Controlled Atmosphere Storage

Controlled atmosphere storage can lengthen the storage life of some apple and pear varieties by several months. Controlled atmospheres (CA) slows ripening and maintains firmness, but also inhibits some flavor development. Sweetness and tartness are preserved to some extent by the use of CA, but other flavors may be lessened in CA-stored fruit. By lowering the oxygen in an air-tight storage room, fruit respiration is reduced and fruit quality can be maintained. The carbon dioxide concentration is typically allowed to rise, as well, and this contributes to quality maintenance. Oxygen is an essential ingredient in respiration. By reducing its concentration to a very low level, respiration is slowed to the point where the condition of the fruit changes very little. During respiration, fruit give off carbon dioxide, which builds up in a sealed room and also contributes to the slowing of ripening. The necessary equipment adds considerably to the storage costs of apples and pears.

The normal oxygen concentration of air, 21%, is reduced to 1 to 3% by flushing the storage room with nitrogen gas. Specialized nitrogen generators and liquid nitrogen are two methods of flushing a room. If the oxygen level drops below 1%, fermentation is likely, and will impair fruit quality. Good air circulation prevents the occurrence of areas in the room with oxygen concentrations that are too low. The fruit consume oxygen and decrease the concentration to below optimum if air circulation is poor. Bins should be arranged to allow air movement. A leaky storage room leads to above optimum oxygen and poor maintenance of fruit firmness. Some varieties have less tolerance for low oxygen storage and should not be placed in rooms with less than 2% oxygen. Where several varieties will be stored together in one room, maintain oxygen and carbon dioxide at levels suitable for the most sensitive variety. Fruit sensitivity to low oxygen varies with variety, maturity at harvest, and conditions during the growing season.

Carbon dioxide levels need to be maintained below a concentration of 3% during initial stages of storage, and in the case of fruit treated with SmartFresh®, 1% or lower is needed. When carbon dioxide is too high, browning disorders of the skin and flesh occur that make fruit unmarketable. Carbon dioxide can be removed from the atmosphere by specialized scrubbers or by placing bags of hydrated lime (calcium carbonate) inside the room. Bags lined with polyethylene need to be punctured in order to absorb carbon dioxide. To adequately maintain carbon dioxide levels below toxicity, the room should have 0.6 lbs. of hydrated lime for every bushel of fruit (0.6 kg of hydrated lime for 100 kg of fruit). Honeycrisp apples are very sensitive to high levels of carbon dioxide and are considered at risk in CA storage. To maintain optimum atmospheres, a gas analyzer is needed to routinely measure concentrations of oxygen and carbon dioxide.

The application of SmartFresh® increases the sensitivity of apples to carbon dioxide. Maintain carbon dioxide below 1% in rooms that have fruit treated with SmartFresh®. Air circulation is critical with SmartFresh®.

The decision to install a CA room is usually based on the needed duration of cold storage. For fruit that will be marketed four months or longer after harvest, CA storage can prevent losses. For varieties that soften quickly after harvest such as McIntosh, CA storage may be beneficial for shorter storage durations of two to four months.

CA storage can increase chilling sensitivity in some varieties of apple. Therefore, a warmer storage temperature of 36-38°F may be needed to prevent browning disorders. McIntosh is chilling sensitive in some cases, although many storage operators have successfully stored it at colder temperatures to maintain better firmness.

Recommended Levels for Standard Controlled Atmosphere Storage for Apple Varieties
Variety	Oxygen (%)	Carbon Dioxide (%)1	Carbon Dioxide (%) with SmarfFresh® treated fruit
Cortland	2.5	2.5	<2.0
Empire	2.5	2.0	<0.5
Gala	2.5	2.5	<2.0
Golden Delicious	2.5	2.5	<1.0
Honeycrisp	2.5	<1%	<1.0
Idared	2.5	2.5	<1.0
Jonagold	2.5	2.0	<1.0
McIntosh	2.5	2.5	<0.5
Red Delicious	2.5	2.5	<1.0
1Carbon dioxide in the first six weeks of CA storage, after which concentration can increase by 1-2%.