24 MASS. EXPERIMENT STATION BULLETIN 426 



WHAT ABOUT PRESSURE CANNERS? 



So far as pressure canner operation is concerned, it has been found (Esselen, 

 1944) from experimental studies that many of the directions provided for home 

 canning in pressure canners do not allow for sufficient venting and in some in- 

 stances might contribute to underprocessing. A venting time of at least ten 

 minutes is to be recommended for pressure canners of the size commonly used in 

 home canning. It was further pointed out that if the actual temperature in a 

 pressure canner was 235°F. (112.8°C.) because of insufficient venting, whereas 

 the temperature shown by a pressure indicator was 240°F. (116°C.), the required 

 process time should be considerably increased. In some cases the required process 

 tmie at 235°F. (112.8°C.) would amount to as much as 140 percent of that at 

 240°F. (116°C.). Thus a product which required a process time of 30 minutes 

 at 240°F. (116°C.) would have to be processed 42 minutes at 235°F. (112.8°C.) 

 to receive the same degree of sterilization. 



Inaccurate pressure gauges on pressure canners probably contribute to under- 

 processing in many cases. It is important that these gauges be tested at least 

 once a year. Koon (1944) reported that, of 232 pressure canner pressure gauges 

 tested at the VValtham Field Station, only 85 were accurate. The remainder 

 registered from 1 to 7 pounds too low or too high. Many of the safety valves 

 submitted were stuck and failed to blow ofT at pressures dangerous to the operator. 



ATTITUDE OF UNITED STATES DEPARTMENT OF AGRICULTURE AND 

 FOOD TECHNOLOGISTS TOWARD HOME CANNING AND BOTULISM 



In 1928 Jordan and Falk (1928) summarized the situation in regard to home 

 canning and botulism as follows: 



Since the spores of CI. botidinum are relatively common in soil in many 

 parts of the world and must adhere frequenth" to vegetables used for 

 preservation, and since also the temperatures used in the home canning of 

 food have in the past been often inadequate to insure complete steriliza- 

 tion, it might be expected that botulism would have occurred more com- 

 monly than has been the case. How can the rarity of the disease be ex- 

 plained? It is obvious that the factor necessary to bring about the pro- 

 duction and persistence of botulism toxin in canned foods must occur 

 relatively seldom. Numerous instances are on record in which the presence 

 of botulinum toxin has been demonstrated in certain jars or cans of food 

 while other jars or cans of the same lot preserved at the same time and to 

 all appearances in the same manner have proved free of toxin. Among 

 the factors that tend to prevent the production of botulinum toxin are an 

 unsuitable reaction of the culture medium, the maintenance of an unsuitable 

 temperature for germination, the presence of only a scanty number of 

 uninjured spores, and the simultaneous presence of other organisms such 

 as CI. sporogenes. Even when once formed, the toxin of CI. botulinum 

 rhay sometimes be destroyed by the growth of CI. sporogenes, and other 

 anaerobes. 



Without doubt the rarity of botulism and the apparent absence of outbreaks 

 in certain sections of the country are due to some of the imponderables posed 

 by Jordan. 

 In 1943 a press release from the United States Department of Agriculture stated: 



.... the use of the steam pressure canner is the only method it can 

 recommend for the canning of certain types of vegetables. In response to 

 numerous requests by individuals and agencies interested in the canning 



