4 MASS. EXPERIMENT STATION BULLETIN 302 



data were secured by determining the temperature of the mix in the freezer and 

 the percentage of overrun at intervals of 1 minute throughout the freezing process. 

 All of the samples (pint cartons) within a given series used for comparison were 

 drawn at the same temperature and at approximately 85 per cent overrun. The 

 overrun of all samples used for judging was checked by calculations from volume 

 and weight determinations. 



Melting resistance data were secured by subjecting all samples to the same 

 thermal conditions and noting the appearance of the sample and the rate of 

 melting. The samples remained in the hardening room (approximately 0° F.) 

 for several days, after which they were placed on 0.25-inch mesh wire screening 

 over the mouth of a funnel supported by a ring stand. Room temperature varied 

 from 70° to 75° F. The melted ice cream was weighed after the samples had been 

 exposed for l]/^ hours, and photographs were also taken at that time. 



Approximately one and three weeks after being frozen the ice cream was 

 judged by members of the college staff and a class in judging dairy products. 

 Samples within each group used for comparison were placed numerically and 

 a three to one preference was assumed to be significant. 



The term "basic viscosity" in this investigation is defined as the viscosity 

 which persists in an ice cream mix after being stirred by ordinary means until 

 there is no further reduction in consistency. The samples were first tempered 

 to 68° F. in a constant-temperature bath, then agitated by means of a motor- 

 driven paddle consisting of a 0.25-inch mesh wire screen. The device was arranged 

 so as to prevent the incorporation of air and so that the sample could be agitated 

 while remaining in the constant-temperature bath. A constant agitation time of 

 10 minutes was used because a preliminary experiment showed no decrease in 

 consistency of the mix after 10 to 30 minutes of agitation. Basic viscosity deter- 

 minations were made with the MacMichael viscosimeter, when using the disc 

 bob, and rotating the cup which contained 125 cc. of mix at 68°, at a speed of 

 IS r.p.m. Duplicate or triplicate determinations were made. The viscosity measure- 

 ments were made at a fixed time (3 minutes) after the samples were agitated in 

 order to prevent any appreciable recovery in the viscosity of the mix. All results 

 are reported in degrees MacMichael for No. 30 gauge wire. 



EXPERIMENTAL RESULTS 



The Effect of Aging at 68° F. on Bacterial Growth. pH, and Titratable Acidity 



Before studying the effect of aging at a high temperature on the quality of the 

 finished product, it seemed desirable to determine its effects on bacterial growth, 

 pH and titratable acidity as these factors are of importance in determining the 

 practicality of using a high temperature. In order to make this study applicable 

 to commercial conditions, mixes having different initial bacterial counts were 

 used. These differences in initial bacterial count were secured by varying the 

 pasteurization time and by using raw materials of varying quality. Both butter 

 and sweet cream mixes were used in this experiment. Skim milk powder was the 

 chief source of serum solids. The mixes were prepared and processed according 

 to standard procedure as given under experimental methods. After homogeniza- 

 tion, the mixes were cooled rapidly to 68° F. and then aged at this temperature 

 in a constant temperature water bath ( + 0.1°). Bacterial platings and pH and 

 titratable acidity determinations were made on the mix before aging and after 



