Seed Drying
Seed Moisture Management
And Building Pressurized Dryers
Dry air (Relative humidity is
below 30%)
Water leaves the seed
and goes into the dry
air.
Moist seed
Humid air (relative humidity
is above 30%)
Water moves
from the air
into the dry
seed.
Dry seed
To keep seeds dry
they must be
sealed in a
moisture proof
container.
Humid air (70%)
Water
moves both
ways or not
at all.
Moist Seed
The condition of a high equilibrium relative humidity. At equilibrium
this seed will measure an ERH of 70%.
Dry air (30% relative humidity)
Water
moves both
ways or not
at all.
Dry Seed
The condition of a low equilibrium relative humidity. At
equilibrium this seed will measure an ERH of 30%.
Probe
Sample holder
A water activity (Aw)
meter used to measure ERH.
Meter
Water
Activity
Meter
Hand held hygrometer
Hygrometer measuring ERH of seeds in a covered box
Peanut butter jar lid
with hole bored to
fit ½’ pvc pipe
f of ½” pvc pipe coupling
C pipe and coupling form an
eptably air tight port for the
Half of ½” pvc pipe
coupling
¾” length of ½”
pvc pipe
nus americana
s
Peanut butter jar used as an ERH test
chamber
Moisture Content vs Equilibrium Relative Humidity
Isotherm
of Seed Moisture
35.00
30.00
y = 85.294x3 - 99.113x2 + 43.567x
R2 = 0.9779
Free Water
Seed Moisture Content
25
Tightly
Bound
Water
25.00
20.00
Moderately
Bound Water
mc
Poly. (mc)
15.00
10.00
5.00
0.00
0
0.1
0.2
0.3
0.4
0.5
0.6
Equilibrium Relative Humidity
0.7
0.8
0.9
1
Moisture Content vs Equilibrium Relative Humidity
35.00
30
30.00
y = 85.294x3 - 99.113x2 + 43.567x
R2 = 0.9779
25
Seed Moisture Content
25.00
20.00
Isotherm of Seed Moisture
mc in
more variation
Poly. (mc)
mc than ERH
15.00
10.00
5.00
Good ERH for Seed Storage
0.00
0
0.1
0.2
0.3
0.4
0.5
0.6
Equilibrium Relative Humidity
0.7
0.8
0.9
1
Key points of ERH
• Can be used on any seed lot regardless of
–
–
–
–
•
•
•
•
•
•
Viability
Purity
Stage of extraction
Species
No conversion charts needed
Traceable standards available for calibration
Non destructive
Fast – Results obtained in a few minutes
Very easy to use
Low cost equipment ($200 to $2500)
Drying seeds: Key points
• Seeds that can be dried to low moisture content are called
orthodox.
• Controlling moisture is the single most important factor in
preserving seed viability.
• A pressurized drier is the fastest and most efficient way to
quickly and uniformly dry orthodox seeds to preserve
maximum viability.
– The air in the drier should be 30 to 35% RH.
• This is most easily done by putting the drier in a dry room that is at 30%
relative humidity (test the humidity with a hygrometer).
– Seeds will be dry in 24 to 48 hours.
– Test the seed with a hygrometer to know they are dry.
Water escapes from
surface seeds only to dry
air.
Seeds in a container
or pile can only dry at the
surface of the seed mass.
Seeds at the lower layers stay moist
and are vulnerable to deterioration.
Water escapes to
atmosphere
Seeds in a pressurized drier
can dry quickly and
uniformly because air passes
around all the seeds carrying
away the moisture.
Fan
Bottom plenum is
pressurized
with dry air (30%rh)
Seeds at the lower layers
finish drying first (drying
front).
5 gallon bucket
paint strainer
holds the seeds.
A five
gallon
bucket
serves as
the
plenum.
Drier viewed
from the top.
Multiple trays can be
stacked to dry larger
lots or several lots
simultaneously.
A collar is
needed between
each tray to seal
in the air and keep
the drier pressurized.
Collars are made
by cutting out
the center of the
bucket lid and
lining the edge
with weather
stripping.
Shaded pole blower
motor mounted to
bottom of bucket.
Blower needs to
operate between 1
to 2 inches water
column static
pressure (wc sp)
Flexible
cable
connector
Six foot extension cord
makes a ready made
power cord.
Wooden blocks hold the
bucket off the work
surface to allow air to
enter the fan. Screws
are place through the
mounting brackets on
the fan into the blocks
which secures both
blocks and fan motor.
Air inlet for the fan
motor. (The safety
screen removed for
illustration.)
The blower motor
Barrel size, 22”
diameter, strainer
for larger seed lots
No collar was
required for these
particular trays.
Vendor and Material List
• usplastic.com
– ES Strainer
• 600, 400, 200, 100 mesh (higher number is larger hole)
• 5 gallon pail size: $4.31 each
• 55 gallon drum size, 22”: $12 to $13 each
– 5 gallon Bucket $4.30 + lid $1.82
• grainger.com
– Induced draft blower Dayton 4C723 $122.50
– Extension cord, Romex connector, wire nuts
• Local sources
– Weather stripping, pan head screws, 2x2 wood blocks, hardware
cloth to cover blower inlet
Long term storage
• Must first be dried with air at 30% relative
humidity or less until dry
• Seal in a moisture proof container
– 4 to 6 mill poly bag
– Plastic bottle or plastic dish with a tight lid
• Freeze the seed