VIDALIA ONION RESEARCH LABORATORY

UNIVERSITY OF GEORGIA

TIFTON CAMPUS

International Onion Short Course

April 26, 2005

Presenter:

Joel Thad Paulk, Agricultural Research Assistant I

Vidalia Onion Research Laboratory

Horticulture Department

University of Georgia

Tifton Campus

Slide #1: Controlled Atmosphere Onion Storage

Slide #2:  What is “Controlled Atmosphere” Storage?

The composition of the air we breathe is approximately 79% nitrogen, 21% oxygen and 0.03% carbon dioxide.  In ordinary storage, fresh produce is held in this air at reduced temperatures.  In CA storage, the composition of the air is changed and controlled.

Slide #3: Why was CA storage needed?

1. Vidalia onions can be grown for harvest only during late April, May and June.
2. Earlier or later planting dates will not effect this marketing window.
3. More onions could be marketed if there is a way to store them until the market could absorb additional amounts.
4. CA storage makes this opportunity possible.

Slide #4: What impact has CA storage had on the Vidalia onion industry?

In the 1970’s, there were 400 to 600 acres grown with a value of $750,000.   By 1983, there were 6,000+ acres being grown with a value of 20,000,000.  By the mid 1990’s, there were 12,000 to 16,000 acres being grown with a value of $50,000,000.  The growth in the 1990’s is attributed primarily to the ability to store onions in CA and extend their marketing period.

Slide #5: Requirements to get the optimum benefit from CA storage of onions.

Slide #6:  In order to have a successful Controlled Atmosphere storage season, there are several things that must be handled carefully.

1. You must harvest fully mature onions.  When the onions are fully mature, they should be undercut and allowed to air dry for two or three days.  Undercutting initiates and/or hastens the onion’s change from growth to dormancy.  After air drying the tops should be clipped to approximately 2 inches from the bulb and the roots clipped to approximately 1 inch from the base plate.
2. Handle onions with care.  Care must be taken to prevent damage to the onion at harvest, transport to the packing shed, during the curing and grading process and placing them in storage.  The less bruising and other damage to the onion will give you a chance to have a more successful storage cycle.
3. Thoroughly cure the mature onions.  In order to maintain optimum quality during storage, the onion must be fully cured and in a state of dormancy.  An onion that is not fully cured will breakdown during storage.  Thorough curing is characterized by the complete drying of the root, the stems and several layers of bulb scales.  In a properly dried onion, the neck tissue should not slide back and forth when squeezed between the thumb and forefinger.
4. Onions must be free of defects and disease.  Any onion that shows signs of mechanical or other injury should be removed during the grading process. All diseased onions should be removed during the grading process before going into storage.  You will only get out as good an onion as you put in.
5. Storage rooms must be sanitary.  Storage rooms should be cleaned and disinfected prior to start of the storage season.  All rotten onions, dirt and other plant tissue should be removed.  The room can be washed down with a bleach solution and rinsed.
6. Establish the CA atmosphere as rapidly as possible.

Slide #7: Requirements for Successful Onion Storage cont’d.

      1.  Gradually warm rooms over period of days before removing onions from storage.

           This will facilitate in the onions not sweating when removed from the storage

           room.

      2.  Use forced air dryers to keep moisture from forming on the onions when removed

           from storage if you are unable to warm the rooms.

Slide #8: Requirements for a CA system.

1. You must have and air tight refrigerated storage room.  If the room has leaks, you will not be able to control the atmosphere.  You will constantly be flushing to get rid of the excess oxygen and you will not be able to control the relative humidity.
2. You will need a nitrogen source.  You can supply nitrogen by using liquid nitrogen or you can install a Hollow Fiber Membrane Separator and produce the needed nitrogen on site.  It is cheaper to produce your own nitrogen than to use liquid nitrogen.
3. You will need a carbon dioxide source.  Carbon dioxide can be supplied by purchasing it in gas cylinders.  The onions themselves will generate carbon dioxide through normal respiration.
4. You will need temperature and relative humidity monitors.  It is crucial to maintain temperature.  Onions will freeze in a short period of time when the temperature drops below 31°F.  Relative humidity needs to be maintained between 65% and 70% to reduce onion decay.
5. You will need an oxygen/carbon dioxide monitor to keep check on the atmosphere in the storage room.
6. You will need good air flow.  Sufficient air flow is needed to keep moisture from building up in pockets in the storage room.  Free moisture allows for fugal spread and growth.
7. Pressure relief valves are needed to allow for the expansion and contraction of the atmosphere in the storage room during pressure changes.
8. Shock absorber bags are required for the storage rooms to allow for expansion and contraction of the atmosphere during refrigeration defrost cycles.

Slide #9: The best atmosphere for storing Vidalia Onions has been established at 3% oxygen, 5% carbon dioxide and the balance 92% nitrogen.

Slide #10: What are the benefits of Controlled Atmosphere storage of onions?

1. A carbon dioxide level maintained at 5% inhibits fugal sporulation.
2. An oxygen level maintained at 3% reduces respiration and sprouting.
3. A low temperature of 34°F reduces respiration and helps keep the onion dormant.
4. Low relative humidity inhibits fungal spread.
5. You are able to extend your marketing window by not having to quickly sell the onions in a fresh market arena.

Slide #11: What are some of the problems associated with Controlled Atmosphere storage of onions?

1. Low levels of carbon dioxide allow fungal growth and development.  The carbon dioxide should be monitored and kept at 5%.
2. Elevated temperature increase respiration, thus increasing carbon dioxide production.  Carbon dioxide is a natural result of oxygen consumption in the respiratory process.
3. High carbon dioxide, greater than 7% destroys the inner cells of the onion and produces Translucent Scale.  Carbon dioxide levels must be monitored
4. High oxygen content promotes sprouting.
5. Elevate relative humidity promotes decay.  Free water on the onion creates a pathway for fungal diseases to spread.

Slide #12: What are the effects of carbon dioxide on stored onions?

Slide #13: Botrytis allii is main problem facing onion growers in the Vidalia region.

Today I want to talk about this problem and what can be down to prevent substantial loss of onions during storage.

Slide #14: Previous in-vitro studies showed there is not much difference in growth of Botrytis allii between CA and Air.  A reduction in temperature slows the growth rate of Botrytis allii, but carbon dioxide does not inhibit growth.

Slide #15: Increasing the level of carbon dioxide inhibits germination and sporulation of the Botrytis allii.  While the carbon dioxide does not stop growth the spores will not reproduce.

Slide #16: This slide shows the Botrytis allii at 50°F in a normal room atmosphere.  You can see the grayish brown mycelium which indicates sporulation.

Slide #17: Botrytis allii in-vitro at 50EF in normal CA conditions continues to grow, but does not sporulate.  The carbon dioxide has not stopped growth, but there is no sporulation.

Slide #18: Experiments have been conducted for the past two years to determine the effect of High-CO₂ on onions stored at 34EF.

Two key points were examined in these experiments: 1- Do high concentrations of CO₂ damage the onion and cause Translucent Scale; 2- Do high concentrations of CO₂ kill Botrytis allii?  In the 2002 experiment, the High-CO₂ concentration appeared to have killed the Botrytis allii.

Slide #19:

To determine the effects of High-CO₂, two experiments, storing onions for 10 weeks at 34EF were conducted in 2003.  All onions used for the 2003 experiments were grown at the Vidalia Onion and Vegetable Research Farm.  Savannah Sweet onions were cured and mechanically graded at the Research Farm.  Pegasus onions were brought to Tifton and cured and graded by hand at the Tifton facility.  The first experiment in 2003 (a repeat of the 2002 experiment), consisted of 2 cultivars, Savannah Sweet and Pegasus onions stored at 34EF in high humidity.  Two atmospheres were used, Air and High-CO₂ (20% CO₂ + 80% Air).  The experiment was replicated 4 times.  After 10 weeks the onions were removed from storage and evaluated.  After this initial evaluation, the onions were kept an additional 7 days at 68EF in air and 70% Relative Humidity.

Slide #20: 40 onions for each cultivar and treatment with no visible signs of disease were randomly selected and placed in 8 liter buckets and placed in a refrigerated storage cell.

Slide #21: Atmosphere readings were recorded 3 times each week.  After each reading, the buckets were flushed for 95 minutes with the appropriate atmosphere to obtain a complete air exchange. In this slide you can tell the O₂ level in the Air treatment decreases and the CO₂ level increases.  This is probably the result of the onion tissue breaking down.  As onion tissue breaks down, respiration increases and CO₂ production increases.  CO₂ is a natural by-product of respiration.  O₂ in the High-CO₂ treatment decreases and the CO₂ increases.  This is a result of the CO₂ injury to the onion tissue, injury increases respiration.  Notice the dashed lines, the O2/CO2 analyzer malfunctioned and I didn’t get a replacement for about 4 weeks.

Slide #22: After 10 weeks storage, all onions were visually examined for external symptoms of disease and cut to determine internal symptoms. Evaluations indicated that the High-CO₂ treatment prevented sporulation of Botrytis allii on the exterior of the onion compared with about 85% in the Air treatment.  The High-CO₂ also, produced almost 100% translucent scale in the onions.  There is not much difference between the treatments for internal Botrytis allii. The 2003 experiment yielded the same initial results observed in the 2002 experiment.  In addition to the protocol of the 2002 experiment, onions were kept at 68EF in air for an additional 7 days after removal from the designated treatments to determine if Botrytis allii spores would develop on the High-CO₂ treatments when exposed to air. Visual observations of this experiment exhibited less Botrytis allii in the High-CO₂ treatment compared with that of the Air treatment.  However, the Botrytis allii continued to sporulate on the High-CO₂ treatment when exposed to room air.

Slide #23: This slide shows the Pegasus and Savannah Sweet, Air treatment inside the containers after 10 weeks storage.  You can see the fungal growth on the exterior of the onions.

Slide #24: These are the same onions in the previous slide that have been spread out.

Slide #25: This slide shows Pegasus and Savannah Sweet, High CO₂ treatment inside the containers after 10 weeks storage.  The exterior of the onions do not show any fungal growth.

Slide #26: These are the same onions in the previous slide that have been spread out.

Slide #27: This slide shows the Pegasus and Savannah Sweet, Air treatment after 7 days at 68EF in air.  The spores continue to develop.

Slide #28: These are the same onions in the previous slide that have been spread out.

Slide #29: This slide shows Pegasus and Savannah Sweet, High-CO₂ treatments after 7 days at 68EF in air.  You can see after the onions were exposed to air, the Botrytis has resumed growth on the exterior of the onions.  But, the onions still show less fungal growth than the Air treatment.

Slide #30: These are the same onions in the previous slide that have been spread out.

Slide #31: This slide shows an onion with Translucent Scale.  The High-CO₂ concentration has damaged the cell membranes and the contents have leaked out.  The tissue has a translucent, watery appearance.

Slide #32: This is a close-up of an onion with Translucent Scale.

Slide #33: The second experiment in 2003 consisted of 2 cultivars, Savannah Sweet and Pegasus.  Both cultivars were grown at the Vidalia Onion Vegetable Research and Education Center .  The Savannah Sweet onions were cured and graded at the Research Center .  The Pegasus onions were brought to Tifton and cured for 48 hours and hand graded to select bulbs to be used in the experiment.  The onions were stored at 34EF in 70% Relative Humidity for 10 weeks.  A second group of onions were allowed to set at 68EF in air for 12 days after removal from the storage period.  Three atmospheres were used, Air, 3% O₂ + 5% CO₂ + 92% N₂  (Recommended Level) and 3% O₂ + 10% CO₂ + 87% N₂ (High CO₂). 40 bulbs of each variety were randomly selected and placed in 1 bushel containers and replicated 4 times.

Slide #34: The containers were placed in CA storage cells.  Both cultivars were placed in the respective storage cells together.  These storage cells were monitored and maintained using a David Bishop computerized atmosphere system.  Temperature, atmosphere and RH% were monitored and maintained with the computerized system throughout the storage period.

Slide #35: After 10 weeks, the crates were removed from respective storage atmospheres and evaluated visually for presence of diseases.  Both cultivars had Botrytis allii in all of the treatments.  The onions had a high percentage of Botrytis allii going into the storage cycle.  The CO₂ treatments exhibited the same effect observed in the previous bucket experiment.  External Botrytis allii was lower in the CO₂ environments than in Air.  The Savannah Sweet Air treatment had the highest percentage of external Botrytis (12.5%), followed by the 5% CO₂ (3.75%) and 10% CO₂ Level (1.25%).  Pegasus had a minor amount of external Botrytis, Air (1.25%), 5% CO₂ (0%) and 10% CO₂  (0.63%), but this is probably due to the handling.  Approximately, 90% of the Pegasus onions were discarded for symptoms of Botrytis when being selected for this experiment, whereas, the Savannah Sweet were mechanically graded at the Onion Farm.  Both cultivars had a high percentage of Translucent Scale in the 10% CO₂ treatment (Savannah Sweet 100% and Pegasus 98%).  Savannah Sweet had a higher percentage of internal Botrytis than Pegasus, but that is probably due to bulb selection prior to the experiment.

Slide #36: After 12 days at 68EF in air, Botrytis continued to grow on the Air treatment.  When the CO₂ treatments were exposed to air, you can see that the Botrytis has resumed growth.  The Translucent Scale percentages in the 10% CO₂ treatments were basically unchanged.  The CO₂ treatments showed an increase in internal Botrytis development with Pegasus having a lower percentage than Savannah Sweet.  But, once again this is probably due to bulb handling and selection prior to the start of the experiment.

Slide #37: This slide shows the two cultivars from the Air treatment after 10 weeks at 34EF.  You can see that they look better than the previous pictures from the bucket experiment.  The Relative Humidity was maintained at a lower percentage.  However, fungal growth is apparent.

Slide #38: This slide shows the two cultivars from the 5% CO₂ treatment.  The appearance is much better than that of the Air treatment.  There is no visual sign of Botrytis.

Slide #39: This slide shows the two cultivars from the 10% CO₂ treatment.  There is minimal amount of Botrytis.

Slide #40: This slide shows the two cultivars from the Air treatment after 10 weeks storage plus 12 days at 68EF in air.  The Botrytis allii fungus continues to develop.

Slide #41: This slide shows the two cultivars from the 5% CO₂ treatment after 10 weeks storage plus 12 days at 68EF in air.  You can see that after the onions are placed in air, the Botrytis continues to grow.

Slide #42: This slide shows the two cultivars from the 10% CO₂ treatment after 10 weeks storage plus 12 days at 68EF in air.  You can see that after the onions are placed in air, the Botrytis resumes growth.

Slide #43: In conclusion,

1. Current CA recommendations will slow growth and prevent germination and sporulation of Botrytis allii.
2. When onions are removed from CA conditions and placed in air, if the onions have Botrytis allii spores on them, they will continue to grow, germinate and sporulate.
3. High levels of carbon dioxide, greater than 7% are detrimental to onions.  The high carbon dioxide level disrupts the inner cells in the onion, therefore causing Translucent Scale.

Slide #44: In order to have a successful storage season, you must reduce or eliminate harvest injury.

1. Remove protruding nails or staples and smooth the rough edges on field containers.
2. Use care in dumping onions from one container to another.
3. Use padding on all impact areas when possible.
4. Don’t overfill containers!  Severe damage and result when onions are stacked.

Slide #45: Properly prepare for the post-harvest season.

1. Harvest fully mature onions.
2. Properly cure onions.  An improperly cured onion will not store well.
3. Lower onion bulb temperature as rapidly as possible to reduce respiration.
4. Properly grade onions.  Remove and diseased or damaged onions.
5. Place graded onions in properly sanitized storage cells.
6. Establish CA conditions as quickly as possible.

Slide #46: Questions?

Submit questions with regard to this presentation to:

Joel Thad Paulk, Agricultural Research Assistant I

Vidalia Onion Research Laboratory

Horticulture Department

University of Georgia

Tifton Campus

Phone: (229) 386-7445

Fax: (229) 386-7444

e-mail: paulk@tifton.uga.edu

Material for this presentation was produced by:

Joel T. Paulk¹, Albert C. Purvis¹, and Jason Brock²

¹Horticulture Department, Coastal Plain Experiment Station

²Plant Pathology Department, Rural Development Center

The University of Georgia – Tifton Campus

Tifton , Georgia 31793

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Last modified: May 23, 2005