44 



BULLETIN 956, IT. S. DEPARTMENT OE AGRICULTURE. 



The nature of the pack is one of the most important factors af- 

 fecting the rate of change of temperature. In any material of 

 any composition which is so packed that there is a free liquid filling 

 in the interspaces between the pieces of material in the pack there 

 is a very rapid change in temperature during processing. The tem- 

 perature of the material approaches the temperature of the bath 

 or retort very quickly. Any variation in the method of packing 

 which interferes with convection alters the rate of change of tem- 

 perature in the center of the can. The proportion of liquid to 

 material is important, as has been shown under " Sweet corn." The 

 fineness of division of the material is of importance because of the 

 increased effect of surface tension in finely divided material. The 

 fineness of division also affects the proportion of liquid to material. 



The blanching or precooking affects the temperature changes if it 

 in any way alters the nature or proportion of free liquid in the ma- 



^70 



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Fig. 51. — Time-temperature relations for different vegetables when processed in No. 3 

 tin cans at 100° C, as compared with distilled water. Curves A, B, and C were plotted 

 from readings made at intervals of 1 minute, and curves D to H were plotted from 

 readings made at intervals of 5 minutes. A, Distilled water ; B, string beans ; C, soy 

 beans ; D, cabbage ; E, tomatoes ; F, pumpkin ; G, corn ; H, sweet potatoes. 



terial. Blanching the leafy vegetables would enable a closer pack 

 to be made and would thus make the rate of change of temperature 

 slower. 



The heating and cooling medium is of importance. When the 

 container is heated or cooled in the air the process is very slow. 

 When it is heated in water or steam and when cooled in water, the 

 process is very rapid, depending upon the other factors already 

 pointed out. 



Figure 51 shows curves for a large number of vegetables proc- 

 essed in No. 3 tin cans at 100° C. They fall pretty distinctly into 

 two groups. The group having a free liquid with a consequent 

 rapid rise in temperature contains by far the larger proportion of 

 fruits and vegetables. This group includes string beans, Lima 

 beans, soy beans, peas, and asparagus. In the second group, having 



