4 BULLETIN 1022, U. S. DEPARTMENT OF AGRICULTURE. 
Figures were given for the vacuum produced by heat exhaustion ~ 
in No. 24 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- 
Seay hice was emphasized, and other factors influencing vacuum were 
enumerated. 
In 1917 Bitting and Bitting (7) reported upon peer in: 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 1 increase above bee pres- 
sure 1n 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 St 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 
i TO PRESSURE AND VACUUM. 
The problem always before the canner 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 procedur : 
The chief lice factor in the can is the temperature. Proper 
temperature maintained for a length of time sufficient to prevent 
