156 DAIRY TECHNOLOGY 



When cream is frozen very quickly, less swell is secured. 

 The reason for this seems to be that the swell is produced 

 only while the cream is passing through a few degrees of 

 temperature just before it freezes. Only during this short 

 time is it sufficiently viscous to retain the air that is beaten 

 into it. According to Washburn, 1 when the temperature 

 of the cream reaches 34 F., the cream begins to foam up 

 and continues to increase in volume until a temperature 

 of 29 or 28 F. is reached. At this point the temperature 

 remains constant for from four to fifteen minutes. During 

 this time the latent heat is being extracted from the cream 

 by the freezing mixture. The swelling of the cream, that 

 begins at about 34 F., is most rapid toward the end of 

 this period of lowering temperature. Just before the cream 

 freezes, the swell is very rapid. 



The effect upon the swell, of over-freezing, is reported 

 by the same authority. A series of trials was made in a 

 double-disc continuous freezer, run at the rate of 225 

 revolutions per minute. It was found that when the ice 

 cream was drawn off at a temperature of 29 F., the swell 

 was 70 per cent; at 28 F., 60 per cent; at 27 F., 50 per 

 cent; at 26 F., 43 per cent; at 25 F., 40 per cent. The 

 ice cream drawn off at 29 F. was too soft, but that at 28 

 was entirely satisfactory. 



The speed of the rotator or beater also influences the 

 overrun or swell. Some freezers have facilities for varying 

 the speed, making it slow at first, then during the latter 

 part of the freezing period increasing the speed. The 

 higher the speed up to a certain limit (250 rev. per min.), 

 the more air is beaten into the ice cream. High speed 

 during the first part of the freezing period may cause the 

 cream to churn. 



1 Vermont Bui. 155. 



