516 MANUAL OF MILK PRODUCTS 



Swell. 



The overrun, or the amount (volume) of ice cream obtained 

 in excess of the amount (volume) of total mixture put into the 

 freezer, constitutes the " swell." This increase in volume is due 

 almost wholly to the incorporation of air into the product and, 

 to a very slight degree indeed, to the expansion of the cream 

 due to freezing. The amount of swell obtained is a sort of 

 component resulting from several cooperating factors. 



A viscous cream retains more of the air which is whipped 

 into it than does a cream relatively non-viscid ; hence the more 

 viscid it is, the greater the swell, other things being equal. The 

 viscosity of a cream increases very noticeably from the hour of 

 its separation or pasteurization for about six hours, and slowly 

 thereafter for several days, especially if it is held cold. The 

 cause for this increased viscosity is but poorly understood, but it 

 seems to be due to a slight thickening not unlike the clotting of 

 blood. 



The rate of freezing is an important factor in this matter of 

 increase in volume ; for the reason that if the freezing be done 

 too rapidly, too little time elapses to admit of its thorough whip- 

 ping or beating, during the interval after the cream becomes 

 'cold enough to whip and before it becomes frozen. In other 

 words, the temperature drop through the few critical degrees of 

 cream whipping is too rapid to admit of a thorough incorpora- 

 tion of the air. The ordinary mixture of cream and sugar used 

 in ice cream making is too thin and sloppy to retain any appre- 

 ciable amount of the air whipped or beaten into it before it 

 reaches about 34 F. At this temperature it begins slowly to 

 foam up and gradually increases in volume until the tempera- 

 ture of 29 to 28 is reached, at which point (its true freezing 

 point, which varies from 29 to 28, depending on the amount 

 of sugar in the solution, and in general practice, too, on the accu- 

 racy of the thermometer used) the temperature ceases to drop 

 and stands for some minutes without becoming any colder, as 



