28 



BULLETIN 1022, U. S. DEPARTMENT OF AGRICULTURE. 



From the curves it is obvious that only a short exhaust is necessary 

 for peas, two to four minutes being ample if sealing follows imme- 

 diately. Emphasis must be laid on prompt sealing, however, as the 

 air of the head space cools more rapidly than the material, and unless 

 sealing is effected at once abnormal or too high pressures are de- 

 veloped during the processing and too low vacuums are obtained on 

 cooling. 



The cooling curves are of interest in that they show again how. 

 in material like peas, the starch and other substances which are 

 cooked out alter the viscosity of the liquor, causing the form of the 

 cooling curves when temperatures are reversed to vary considerably 

 from that of the heating curves. This is more marked in peas than 

 in string beans. The curves also show how the viscosity is increased 



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TIME IN MINUTES 



Fig. 16. — Experimental time-temperature curves for peas in No. 3 tin cans, starting at 

 different uniform temperatures and processed at 100°, 116°, and 121° C. Curve for 

 can starting: A, At 24° C. and processed at 100° C. ; a, at 24 J ° C. and processed at 

 116° C. ; a', at 24*° C. and processed at 121° C. ; C. at 80° C. and processed at 100° 

 C. ; c, at 80° C. and processed at 116° C. ; c', at 80° C. and processed at 121° C. 



by the higher processing temperatures. The viscosity will also be 

 affected by the degree of maturity of the peas and the length of the 

 processing period. 



Figure 16 shows curves for the No. 3 can and illustrates how, be- 

 cause of the rapidity with which temperature changes occur, process- 

 ing periods for No. 2 and No. 3 cans need differ but little; that is. 

 four or five minutes. 



There are several factors which will make the temperature change 

 during processing vary slightly. If in any way a number of peas 

 get broken the starch will come out of these more readily and will 

 make the rate of change of temperature slightly slower. The more 

 mature the peas the greater will be this effect. The proportion of 

 peas to liquor will have a small effect also. If the proper quantity 



