320 PRESERVATION METHODS 



mize internal corrosion by the elimination of as much air as possible 

 from the canned product, to prevent end bulging which may be caused 

 by changes in altitudes, and to extend the shelf-life of the product. 



A vacuum may be obtained in canned fishery products in a variety of 

 ways. Perhaps the oldest method and one which is still in use in certain 

 operations is the steam exhaust. In this method the cover is usually 

 clinched loosely on the can, and the packed can is then either preheated 

 or passed through an exhaust box to heat the contents of the can and 

 drive off the air, replacing it with steam. The can is then sealed while hot 

 and processed. During cooling, a partial vacuum is created by conden- 

 sation of the water vapor in the headspace and by product contraction. 



A related method of obtaining partial vacuum is hot filling the product 

 or a portion of the product, such as the added oil or brine. Another 

 method is the ''steam-vac" closure. Here a steaming device built into 

 the closing machine blows steam, under 5 to 10 pounds pressure, into the 

 headspace area of the can prior to closing, thus evacuating the air and 

 replacing it with steam. This method is still used to a large extent on 

 canned salmon and is often used in combination with a hot oil or brine fill. 



The best and most consistent method of obtaining can vacuum is 

 through the use of vacuum closing equipment. Here the can is actually 

 sealed in a vacuum chamber built into the closing machine. The addition 

 of the vacuum chamber to the closing equipment necessarily complicates 

 and raises the cost of such equipment. 



Retorts. A retort is a chamber in which canned foods may be processed 

 under superimposed steam pressure. There are two basic types of retorts — 

 still (nonagitating) and agitating. In addition, retorts may be either batch 

 type or continuous. The vast majority of fishery products are processed 

 in still, batch-type horizontal or vertical retorts. While the size, design, 

 and capacity will vary, depending on the application, all retorts should 

 have the following related equipment : 



Automatic Controller to accurately maintain processing temperatures. 



Indicating Thermometer with corresponding pressure scale suitabl}- ad- 

 justed so as to compensate for altitude. 



Recording Thermometer for establishing accurate records of actual 

 processing. 



Pressure Gauge for accurate knowledge of internal retort pressure dur- 

 ing processing and cooling. The separate pressure gauge also pro- 

 vides a means for cross-checking the thermometers. 



Retort Crates, Baskets, Trays, or Gondolas for holding stacked cans dur- 

 ing processing. These crates are made of strap iron or perforated 

 metal and should be so designed as to allow uniform steam flow. 



