CHAPTER 7: STORAGE OF HORTICULTURAL CROPS - 4

Controlled atmosphere ( C.A. ) storage

Controlled or modified atmosphere storage should be used as a supplement to, and not as a substitute for, proper temperature and relative humidity management. Some simple methods for modifying the composition of air in the storage environment are listed below (from Kader, 2002). Air coming into the storeroom or being re-circulated within the room must pass through a monitoring and control system.

Oxygen gas control:

to DECREASE:

• purging with nitrogen
• from liquid nitrogen through an evaporator
• from a membrane system nitrogen generator
• from a molecular sieve system nitrogen generator

Carbon dioxide control:

to INCREASE:

• dry ice
• pressurized gas cylinder

to DECREASE:

• molecular sieve scrubber
• activated charcoal scrubber
• sodium hydroxide scrubber
• hydrated lime (use 0.6 kg of hydrated lime to treat the air used to ventilate 100 kg of fruit. Air can be directed to pass through a box, located inside or outside the C.A. storeroom).

Source: Vigneault, C., Raghavan, V.G.S., and Prange, R. 1994. Techniques for controlled atmosphere storage of fruits and vegetables. Research Branch, Agriculture and Agri-Food Canada , Technical Bulletin 1993-18E.

To regulate CO₂ levels in a CA room, one of the simplest methods is the hydrated lime (Calcium carbonate) Ca(OH) 2 “scrubber”. The scrubber is constructed using an insulated and airtight plywood box located outside the CA room. The box should contain enough lime for the entire storage period, but new lime can be added if CO₂ absorption drops.

The CO₂ and hydrated lime reacts in a 1:1 ratio to form limestone and water. Lime with a smaller particle size (fine mesh) is more efficient at reacting with CO₂ than lime with a coarse mesh. The following will maximize the effectiveness of your small-scale scrubber:

• Lime should be packaged in 25 kg bags that have no polyethylene liner.
• Each bag should be only partially filled (50%) to avoid the outer layer hardening and reducing efficiency over time.
• Lime bags should be stacked on a pallet equipped with shelves that allow a 10 cm space between layers to maximize air circulation.
• 50% of the recommended lime for the anticipated storage period can be placed on pallets on the floor of the storage room (this will consume the initially high CO₂ produced by the fruit).

To keep CO₂ below 2%, about 12 kg of lime per metric tons of apples is recommended for 3 to 4 months of storage.

Source: Vigneault, C. et al. 1994. Techniques for controlled atmosphere storage of fruits and vegetables. Research Branch Agriculture and Agri-Food Canada .

For preventing the buildup of ethylene, a simple “scrubber” can be constructed and added to the air supply line.

Ethylene control:

to DECREASE:

• potassium permanganate
• activated charcoal
• catalytic oxidation

Controlled atmosphere storage of pallet loads of produce is also possible using a semi- permanent set-up for creating a gas-seal. Any number of pallets can be accommodated inside a plastic tent made using 7 to 8 mil polyethylene sheeting. A better gas barrier at the floor (compared with the traditional tube in trough method illustrated below) can be obtained by laying a sheet of 4 to 5 mil polyethylene on the floor and covering it with wood panels. The seal is then made by joining the tent plastic with the floor plastic (using tape).

Typical layout of a C.A. tent:

Source: McDonald, B. 1982. Controlled atmosphere storage using plastic tents. International Institute of Refrigeration.

Another simple practice that can be used to store produce under controlled atmosphere conditions involves constructing plastic tents using 20 mil polyvinylchloride (PVC) sheeting suspended over traditional pallet racks inside a refrigerated cold storage warehouse. The diagram below shows two C.A. tents. The top tent has been sealed by closing zippers on both sides of the door and immersing the bottom of the door and adjacent floor panel in a trough of water. The trough is made using a 6 inch diameter PVC pipe (with 1/3 of the pipe cut away), which also allows supply and sampling gas lines and temperature probe wiring to pass into the tent. All seams and joints of the tents are heat-welded.

The bottom tent is shown with the door rolled open and one pallet load of produce loaded into the tent.

Source: Leyte , J.C. and Forney, C.F. 1999. Controlled atmosphere tents for storing fresh produce in conventional refrigerated rooms. HortTechnology 9 (4) 672-675.

A low cost plastic tent fashioned from clear polyethylene sheeting can be used for controlled atmosphere storage of bunches of green bananas. A small fan serves to circulate the C.A. storage air (2% O₂ and 5% CO₂) through a chamber of potassium permanganate on aluminum oxide (Purafil). Ripening is delayed as ethylene is scrubbed from the storage air. The shelf life of bananas under these conditions is four to six weeks at ambient temperatures.

Source: Shorter, A.J. et al. 1987. Controlled atmosphere storage of bananas in bunches at ambient temperatures. CSIRO Food Research Quarterly 47:61-63.

The following tables are a summary of controlled atmosphere (CA) and modified atmosphere (MA) requirements and recommendations.

A summary of controlled atmosphere (CA) and modified atmosphere (MA) requirements and recommendations for apples

Average optimum levels of popular apple varieties.

Source: Kuprferman, E. 1997. Controlled atmosphere storage of apples. CA ’97 Proceedings, Vol. 2, pp 1-30. Postharvest Horticulture Series No. 16, University of California , Davis .

A summary of controlled atmosphere (CA) and modified atmosphere (MA) requirement and recommendations for pears

Optimum storage temperature is –1 to 0 ° C, unless otherwise indicated. (CA-disorder abbreviations are: IB=internal breakdown or browning; BH=brown heart; PBC=pithy brown core; CF=core flush; Cav=cavity, usually lense shaped; CI=chilling injury).

‘Ya Li’ may show a type of chill injury at temperatures <5°C.

Source: Richardson , D.G. and E. Kupferman. 1997. Controlled atmosphere storage of pears. CA ’97 Proceedings, Vol. 2, pp 31-35. Postharvest Horticulture Series No. 16, University of California , Davis .

A summary of CA requirements and recommendations for fruits other than apples and pears

¹ Usual and/or recommended range; a relative humidity of 90-95% is recommended.

² Specific CA combination depends on cultivar, temperature, and duration of storage. These recommendations are for transport and/or storage beyond 2 weeks. Exposure to lower O₂ and or higher CO₂ concentrations for shorter durations may be used for control of some physiological disorders, pathogen, and/or insects.

Source: Kader, A.A. 2001. A summary of CA requirements and recommendations for fruits other than apples and pears. pp 29-70. Postharvest Horticulture Series No. 22A, University of California , Davis .

A summary of CA requirements and recommendations for 34 harvested vegetables

	Temperature1		Atmosphere2
Vegetable3	Optimum	Range	%O2	%CO2	Application3
Artichokes	0	0 - 5	2 - 3	2 - 3	++
Asparagus	2	1 - 5	Air	10 - 14	+++
Beans, green snap	8	5 - 10	1 - 3	3 - 7	+
processing	8	5 - 10	8 - 10	20 - 30	++
Broccoli	0	0 - 5	1 - 2	5 - 10	+++
Brussels sprouts	0	0 - 5	1 - 2	5 - 7	+
Cabbage	0	0 - 5	2 - 3	3 - 6	+++
Chinese cabbage	0	0 - 5	1 - 2	0 - 5	+
Cantaloupes	3	2 - 7	3 - 5	10 - 20	++
Cauliflower	0	0 - 5	2 - 3	3 - 4	+
Celeriac	0	0 - 5	2 - 4	2 - 3	+
Celery	0	0 - 5	1 - 4	3 - 5	+
Cucumbers, fresh	12	8 - 12	1 - 4	0	+
pickling	4	1 - 4	3 - 5	3 - 5	+
Herbs4	1	0 - 5	5 - 10	4 - 6	++
Leeks	0	0 - 5	1 - 2	2 - 5	+
Lettuce (crisphead)	0	0 - 5	1 - 3	0	++
cut or shredded	0	0 - 5	1 - 5	5 - 20	+++
Lettuce (leaf)	0	0 - 5	1 - 3	0	++
Mushrooms	0	0 - 5	3 - 21	5 - 15	++
Okra	10	7 - 12	Air	4 - 10	+
Onions (bulb)	0	0 - 5	1 - 2	0 - 10	+
Onions (bunching)	0	0 - 5	2 - 3	0 - 5	+
Parsley	0	0 - 5	8 - 10	8 - 10	+
Pepper (bell)	8	5 - 12	2 - 5	2 - 5	+
Pepper (chili)	8	5 - 12	3 - 5	0 - 5	+
processing	5	5 - 10	3 - 5	10 - 20	++
Radish (topped)	0	0 - 5	1 - 2	2 - 3	+
Spinach	0	0 - 5	7 - 10	5 - 10	+
Sugar peas	0	0 - 10	2 - 3	2 - 3	+
Sweet corn	0	0 - 5	2 - 4	5 - 10	+
Tomatoes (green)	12	12 - 20	3 - 5	2 - 3	+
ripe	10	10 - 15	3 - 5	3 - 5	++
Witloof chicory	0	0 - 5	3 - 4	4 - 5	+

¹ Optimum and range of usual and/or recommended temperatures. A relative humidity of 90% to 95% is usually recommended (except for bulb onions).

² Specific CA recommendations depend on cultivar, temperature, and duration of storage.

³ Potential for application can be high (+++), moderate (++), or slight (+).

⁴ Herbs: chervil, chives, coriander, dill, sorrel and watercress.

Source: Saltveit, M.A. 2001. A summary of CA requirements and recommendations for vegetables. pp 71-94. Postharvest Horticulture Series No. 22A, University of California , Davis .

A summary of CA and MA recommendations for selected fresh-cut fruits and vegetables

James R. Gorny
International Fresh-cut Produce Association
Davis , CA 95616 USA

Table 1. Fresh-cut Vegetables

Table 2. Fresh-cut Fruit

Source: Gorny, J.R. 2001. A summary of CA and MA requirements and recommendations for fresh-cut (minimally processed) fruits and vegetables. pp 95-145. Postharvest Horticulture Series No. 22A, University of California , Davis .