30 



BULLETIN 1022, TJ. S. DEPARTMENT OF AGRICULTURE. 



The outstanding feature of these curves is the fact that by far 

 the greatest reduction in pressure during the 45-minute process period 

 occurred as the result of the first two minutes of the exhaust. In the 

 case of the No. 2 can this was a reduction of about 7 to 8 pounds 

 per square inch during the first two minutes, which was lowered 

 only about 1.8 pounds further by a 6-minute exhaust. The curves 

 for the No. 3 can show the same thing, except that the maximum 

 pressure developed in the can not exhausted was not so great. It 

 is obvious that the temperature of the head space is the greatest 

 factor in determining the vacuum and pressure, for the average of 

 the can at the end of two minutes could not be high enough to account 

 for the low pressure and high vacuum thus obtained. The effective- 

 ness of a short exhaust, as in the case of peas, would be realized only 

 by immediately sealing, as will be shown later by the results of 

 vacuum tests on cans in lots which were exhausted in the steam 

 box for the different exhaust periods and then removed and sealed at 

 once. 



VACUUM STUDIES. 



The figures for the vacuum obtained in the cans used in the pres- 

 sure tests which are of interest are presented in Table 8 : 



Table 8. — Vacuum tests with tomatoes in No. 2 and No. 3 tin cans. 





Temperature (°C). 



Time (minutes). 



Barometer reading. 



Vacuum 



Size of cans. 



Material 

 at the 

 start. 



After 

 cooling. 



Exhaust. 



Length of 

 process. 



At 

 sealing. 



After 

 cooling. 



(inches 

 of mer- 

 cury). 



No. 2 



No. 3 



f 24 

 \ 23i 

 I. 23 



f 25i 

 \ 25 

 1 24 



16J 



17 



19 



22| 



22 



21J 



2 

 4 

 6 



2 

 4 

 6 



45 

 45 

 45 



45 

 45 

 45 



29.85 

 29.80 

 29.39 



29.86 

 29.80 

 29.73 



29.80 

 29.73 

 29.39 



29.85 

 29. SO 

 29.39 



15 

 16| 



m 



15 

 15 





16 



With the increase in exhaust there is slight increase in vacuum, 

 which corresponds well with the figures on pressures, and they are 

 found to be fairly high. • « 



The average vacuum readings upon No. 3 cans in lots, exhausted 

 for different periods in the steam box and then sealed in the mechani- 

 cal sealing machine, were as follows : 



l-minute exhaust 3 inches. 



2-minute exhaust : 34 inches. 



3-minute exhaust = 4 inches. 



4-minute exhaust 64 inches. 



5-rninute exhaust S inches. 



It is seen that these values are considerably below those in which 

 the cans were sealed while still in the retort, and they illustrate how 



