United States Department of Agriculture
Forest Service
Tech Tips
National Technology & Development Program • Reforestation and Nurseries • October 2013 •
2400 • 1424–2310P–MTDC
Improved Seed Crusher and Vacuum Pickup Head
for Seed Screening
Keith Windell, Project Leader and
Joshua Bronson, Manager of the Forest Service Southern Region Resistance Screening Center
T
Highlights…
he U.S. Department of Agriculture, Forest Service,
Southern Region Resistance Screening Center (RSC)
in Asheville, NC, is responsible for testing pine seed
intended for export. Seedlots are tested for the presence of
the pitch canker pathogen Fusarium circinatum (a quarantined pathogen in pines). RSC established a process following the protocol set by the International Seed Testing Association (ISTA).
• Scientific evaluation of tree seeds
provides important information on
disease resistance.
• The seed screening process requires
crushing and testing of seeds in a
sterile environment.
• During 2010 through 2012, MTDC
addressed safety and efficiency concerns by changing the sterilization
method for tools and containers, by
developing a higher capacity seed
crusher, and by fabricating seed
vacuum pickup heads.
RSC Seed Screening Process
The RSC process requires testing 400 seeds per
seedlot. To crush seeds, the lab currently uses a 5¼- by
5¼- by 1½-inch plastic box with lid (figure 1), sterile blotter paper, and a custom-designed template to
maintain seed spacing (figure 2) in the crushing device
(figure 3). After placing the sterile blotter paper and
template in the box, one seed is added per hole (total
of 25). The crushing device is pressed by hand until the
seeds are crushed. The crushing device and template
are removed and a semi-selective media (pentachloronitrobenzene broth) is put directly on each seed. The
boxes are incubated for 7 days with the lid in place
(figure 4). A trained pathologist observes the seeds and
transfers suspect fungal colonies. Isolates from the fungal colonies with growth characteristics of F. circinatum are transferred to carnation-leaf agar and incubated
for at least 3 days prior to identification. The results of
this test determine if a seedlot is (or is not) suitable for
export.
Figure 1—Resistance Screening Center original seed box with sterile blotter
paper.
For additional information, contact: USDA Forest Service, MTDC; 5785 Hwy. 10 West; Missoula, MT 59808–9361.
Phone: 406–329–3900; fax: 406–329–3719; email: wo_mtdc_pubs@fs.fed.us
The seed crushing tools and storage containers are sterilized between each use. The current methods for sterilization
are to wipe surfaces with alcohol or to immerse the equipment
in a cooler filled with 95 percent ethanol (figure 5) followed by
flaming or air drying.
Figure 2—Resistance Screening Center original seed crusher template in a box.
Figure 5—Cooler filled with 95 percent ethanol for sterilization.
Figure 3—Resistance Screening Center original seed crusher and template.
The Missoula Technology and Development Center
(MTDC) worked with RSC to make the seed screening process more efficient by developing a higher capacity seed
crusher and a safer sterilization method. RSC already owned
an autoclave that could be used for sterilization (figure 6). An
autoclave works by increasing the temperature of the sealed
enclosure through the use of pressure and water (in the form of
steam). The pressurized container permits the temperature of
Figure 4—Resistance Screening Center seed boxes being incubated.
Figure 6—Autoclave owned by the Resistance Screening Center.
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the steam to rise above 212 °F when additional heat is added.
The moist hot air sterilizes whatever is in an autoclave. This
autoclave can reach 221 °F, which is the necessary kill temperature for F. circinatum. However, this temperature presents
a melting problem for most clear plastic boxes. The opening
to the autoclave is 14 inches wide, but with internal racks
installed, the opening becomes 10 inches high by 14 inches
wide by 22 inches deep. The initial plan was to size any new
seed crusher design or seed box to fit into the autoclave.
To speed up the process, more seeds needed to be crushed
and accurately placed on a grid pattern per cycle time. The
seeds are spaced about 1 inch apart to allow for accurate test
results. To crush more seeds each cycle, a larger plastic box was
sought. The plastic had to allow sufficient light transmission for
stacked test containers to incubate possible infestation. The container material chosen also had to be compatible with a viable
sterilization method.
A multitude of relatively inexpensive, polystyrene or
acrylic clear boxes with lids are readily available but would
have to be sterilized before use. Unfortunately, these plastics
will melt in an autoclave.
Comparing Sterilization Methods
Sterilization could be accomplished with ethylene oxide
(gas), but RSC chose not to pursue that method for safety
reasons. Other chemical disinfectants are available but they
appear to be more hazardous than the ethanol currently being
used. Dry heat can be used as a sterilization method, but
polysulfone, a good plastic choice that can tolerate heat, fails
the optical clarity requirement. Microwaving is not readily used in commercial operations; success with this method
is unknown. Irradiation with gamma rays is a widely used
method to sterilize plastic containers such as petri dishes and
bioassay dishes. However, this method requires equipment
that is not very portable and is expensive.
Another sterilization method considered was ultraviolet
(UV) light. The use of inexpensive ($100 to $400 in 2012)
UV-C germicidal lights (used by industry for sanitizing)
seemed like a solution until it became known that manufacturers of these lights don’t guarantee to “kill everything.”
The Food and Drug Administration (FDA) approved
pulsed UV light in the production, processing, and handling
of food (Title 21 of the Code of Federal Regulations, Part
179). Xenon Company thought its new “pulsed” UV system
might work for this application. A basic unit (without containment box) costs $20,000 in 2012. A containment box of
highly polished aluminum was recommended as the best way
to save money. This containment box (RS3000C) with all
safety mechanisms costs $28,000 in 2012. The replacement
UV bulbs cost $700 in 2012 and last 600 to 1,200 hours.
Xenon said exposure times of only a few seconds should be
adequate for sterilizing the inside of the box. If the outside
needs to be sterilized, a more elaborate tunnel-type chamber might be needed. This method seemed to be a little too
expensive given the number of boxes that would need to be
sterilized and the uncertainty of the outcome.
MTDC decided sterilization in an autoclave would be
the best method if a suitable container could be found. Polymethylpentene (PMP) is an autoclavable plastic and is used in
some small, reusable petri dishes. Unfortunately, RSC needs
a larger container for seed crushing. A custom injection mold
could be made, but the cost would be excessive for the number of containers that are needed.
MTDC chose a sterilized box (with lid) that is used only
once before disposal. Not many size choices are available, but
a sterilized polystyrene bioassay dish with lid was located that
measured 245 by 245 by 25 millimeters (figure 7). These lab
dishes are sterilized by irradiation and have enough room for
53 seeds on about a 1-inch grid pattern. A Corning lab dish
(product number 431111, $148 for 16 in 2012) can be ordered
at <http://www.corning.com>. A Thermo Scientific Nunc lab
dish (same size as the Corning dish) is available through distributors, such as Cole-Parmer (<http://www.coleparmer.com>
product number EW–01929–00, $171 for 16 in 2012).
Figure 7—New sterile clear plastic lab dish with seeds.
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Improving the Seed Crusher
MTDC built a seed crusher from an idea pioneered
by
the
RSC lab. To simply “upsize” the design to accomTo successfully crush this increased number of seeds
modate the additional seeds would have resulted in a heavy
requires more force than is readily available by human hand
strength. The seed crusher could be hit with a mallet, but the device. For repeated cycling through an autoclave, stainless
steel is the best steel to combat oxidation but is very expenuneven force levels would lead to increased occurrence of
sive. Instead, an autoclavable ultra high molecular weight
cracks in the plastic dish. The crushing depth also has to be
precisely controlled so all the seeds will be split open without (UHMW) opaque polyethylene was selected for the seed
crusher.
destroying the dish.
Rather than use bolts as the lab’s prototype did, MTDC
To solve these problems, MTDC modified an arbor press
3⁄8-inch diameter stainless steel pins to do the precision
chose
(figure 8) and included provisions for precise depth control.
crushing. The pins are pressed into undersized holes. This
Two parallel rigid plates were fabricated to hold the seed
crusher assembly firmly and spread the force evenly over the press and shrink fit is critical. Too little bore pressure and the
pins would not stay in place during the thermal cycling in
seeds.
the autoclave; too much pressure and the plastic plate would
distort or crack. The prototype seed crusher assembly (figure
9) was sent to RSC for testing. Reports were favorable and,
eventually, 12 sets of seed crusher assemblies were delivered.
Precision
height
adjustment
Upper seed
crusher plate
Stainless
steel pins
Figure 8—Modified arbor press with seed crusher assembly.
Sterile
clear
plastic
lab dish
Lower seed
crusher plate
Figure 9—Prototype seed crusher assembly: upper seed crusher plate,
stainless steel pins, lower seed crusher plate, and sterile clear plastic lab dish.
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Increasing Seed Loading Efficiency
Manually loading each of the 53 seeds into the seed crusher
assembly becomes tedious. MTDC conducted product research
for better technology to sow seed trays. Manual designs are
available, such as the Shutterbox seeder <http://www.stuewe
.com/products/shutterbox.php>, but these may be more applicable for larger seeds like corn. Smaller seeds like pine can be
handled better with vacuum-style pickup heads, such as the
Seed E-Z Seeder <http://sezsdr.com/economical-labor-saving
-greenhouse-equipment/>, the Evergreen Vacuum Seeder
<http://www.stuewe.com/products/evergreen_vacuum
.php>, or the HP Vacuum System <http://www.hoffmanmfg
.com/products/3-4-hp-vacuum-system.hmtl>. Hoffman Manufacturing, Inc., offers standard vacuum units, but vacuum pickup
heads can be custom built to the customer’s specification.
MTDC fabricated a number of experimental pickup
heads to be used with a Hoffman vacuum unit (model number
VPS075, $1,784.50 in 2012) (figures 10 and 11). Drawings are
available by contacting MTDC’s drafting department (dmucci@
fs.fed.us, 406–329–3999). Ask for the “Seed Crusher Assembly,” drawing number MTDC-1086, which also includes
details for the seed vacuum pickup heads and the arbor press.
Figure 11—Seed vacuum pickup head with upper and lower seed crusher plates.
After some trial and error, MTDC developed a working
seed vacuum pickup system that was sent to RSC for evaluation. An operator’s manual is available to help employees
quickly dial in the information for the specific seed species
they are trying to pick up during vacuum seeding.
Contact MTDC (wo_mtdc_pubs@fs.fed.us, 406–329–
3978) for the “Seed Vacuum Pickup System: Operator’s
Manual” (1224–2824–MTDC).
Conclusions
RSC considers the project
a success. Feedback has been
very positive, even though
extended operational testing
has not taken place. For the
latest update on the seed
crusher’s performance, contact
RSC manager Joshua Bronson
(jjbronson@fs.fed.us,
828–667–5089, ext. 202).
Figure 10—Seed vacuum unit with MTDC vacuum pickup heads: a seed vacuum pickup system.
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About the Authors
Library Card
Keith Windell is a project leader for reforestation,
fire, and residues projects. He has a bachelor’s degree in
mechanical engineering from Montana State University. He
has worked for the California Department of Forestry; U.S.
Department of the Interior, Bureau of Land Management; and
the USDA Forest Service.
Joshua Bronson is the manager of the Resistance
Screening Center (RSC) in Asheville, NC, (part of the U.S.
Department of Agriculture, Forest Service, Forest Health
Protection program, Southern Region). Bronson received a
bachelor of science degree in forestry from Colorado State
University in 1996 and a master’s degree in plant pathology
from the University of Wisconsin-Madison in 2004. Before
joining the RSC in 2006, he worked as a biological science
technician at the Forest Service, Oconto River Seed Orchard
in White Lake, WI.
Windell, Keith; Bronson, Joshua. 2013. Improved seed
crusher and vacuum pickup head for seed screening. Tech
Tip 1424–2310P–MTDC. Missoula, MT: U.S. Department of
Agriculture, Forest Service, Missoula Technology and Development Center. 6 p
The Resistance Screening Center (RSC) technicians
evaluate seeds for resistance to diseases, including pitch canker. Information from RSC laboratory tests is valuable to tree
improvement specialists, seed orchard managers, scientists,
research institutions, and private industry. The seed screening process requires crushing and testing of seeds in a sterile
environment. This tech tip explains how safety was improved
and efficiency was increased by changing the sterilization
method for tools and containers, improving the seed crusher,
developing a seed crusher assembly, and fabricating seed
vacuum pickup heads.
Keywords: autoclave, Fusarium circinatum, International Seed Testing Association, irradiation, laboratories,
polyethylene, polystyrene, prototypes, safety at work, seed
lots
For additional technical information, contact MTDC:
USDA Forest Service
Missoula Technology and Development Center
5785 Hwy. 10 West
Missoula, MT 59808-9361
Phone: 406–329–3900
Fax: 406–329–3719
Electronic copies of National Technology and Development documents are available on the Internet at:
http://www.fs.fed.us/eng/pubs
Forest Service and Bureau of Land Management employees
can search National Technology and Development documents, CDs, DVDs, and videos on their internal computer
networks at:
http://fsweb.mtdc.wo.fs.fed.us/search/
http://fsweb.sdtdc.wo.fs.fed.us/
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