126 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE 



Vacuum-Chamber Method 



Another process of removing air from 5-gallon cans and replacing 

 it with carbon dioxide or nitrogen can be termed the vacuum-chamber 

 method. This method requires equipment similar to that required in 

 vacuum canning. One type of equipment has a flat plate with a space 

 on it for one or more cans. Cans are placed on the plate and are 

 covered by a heavy metal chamber or bell provided with a counter- 

 poise. For the evacuation of the air in a chamber, a vacuum pump 

 is provided. The effectiveness of the equipment depends largely on 

 the closeness of the joint between the chamber and the plate, which 

 governs the leakage of air inward, and on the degree of vacuum to 

 which the pump can exhaust the air in the chamber and the can or cans 

 thereunder. 



For a single-unit chamber the pump should have a capacity dis- 

 placement of 100 cubic feet per minute, and should produce a vacuum 

 of 29.5 inches of mercury. The highest degree of vacuum can be 

 attained by the use of any of a number of rotary pumps. The next 

 type of pump, in order of attainment of vacuum, is the two-stage 

 steam ejector. At least 75 pounds per square inch of steam pressure 

 is necessary for efficiency. The first cost is lower than those of rotary 

 or reciprocating pumps, but the demand on steam between evacuations 

 diminishes the utility of this device. Single-stage reciprocating 

 pumps will not produce the required vacuum, but double-stage 

 reciprocating pumps will. Some double-acting reciprocating pumps 

 can be easily and inexpensively converted to double-stage pumps. 



Modern gassing chambers of the bell or horizontal type hold the 

 lid away from the can during the operation by means of an ex- 

 ternally operated chuck or. by an electromagnet. The procedure is 

 carried out as follows : The can is filled with a weighed amount of 

 dehydrated vegetables, and a lid is set in place. The chamber is 

 closed. A vacuum of at least 29.5 inches is drawn on the chamber and 

 this will take 20 to 40 seconds. The valve to the vacuum pump is then 

 closed, and a valve to the cylinder of gas is opened, releasing the 

 vacuum. A pressure of 1 to 2 pounds per square inch is built up in the 

 bell, which will prevent the entrance of air when the chamber is 

 opened. The chamber is opened and the can is removed and hermeti- 

 cally sealed immediately. Cans should be handled by the corner 

 edges, since any bellows action of the sides caused by handling will 

 draw air into the can before it is sealed. 



In practice, the time for evacuation is 20 to 40 seconds, for breaking 

 the vacuum about 20 seconds, and for a cycle including opening and 

 closing of the chamber and sealing a pair of cans, about 90 seconds. 

 In tests oxygen content of the atmosphere in cans of carrots and 

 cabbage sealed by means of a double-stage reciprocating pump, with a 

 29.5-inch vacuum, and one chamber, was found to be 0.5 to 1.6 percent. 

 Three to four cans may be evacuated, gassed, and sealed per minute in 

 modern-type machines (fig. 62) . 



Gassing by Means of "Dry Ice" 



The solid carbon-dioxide snow or "dry ice" method has been de- 

 veloped at the Western Regional Research Laboratory as a means of 

 gassing carrots or cabbage with a minimum amount of equipment. 

 The method has not yet been tried out commercially. Unlike water 



