4 BULLETIN 1022, U. S. DEPARTMEXT OF AGRICULTURE. 



Figures were given for the vacuum produced by heat exhaustion 

 in No. 2| cans sealed at different temperatures. Tables were given 

 showing the effect of the fill of the can on vacuum, as well as tables 

 showing the amount of vacuum with different sealing temperatures. 

 The relative merits of heat exhaust and mechanical exhaust were 

 discussed, the relation of exhaust and head space to erosions and per- 

 forations was emphasized, and other factors influencing vacuum were 

 enumerated. 



In 1917 Bitting and Bitting (7) reported upon further studies 

 along this line. Attention was called to the pressure developed in 

 tin cans during processing, which showed very considerable diver- 

 gence from the pressure calculated from physical laws. These inves- 

 tigators stated also that gases incorporated in the tissues of fruits and 

 in cold-storage products cause a decided increase above normal pres- 

 sure in processing. 



Tables showing the absolute pressure of saturated steam at various 

 temperatures and also tables showing the expansion of water were 

 given by Bitting and Bitting. They discussed also the pressures 

 developed when cans were filled with different quantities of hot and 

 of cold water. 



The possible relation of exhaust and vacuum to the black discolora- 

 tion of corn was pointed out more recently by Bigelow (2). 



The material presented in these papers is rich in suggestion and 

 of much practical importance. Through them the practice of ex- 

 hausting is placed upon a sound scientific basis. 



In the present work the writers have found it necessary for their 

 purposes to cover in an experimental way some of the same, ground 

 included in the above-mentioned investigations. At the points of 

 contact between the work of the earlier investigators and that herein 

 reported more detailed references will be made. 



NEED FOR STUDY OF INITIAL TEMPERATURE IN ITS RELATION 

 TO PRESSURE AND VACUUM. 



The problem always before the camier is how to get the best quality 

 in the finished product. In striving for the best quality many things 

 are to be considered, such as selection of the best varieties, prompt 

 transfer from the field to the place of canning, and proper treat- 

 ment previous to filling the can. But the thing of greatest impor- 

 tance is to secure the proper condition in the can so as to protect 

 the food against microorganisms and at the same time develop and 

 preserve the desirable flavors in the product. A thorough knowl- 

 edge of conditions in the can, therefore, will facilitate the choosing 

 of the best procedure. 



The chief altering factor in the can is the temperature. Proper 

 temperature maintained for a length of time sufficient to prevent 



