ground while excessive chain speed tends to snatch the
vines, leaving the pods in the ground.
Windrowing rods or inverting bars should be adjusted to
form a uniform, fluffy and well aerated window with no vines
remaining in the wheel middle.. Pods should not touch the soil.
Prolonged exposure during hot, dry periods may result in over-
drying of pods in contact with soil in full sunlight. Even with
the tunnel type of window, pods should be off the ground. When
re-shaking is necessary, a low shaker with very gentle action
should be used with extreme care.
Combining efficiency depends upon several variables
including window condition, cylinder speed, forward travel
speed, and internal combine adjustments. The quality of peanuts
for seed and market is greatly influenced by the operators's
proficiency in the use of the peanut combine.
Peanuts are normally between 35% and 50% moisture at
digging. However, harvest under these.conditions is practiced
only where ample drying facilities are immediately available.
Even if drying facilities are available, the drying cost and
drying time will be greater because more fuel and time are needed
to dry these more moist peanuts. At high moistures, the vines
and stems may be too tough for good pod removal and too much pod
damage along with too many loose shelled kernels can result.
Generally peanuts are allowed to remain in the window for
2 to 4 days, depending upon weather conditions. The moisture
content at combining is usually between 18% and 24% moisture and
the peanuts will rattle in the hull. Mechanical damage at this
moisture range is less severe than for very moist or dry peanuts.
Excessive field losses will result if peanuts are allowed
to field dry to below 18% moisture. Attempting to field dry
peanuts to 9% so that mechanical drying will not be required is
usually unprofitable. Combine losses increase sharply as the
vines, stems, and pods become drier. The increased harvest
losses will generally cost more than what mechanical drying would
have cost at higher moisture contents. Also, leaving the peanuts
in the window extended periods of time increases the chance for
exposure to adverse weather, insect infestation and aflatoxin
Combining requires an experienced operator. A poorly
adjusted combine can result in excessive losses and damage. The
manufacturer's recommended speed and adjustments should be
followed as closely as possible. Adjustments are also necessary
as field moisture and picking conditions change during the day.
The best measure of combining efficiency is the quality of
peanuts coming in to the combine bin rather than the amount
harvested in a given time. Speed can be the enemy of peanut
quality. Fast moving combine parts may damage a high percent of
hulls and kernels. Both visible and non-visible damage opens the
door to insect and mold infestations.
Proper drying (curing) of peanuts is an important part of
the overall production program. Along with the harvest opera-
tions, proper peanut drying has more influence on peanut quality
than any other operation. Adequate drying procedures must be
used to maintain desirable flavor, texture, germination and
Natural vs. Mechanical Drying
Peanut kernel moisture at digging normally ranges from 35%
to 50%. For safe storage this moisture must be reduced to 8% to
10%. The drying process must begin immediately after digging to
prevent molding and spoilage under normal harvest weather condi-
tions. Moisture must be removed at the proper rate to success-
fully dry peanuts. If moisture removal is too slow peanuts will
mold, while drying too rapidly will cause lowered peanut quality
due to off-flavor and hull-splitting.
Natural drying in the window will not adversely affect
quality if climatic conditions are favorable. Mild temperatures
and relatively dry air without rain for 5 to 10 days are favor-
able natural drying conditions. However, these conditions do not
occur in combination for sufficient periods of time over the
peanut producing areas of Florida to harvest the entire peanut
crop. The ever-present threat of adverse weather during the
natural drying process makes window drying risky. Rain during
the last days of the process encourages pod loss and splitting,
hull discoloration, and serious attack by mold and insects.
A combination of natural and mechanical drying offers the
best approach for Florida's peanut producing areas. Using this
method, peanuts are left in the window from 2 to 4 days after
digging or until kernel moisture content ranges from 18% to 24%.
At this moisture content the kernel will rattle in the hull.
Once this moisture range is obtained, peanuts should be combined
and placed in mechanical dryers for final drying.
Mechanical Peanut Drying
For the mechanical drying systems to operate properly, an
understanding of some of the principles of heated air drying is
necessary. Peanuts are placed in the drying bin or trailer and
the drying process is begun. Air at the desired temperature,
relative humidity and volume is forced up through the peanuts. A
drying zone is created in the trailer at a point where the air
enters. As the drying process continues, this drying zone moves
upward through the batch of peanuts until it reaches the top
layers, thus completing the drying process. In Figure 1, peanuts
above the drying zone are still at relatively high moisture
content while those below the drying zone are dry enough for safe
storage. The drying process must be continued long enough for
the drying zone to move upward through the entire bin of peanuts
and dry the top layer to a safe storage level. The combination
of drying air temperature, relative humidity, and flow rate
determines the degree of drying during a particular drying time
for a load of peanuts at a given initial moisture content. Pro-
longed exposure to drying air with a high temperature, low rela-
tive humidity and high flow rate can cause overdrying which
decreases overall quality. Exposure to drying air with low
temperature, high relative humidity, and low flow rate can cause
inadequate drying which can cause peanuts to mold or to deterio-
rate before the drying process is completed.
The following sections describe the recommended levels of
air temperature and air flow.
.WETTEST PEANUTS ON TOP
(LAST TO DRY)
Figure 1. How Peanuts Dry in the Trailer.
Drying Temperature and Relative Humidity
Air is a mixture of several gases and it contains heat.
When air is dry, it occupies a given space alone. As it occurs
naturally, air is rarely completely dry. Air shares space with
water vapor. The amount of space occupied by the water vapor is
a major concern when drying peanuts. The heat content of the air
is also important because the heat is needed to change liquid
water (in the peanuts) to vapor. The vapor then mixes with the
drying air and is removed.
The air temperature determines the heat content and to a
large extent the total water-carrying capacity of the drying air.
As air is heated, the heat content and the water-carrying capa-
city both increase. The relative humidity of the air can be
thought of as the amount of water vapor in the air relative to
the maximum amount it is capable of holding at the same tempera-
ture. Low relative humidities indicate the air has high water
vapor holding capacity (high drying potential), whereas high
relative humidity indicates low water vapor holding capacity (low
Peanuts should not be exposed to drying air with a
temperature above 95F (350C) or a relative humidity below 55% to
prevent off-flavoring, higher split content, overdrying and
overall quality deterioration. Under most weather conditions
during the peanut season the 950F (350C) maximum dryer thermostat
temperature setting is recommended. However, in Florida as
the season progresses into late fall, the prevailing natural air
temperature and humidity may be low enough to require a tempera-
ture below 950F (350C) to keep the drying air humidity above 55%.
Table 1 illustrates the reductions in relative humidity of the
heated drying air produced by a given temperature rise for
various natural air temperatures and relative humidities.
As a general rule, the temperature rise should be limited to
no more than 100to 150F (5.60to 8.30C). For example, in Table 1,
Table 1. Relative Humidity of Heated Air With Given Temperature
Rise, and Relative Humidity of Natural Air
Outside Temp. Relative Humidity of Natural Air
Temp. Rise. 100% 90% 80% 70% 60%
55 ( 2.8) 83% 75% 67% 58% 50%
55F 10 ( 5.6) 70% 64% 57% 49% 42%
(12.80C) 15 ( 8.3) 59% 53% 47% 41% 35%
20 (11.1) 50% 45% 39% 34% 30%
F ( C)
5 ( 2.8) 84% 77% 67% 58% 58%
600F 10 ( 5.6) 70% 63% 57% 49% 42%
(15.60C) 15 ( 8.3) 60% 54% 48% 42% 36%
20 (11.1) 50% 46% 40% 35% 30%
F ( C)
5 ( 2.8) 84% 77% 68% 59% 51%
70F 10 ( 5.6) 71% 64% 57% 50% 43%
(21.1C) 15 ( 8.3) 61% 54% 49% 42% 37%
20 (11.1) 52% 47% 42% 37% 32%
F ( C)
900F 5 ( 2.8) 80% 76% 69% 60% 51%
if the outside air temperature
humidity of the outside air is
(5.60 C) will be sufficient ti
o lower the
1C) and the relative
ierature rise of 100 F
drying air relative
humidity to 57%, whereas a 150F (8.30C) temperature rise will
lower the drying air relative humidity to 49%, which is below the
55% minimum safe relative humidity. In addition to an upper
limit thermostat control of 950F (350C), many installations have
wired a humidistat in series with the thermostat to cut off heat
when relative humidity drops to 55%. This arrangement is very
effective in preventing drying too rapidly and losing quality if
the calibration of the humidistat remains correct.
Unfortunately, humidistats generally must be recalibrated at
lease once per season.
Proper Air Flow Rates and Drying Depths
Air must be forced through the peanut mass at rates
sufficient to remove moisture and prevent mold. This rate has
been established by-research to range from 50 to 75 cubic feet
per minute (C.F.M.) for each square foot (15.2 to 22.9 cubic
meters per minute for each square meter ) of drying floor while
operating against a static pressure head of 1 inch of water (249
Pascal). For a typical four-trailer drying system using 8 x 20
foot (2.4 x 6.1 meter) trailers, the drying area per trailer
would be 160 square feet (14.9 square meters). A total of 640
square feet (59.5 square meters) would be used for the four-
trailer system. The minimum air flow rate required for this
system would be 640 square feet (59.5 meters) multiplied by 50
cubic feet per minute (15.2 cubic meters per minute) which is
32,000 cubic feet of air per minute (61.0 cubic meters per
minute). The fan or fans selected must be capable of providing
this air flow rate while operating against the static pressure of
1 inch of water (249 Pascal). The manufacturer's fan rating
tables, for the particular fan used, can be used to determine the
air flow rate for the operating static pressure required.
The depth at which peanuts may be dried depends upon the
initial moisture content. Under no circumstances should drying
depth exceed 4 feet (1.2 meters). A 4-foot (1.2 meters) drying
depth may be used for peanuts of 25% initial moisture content or
less. The depth should be reduced to 3 feet (0.9 meters) for
moisture contents ranging from 25% to 35%. A 2-foot (0.6 meters)
drying depth should be used for peanuts in excess of 35%
It is important to distribute peanuts evenly over the
entire drying bin to insure uniform air flow and drying. Plenum
chambers, air ducts, control gates and transition sections should
be well constructed and free of air leaks. Baffles or slide
valves should be used to adjust air distribution proportionately
between trailers or drying bins. Uniform trailer construction
and proper duct openings are desirable.
Peanut Response to Drying
When peanuts are placed in the drying bin and air at the
desired temperature, humidity, and rate is forced through them, a
drying pattern similar to the one illustrated in Figure 2 will
occur. Peanut hulls tend to lose moisture rather rapidly during
the early stages of drying while kernel moisture is removed at a
lower rate. After the drying process has proceeded for several
hours, moisture losses from the hull and kernel tend to equalize.
This point is represented where the two lines cross in Figure 2.
The time required for equalization to occur depends primarily
upon the initial moisture content of the peanuts.
After the equalization point is reached, additional
moisture loss from hulls is minimal. However, kernel moisture
continues to decrease. Moisture removed from the kernels is
transferred to the hulls before being removed completely from the
S M -- HULLS
0 20 40 60 80 100
Figure 2. Drying Characteristics of Peanut Hulls and Kernels
with Air at Recommended Drying Rate.
Time required to dry peanuts at various moisture contents
can be estimated by use of Figure 2. For example, if peanuts
initially at 20% moisture content are dried to 10% moisture
content, approximately 50 hours (67 17) of drying time will be
required. The moisture content of the peanuts must be actually
measured to determine the end of the drying cycle.
Managing the Drying System
Quality control is dependent upon frequent moisture
checks as peanuts approach the 12% moisture level. Under average
climatic conditions, heat may be cut off, leaving the fan in
operation to cool the crop when peanuts reach 10% to 10.5%
moisture. Drying continues as the coasting effect lowers the
moisture another 1% to 1.5% below the cut-off point. This effect
is due to unequal moisture levels between the hulls and kernels
as previously mentioned. The average moisture for any lot should
not be allowed to go below 8.5% with no portion of the lot
containing less than 7% moisture or more than 10%.
Intermittent Drying Systems
These drying units are the same as conventional ones, but
are controlled by a timing device to regulate time on and time
off for both fan and heater. Peanut dryers may be turned off 15
to 20 minutes of each hour with an intermittent timer. The 25%
to 33% reduction in power and fuel input may result in a 10% to
12% increase in drying time. Intermittent curing/drying is
gaining in popularity because of fuel and electricity savings.
Peanuts containing more than 25% moisture should be cured by
continuous drying to prevent any delay in reaching the desired
Work on the use of solar energy to heat the drying air
needed for drying peanuts has been conducted in the southeast for
several years. Solar drying systems have been shown to save 50%
to 75% of the LP gas drying energy. This fuel savings would
require several years to pay for the capital expenditure for the
solar drying system. A standard peanut drying fan must be used
to mix ambient air with solar heated air and force it through the
peanuts to be dried. Therefore the electric fan energy is not
reduced using the solar drying system. For 24-hour drying, a
solar heat storage system is required for drying at night or
during periods of limited sunlight. To prevent disastrous drying
delays the system should be capable of adding conventionally
heated air to the drying air whenever needed. To insure proper
drying and quality maintenance, an extensive system of sensors
and valves to monitor and control the air conditions throughout
the solar drying system is required. As the cost of fossil
heating fuels continues to increase, the application of solar
drying will become more feasible. If the solar drying system is
designed for multiple uses like heating buildings and drying
several crops, the economics could also be improved.
Cost of Owning and Operating Peanut Drying Equipment
There are two major factors affecting operating costs of
a particular peanut drying system: (1) initial moisture content
from which the peanuts are to be dried and (2) the number of tons
of peanuts to be dried per year.
The following economic analysis (Table 2) gives a break-
down of the various cost involved in purchasing and operating a
typical two-Trailer peanut drying system.
The following costs are for a small farm operation drying
70 tons (63.6 metric tons) or less per year. Large commercial
plants specializing in quantity processing may operate at lower
costs. All of the cost figures presented here should be used
only as examples, keeping in mind that figures will vary consid-
erably depending upon the individual operation. Figures presented
in Table 2 are based on drying peanuts with an initial moisture
content of 20%. The total cost will be slightly higher or lower
for peanuts at higher or lower initial moisture contents.
Table 2. Estimated Cost of Owning and Operating Peanut Drying
89 KWH per 4 tons (4.4 metric tons) at $0.08
70 gal (0.3 cubic meters) per 4 tons at $0.80
1 hour per 4 tons (4.4 metric tons) at $3.50
Variable Cost for Drying 4 tons (4.4 metric tons)
Variable Cost for Drying 1 ton (1.1 metric tons)
1 Dryer & Timer (cost $2,200; salvage value
$150; 10-yr. life)
1 Shelter (cost $2,000; 25 yr. life)
Trailers, 2 (cost $3,400 salvage value
$340; 10-yr. life)
Interest (1/2 combined cost of equipment,
$7,600 x 14.0% interest)
Repairs, maintenance, taxes and insurance
(estimated at 3% of new cost)
Fixed Cost $1,351.00
Total Drying Cost Per Ton at Various Volumes
Tons Dried Per Year
i 30 (27.2) 40 j(3637 54 60 (54.5) 70 (63.6)
Variable Cost $16.66 $16.66 $16.66 $16.66 $16.66
Fixed Cost $45.03 $33.78 $27.02 $22.52 $19.30
Total Cost $61.69 $50.44 $43.68 $39.18 $35.96
Storage facilities for peanuts should be weatherproof and
free from insect and disease bearing litter. Bins should have
provisions to control air circulation and prevent condensation.
Adequate air space should be provided between the surface of
stored peanuts and roof or ceiling of the building. Avoid
storage of seed peanuts in buildings where temperatures may
become too high. Buildings having exposed sheet metal roof or
side walls may reach excessive temperatures during fall months
where no aeration is provided.
The latest recommendations as to amount and kind of
insecticides to use on stored nuts should be checked and followed
carefully. This is extremely important, since most peanuts are
used for human consumption.
Machinery and handling methods must be carefully planned
to maintain quality peanuts. Belt type conveyors and elevators
should be used whenever possible. The auger and chain-cleat
types may crush pods and divide kernels which invite insect
infestation and lower both grade and germination. If pneumatic
conveyors are used, they should be constructed to prevent
cracking of pods and kernels by impact. Sharp turns in conveyor
ducts must be avoided and air speed must be carefully controlled.
When necessary to walk over the surface of stored
peanuts, board walkways should be provided. Boards prevent
crushing and cracking damage to nuts which could also invite
insect infestation. Peanuts should be removed from bins by gravi-
ty flow when possible; otherwise, care should be exercised not to
damage nuts in handling.
This public document was produced at a cost of $576.60, or 34 cents per copy, to improve peanut harvesting, drying
and handling and to provide basic principles of post harvesting technology. 12-1.7M-85
COOPERATIVE EXTENSION SERVICE, UNIVERSITY OF FLORIDA, INSTITUTE OF FOOD AND AGRICULTURAL
SCIENCES, K. R. Tefertlller, director, In cooperation with the United States Department of Agriculture, publishes this Infor-
mation to further the purpose of the May 8 and June 30, 1914 Acts of Congress; and Is authorized to provide research, educa-
tional Information and other services only to Individuals and Institutions that function without regard to race, color, sex or
national origin. Single copies of Extension publications (excluding 4-H and Youth publications) are available free to Florida
residents from County Extension Offices. Information on bulk rates or copies for out-of-state purchasers is available from
C. M. Hinton, Publications Distribution Center, IFAS Building 664, University of Florida, Gainesville, Florida 32611. Before publicizing this
publication, editors should contact this address to determine availability.