146 HARVESTING, STORING, AND MARKETING 



Some tender varieties as Mcintosh or Northern Spy should prob- 

 ably not be run over a packing table, but should be sorted from long 

 tables with canvas bottoms in order to avoid bruising. In fact, such 

 varieties should not be packed in barrels. 



IV. MODIFIED ATMOSPHERE STORAGE FOR APPLES 



AND PEARS 



As Described by the Department of Pomology, Cornell University 



What Is Modified'Atmosphere Storage f Fruits are living organisms 

 which carry on the fundamental chemical reaction of respiration. Car- 

 bohydrates are oxidized in the presence of oxygen with the subsequent 

 production of water, carbon dioxide, and heat. In addition to the 

 thermal effect, any chemical reaction is affected by a diminution in 

 amount of one of the reactants or by allowing one of the end products 

 to accumulate. Hence respiration is diminished in intensity not only 

 by lowering the temperature (as in cold storage) but also by reducing 

 the amount of ox>^gen or allowing the carbon dioxide to accumulate. 



By use of gas-tight structures and controlled ventilation, the de- 

 sired proportions of carbon dioxide and oxj^gen can be easily attained 

 in most cases. For example, suppose that an operator desires an atmos- 

 phere containing 10 percent carbon dioxide and 11 percent oxygen. 

 The respiring fruit in a gas-tight, metal-lined storage cham^ber produces 

 an accumulation of carbon dioxide which finally reaches the desired 

 10 percent value. By difference from the normal air figure of 21 percent 

 oxj'gen, the oxygen value will now be 11 percent, since during the pro- 

 duction of carbon dioxide almost an equal volume of oxj^gen has been 

 consumed. In such a system, frequent determinations of the carbon 

 dioxide are made on a katharometer outside the chamber, and if the 

 carbon dioxide exceeds 10 percent, a port in the door is opened until the 

 proper percentage has been attained. 



With some varieties it may be desirable to have independent con- 

 trol of carbon dioxide and ox>^gen. Control of these two gases by 

 regulated ventilation will provide the desired atmosphere only when the 

 summation of these two gases is 21 percent. For an atmosphere such 

 as 5 percent carbon dioxide and 2 percent oxygen, the oxygen is reduced, 

 by respiration, from the normal figure of 21 percent down to 2 percent, 

 and the excess carbon dioxide is absorbed by means of sodium hydroxide 

 or calcium hydroxide in an atmospheric "scrubber." The oxygen is 

 prevented from falling below 2 percent by ventilation as required. 



