THE COMMERCIAL CANNING INDUSTRY 



169 



temperature of 100° Centigrade (212° F.) being 

 thereby obtained. 



(2) The chemical bath. This consists of a strong 

 solution of somesalt, gen- 



erally calcium chlorid, 

 because of its great sol- 

 ubility. The boiling point 

 of the solution being 

 much higher than that 

 of water, higher temper- 

 atures may be reached by 

 its use than with the 

 ordinary water bath, and 

 consequently a shorter 

 time is required to bring 

 about sterilization. This 

 method was first em- 

 ployed in this country 

 about 1863, but was not 

 a success because the 

 cans of that time were 

 not strong enough to 

 withstand the pressure 

 generated within. The 

 method of use is the 

 same as with the water 

 are boiled for a certain 



Fig. 248. 

 A commercial corn cooker. 



bath, i. e., the filled cans 

 definite period. 



(3) Steam under pressure. This method of steril- 

 ization was introduced about 1870. The tempera- 

 ture in this case may be varied by control of 

 steam pressure. The steam being confined in the 

 retorts, of course the pressure is equal within and 

 without the cans ; thus, unless the outside pres- 

 sure is removed suddenly, the strain on the cans 

 is not great and loss from bursting is small. Most 

 of the modern cans, however, are sufficiently 

 strong to withstand sudden changes without injury. 



There are two modifications of the retort, known 

 as the " wet retort " and the " dry retort." In the 

 former, the kettle is filled with water and steam 

 under pressure blown in, so that the boiling-point 

 of the water is much raised owing to the increased 

 pressure. These kettles are generally cylindrical 

 and are placed in a vertical position, with a heavy 

 lid on the upper end. When in use, this lid is fast- 

 ened down by means of heavy bolts. The kettles are 

 generally provided with three valves, — an intake 

 valve for steam at the bottom, an outlet for water 

 at the bottom and an exhaust valve for steam in 

 the lid. Although spoken of as a " wet retort," it can 

 be used without water in the same way as a "dry 

 retort." In the " dry retort," the steam under pres- 

 sure is blown in, directly replacing the air and 

 coming directly into contact with the cans. 



The Portland type of retort consists of a heavy 

 iron chest, about cubical in shape. One side of the 

 cube is the door, which is hinged and fastens by 

 bolts. With the exception of the door the retort 

 is cast all in one piece, the door forming a separate 

 casting. 



Both types of retorts are provided with ther- 

 mometers and pressure gages. In the use of retorts 

 of either kind it is essential that a current of 

 steam under pressure be passed continuously, this 



"circulation" being eifected by leaving the ex- 

 haust valve slightly open. The temperature may be 

 kept constant by regulating the amount of steam 

 entering the retort and the amount of the exhaust. 



As already mentioned, this method is most effi- 

 cient in its action on the resistant spores of bac- 

 teria, consequently is the safest method to employ 

 in the preparation of canned goods. It is neces- 

 sary, however, to avoid excessive heating, as dam- 

 age to the foods may be done in this way. One 

 result of over-cooking is to produce discoloration 

 of the food substance, a defect which sometimes 

 interferes with the commercial value of the article. 

 Temperatures above 120° C. (248° F.) are rarely 

 used, the best temperature for any material being 

 determined directly by experiment. 



In sterilization of canned foods, it is necessary 

 that the whole contents of the can be subjected to 

 the required temperature for a period of time long 

 enough to destroy all germs whether spore-produc- 

 ing or not. This period of time can be determined 

 accurately only by experimental tests. It is of 

 equal importance to know the length of time 

 necessary for the required heat to penetrate to 

 the center of the cans, this time varying very much 

 with different materials, owing to their different 

 conductivity for heat. Liquids are, in general, good 

 conductors, while solid or semi-solid substances 

 conduct but poorly. Knowledge on this point is 

 absolutely essential in order to prescribe a satis- 

 factory process. 



The vacuum. 



It is customary in the preparation of canned 

 foods to have a partial vacuum in each can, and 







Fig. 249. Improved steel process kettle, manufactured to 

 hold 800, 1,000 and 1,200 two-pound cans. 



for many years it was thought that this vacuum 

 was the principal factor in keeping the goods. 

 While this is untrue, it is desirable to have the 

 vacuum as it allows a means of inspection of the 

 cans. The vacuum is indicated by the concavity of 



