TEMPEEATURE CHANGES, ETC., DURING CANNING OPERATIONS. 49 













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ing than is obtained when the reading is made following processing. In the 

 light of the preceding consideration of the liberation of gases during processing, 

 this result is to be expected. 



(2) The longer the processing period, the lower the vacuum obtained. 



HEAT PENETRATION. 



Figure 37 shows the heat-penetration curves for No. 2 cans of 

 spinach starting at 70° and 80° C, processed for different periods at 

 116° and cooled in water. The entire heating curve is also shown 

 for the No. 2 can, 

 starting at 20° C. and 

 processed at 116° un- 

 til the retort tempera- 

 ture was reached. 



These curves are 

 entirely similar to 

 those obtained for 

 sweet corn, though 

 the rate of tempera- 

 ture change is slightly 

 faster. 



By comparison of 

 these curves with the 

 entire curve, starting 

 at 20° C„ it will be 

 seen that after 60 

 minutes of processing 

 the temperature at 

 the center of the can 

 starting at 80° is only 

 6 degrees above that 

 of the can starting at 20°, and in the case of the can starting at 70° C. 

 it is only 4 degrees higher. 



When the can is cooled in water the temperature at the center of 

 the can continues to rise for one or two minutes only and then falls 

 rapidly. It is obvious, therefore, that here also, as in the case of 

 corn and sweet potatoes, the cooking of the material in the can may 

 be closely controlled by cooling in water. 



PRESSURE AND VACUUM IN GLASS CONTAINERS. 



The studies upon pressure and vacuum as applied to foods in glass 

 have not been completed. As is well known, the development of 

 pressures in glass containers is accompanied by danger and loss, 

 owing to liability to breakage, and this is avoided in present prac- 

 tice by partially sealing at first and then completing the sealing oper- 

 ation after the processing has been done. The escape of air, while 

 nearly or entirely complete in most cases, may be slightly restricted 



Fig. 37. — Experimental time-temperature curves for spin- 

 ach in No. 2 tin cans, starting at different uniform 

 temperatures, processed for different time periods at 

 116° C, and cooled in water at 23° to 24° C. The arrows 

 indicate when the cans were taken from the retort and 

 placed in water. A, Entire heat-penetration curve for can 

 starting at 20° C. and processed at 116° C. Curve for can 

 starting : B, At 70° C. and processed for 30 minutes ; b, at 

 70° C. and processed for 45 minutes ; b', at 70° C. and 

 processed for 60 minutes ; c, at 80° C. and processed for 

 45 minutes ; &, at 80° C. and processed for 60 minutes. 



