Controlled Atmosphere Storage

Altering the atmospheric gas content of the refrigerated room in which fruit are stored can lengthen the effective duration of storage time. The concept of controlled atmosphere (CA) storage originated with work on modified atmospheres around 1920. Research on low-oxygen storage did not begin until around 1930. The initial use of low-oxygen CA storage did not occur until the 1940s. Several temperate fruit respond well to CA storage. Excellent benefits are obtained with apples and pears, while good effects are obtained with cherries, figs, nectarines, peaches, plums, and prunes. Fair results are obtained with apricots. The overall impact of CA storage is to allow retailers to provide specific cultivars over a longer marketing period, thus removing seasonal fluctuation in product availability. Physiological effects of CA on fruit quality include lowering respiration, reducing acidity and chlorophyll losses, and maintaining firmness. A negative effect of long-term CA storage is a reduction in the ability of fruit to produce characteristic flavor and volatile components.

Controlling storage gas levels requires an airtight room and can be accomplished through a number of techniques. The removal of oxygen from an airtight storage room can be accomplished by flushing the room with compressed nitrogen, or in a more expensive procedure, nitrogen can come from the release of the gas from liquid nitrogen as it warms and boils. One of the first methods developed to reduce the oxygen content of ambient air, from roughly 21 to about 3 percent, used a catalytic burner. Another method for producing nitrogen gas is labeled "cracking." In this process, a burner cracks ammonia into nitrogen and hydrogen. The hydrogen is burned, producing water as a by-product. More recently, the methods for CA atmospheric generation rely on gas separation technology. One technique, pressure swing adsorption, utilizes two chambers filled with carbon molecular sieve material that retains oxygen and generates relatively pure nitrogen. As oxygen is adsorbed, the pressure builds on one chamber, and nitrogen is released into the CA room. As one chamber absorbs oxygen, the other chamber is desorbing. The carbon sieve also acts as a scrubber that removes carbon dioxide and ethylene. Another gas separation technique uses ambient atmosphere and separates nitrogen from oxygen through the use of a hollow fiber, molecular sieve membrane system. This system was first developed in the medical field in the 1950s for use in artificial kidneys. For CA purposes, ambient air is compressed, put through a coalescing filter, dried, and filtered for particles, oil contaminants, and carbon-containing materials. Once cleaned and filtered, the air passes through a bundle of hollow fibers. Oxygen, carbon dioxide, and water vapor are "fast" gases that diffuse through the membrane. Nitrogen is slow and thus remains behind within the membrane bundle at purity levels around 98 percent. Some CA atmosphere regimes require oxygen levels of 0.7 percent, and these low levels can be achieved through recirculating the 98 percent pure nitrogen through the hollow fiber sieve a second time.

In addition to regulating oxygen levels, the buildup of carbon dioxide in storage is a concern. Carbon dioxide in the past was mainly controlled by the use of alkaline solutions such as potassium or calcium hydroxide, an activated charcoal scrubber, or adsorption by dry hydrated lime. Hydrated lime is a method still used today. The use of gas separators allows the control of carbon dioxide by dilution with liquid or compressed nitrogen.

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