THE COMMERCIAL CANNING INDUSTRY 



171 



ous. _ There may be the formation of acids— lactic, 

 acetic, and butyric particularly— with formation 

 of gas; acid production (lactic) without gas forma- 

 tion; or putrefactive fermentation. The fermenta- 

 tions vary with the conditions and in many cases 

 are due probably to mixed infection, thereby giving 

 a large variety of products. These fermentations 

 often take place rapidly, and are generally favored 

 by a temperature of 35° to 40° C. (95° to 104° F.). 

 These rapid actions are generally accompanied by 

 evolution of gases, sometimes the pressure of the 

 gases generated being sufficient to burst the cans. 

 In other cases, the action is very slow and but a 

 small amount of gas is produced. 



The sweating of green peas when allowed to 

 stand in boxes has been studied to some extent by 

 Underwood and the writer. Rapid fermentation 

 takes place with the formation of acids and a 

 slimy layer envelops the peas. Because of this 

 action, peas should never be allowed to stand over 

 night or for any length of time before being steri- 

 lized. The bacteria causing these fermentations 

 have been studied by Prescott and Underwood. 



Beans. — The canning of green beans or string 

 beans is done in much the same way as the canning 

 of peas. Baked beans, however, being somewhat 

 denser and more resistant to the penetration of 

 heat, require somewhat longer cooking in order 

 thoroughly to sterilize. They are generally packed 

 together with pork or with the addition of some 

 sauce, as tomato. 



Sweet corn is canned in immense amount in the 

 United States. The corn is cut from the cobs by a 

 machine, mixed with water and a little "brine," 

 and heated in a "cooker," in which it reaches a 

 temperature of about 80° C. (176° P.). Sugar is 

 added in small amount and the heated corn is filled 

 into cans and sealed immediately. The sterilization 

 is done by steam under pressure of thirteen to 

 fifteen pounds, and the time required for steriliza- 

 tion varies with the consistency, percentage of 





Fig. 252. Peacb peelers for canning factory. 



water, starch and the like, variations of fifty to 

 seventy-five minutes being found in different fac- 

 tories. Sweet corn undergoes fermentative changes 

 even more rapidly than do peas, because of its high 

 percentage of sugar, and especially from the fact 

 that the kernels are broken, thus allowing direct 



access of bacteria to the saccharine juices. Unless 

 means were taken to prevent it, fermentation 

 would take place in a short time. An extended 

 study of this fermentation has been made by W. L. 

 Underwood and the writer. Several species of 

 bacteria were discovered in cans of " sour " corn, 

 some of these being able to resist five hours' 

 boiling without being destroyed. Further investi- 

 gation showed the source of these germs to be 



the ears of corn. Bacteriological examination of 

 healthy ears of fresh corn' revealed the presence 

 of germs on the kernels beneath the husks. These 

 bacteria give characteristic reactions with nutrient 

 media, and produce rapid fermentation of sugars, 

 giving rise mainly to lactic acid, but also to forms 

 of butyric and acetic acid. Sterilized sweet corn 

 was converted in a few hours to a mass with strong 

 acid reaction and sour taste. The most favorable 

 temperature is 36° to 40° C. 



The effect of the various steps in the canning 

 process was also investigated. In the "cooker" 

 many bacteria are destroyed, the more resistant 

 ones, however, remaining uninjured. Two-pound 

 cans which were given a heating' at 120° C. (248° 

 F.) for thirty minutes were found to contain living 

 bacteria, and cans so treated frequently become 

 much disturbed within a few days. On the other 

 hand, if the heating process is continued for a 

 sufficiently long time all bacteria are destroyed. 

 The reason for the necessity of the long period of 

 heating is the low conducting power for heat of 

 the corn. Experiments made with maximum regis- 

 tering thermometers showed the time necessary for 

 the temperature applied to record at the center of 

 two-pound cans, as follows: 



Temperature applied,— 



This is strikingly confirmed in a practical way by 

 the fact that souring, in many cases, is found only 

 at the center of the cans, and in a majority of 

 cases the fermentation probably begins at that point. 



