COMMERCIAL CANNING OF FOODS. 3 



ing apparatus. Thus a time and temperature curve is obtained directly. One of the 

 important points learned from the latter apparatus -was the effect of stirring or agitat- 

 ing the contents of cans which ordinarily required long cooking. A can of com in a 

 retort requiring 65 minutes to reach 245° F. requires only 30 minutes when rolled back 

 and forth. The effect of the agitation was a shorter cooking, a brighter color of the 

 com, and a bright can on the inside. The principle is good, but some mechanical 

 difficulties in successful operation have yet to be overcome. 



The penetration of heat in the can is dependent almost wholly upon the ease with 

 which convection currents are set up, occurring most rapidly in products which per- 

 mit the tree circulation of water, weak brine, or sirup between the solids, as in peas, 

 and least rapidly in the absence of free liquid, as in dry-packed sweet potatoes. Prod- 

 ucts ha^'ing a heavy though uniform consistency, like pumpkin and squash, require 

 a long time for heat to penetrate to the center of the can. Heavy tomato pulp takes 

 a much longer time to reach the boiling point than canned tomatoes, and soft ripe 

 fruits, as apricots and peaches, need more time to become sterilized than green fruit, 

 not because the germs are more resistant, but because the heat can not penetrate as 

 readily as when the liquid circulates freely between the solid pieces. Failure to 

 recognize this principle of the movement of heat in liquid, semi-liquid, and solid 

 substances has caused the loss of thousands of cases of foods. Mechanical agitation 

 shortens the period of cooking, especially in foods of heavy body; at the same time 

 it places a greater strain upon the can, with a tendency to increase the number of 

 leakers. 



The varying temperatures and methods used in canning cause strains upon the 

 containers, which may be comparatively light or so severe as to cause leakage. The 

 contents expand as the temperature rises above that at which the sealing was done 

 and contract as it goes below that point. The internal pressure, therefore, reaches 

 the maximum in foods packed cold and processed at the boiling point or above in the 

 open, and, conversely, the vacuum is highest in those filled near the boiling point and 

 stored very near that of freezing. If the processing be done in a retort, the internal 

 pressure ^^-ill be the same as that induced by heating to 212° F., plus the number of 

 pounds used in the retort, but a strain is .produced only by such part of the pressure 

 as is developed in bringing the contents to the boiling point as long as the retort is 

 closed. For example, if a can of peas be sealed at 160° F., placed in a retort, and 

 prfx;e8sed at 240° F., the can will first be subjected to a steam pressure of about 10 

 pounds from ^nthout; the internal pressure will rise until the temperature has reached 

 212 °F., at which point the pressure will show about 5 pounds, and as the temperature 

 approaches that of the retort the pressure will show about 15 pounds, only 5 pounds 

 of which will exort any strain until the retort is opened, when the whole becomes 

 effo<;tive, gradually decreasing as the cooling takes jjlace. 



If, however, when the process is completed, the retort remains closed and a stream 

 of cold water is admitted to chill the (.ans, the first effect is to condense tiie steam 

 in the retort suddenly, thus causing a vacuum. This has the effect of removing 15 

 pounds of atmospheric pressure from the outwido of the can or virtually adding the 

 equivalent of that much pressure within, giving a total of about 30 pounds internal 

 Ktrain. THIh is sufficient to break Hf)mo cans, particularly No. 3 or above, aiul accounts 

 for many slow Ic-aks. The more suddenly the strains occur, the greater th(s jxircentage 

 of leaks. Pr(K;eHHing in retortH is accomplishrtd wilh little loss, if on admitting the 

 sUi&m the preHsuro be gradually ajjplied for a few minutes and in turning off a vent 

 he opened and a little time be given to cool. 



In ordr-r to work the retorts rapidly and avoid the sevcrr; Htrains, air imiy he intro- 

 duced under a prcmnn- equal V) tliat of tlie Ht<-am as the latter is bcMug turn<-d off. 

 \Vat<;r for rrKjling may then be introduced witliout danger. By this metlio<l square 

 can.H, gallon and even 5-gaHon cans may be processctd without the Iohhch formerly 

 f!Xi»<;riencr-d. In the open calcium chlorid or other bath th«! cans are subjected to 

 the maximum intt.Tnai pressure througlujut the; entire operation. 



