EXPERIMENT STATION BULLETINS. 487 



The Oibjeot of istiiclying the influence of cooling upon the niim'ber of 

 b'acteria was to determine wliat influence a chiange from a high tempera- 

 ture G0° C (140° F) to a lower temiperature 10° C (50'° F) would have 

 uipon the number of bacteria in tihe mix. It is a well known fact that all 

 dairy products must be cooled to temperatures below no^mal so as to 

 'keep baicteraal activity at a minimum. Aside from this.wlhat are the 

 adtvantages of cooling? Does cooling per se kill bacteria? 



In 52.17 per cent of tbe samples there was an increase in the number 

 of bacteria after cooling. The increase was from 3.17 per cent to as) high 

 as 400.00 per cent witb an average of 51.45 per cent. In 26.08 per 

 cent of the sannples there was a decrease ranging from 4.76 per cent 

 to as high as 100.00 per cent with an average of 27.29 per cent while 

 21.75 per cent of the samples showed no change. 



These data would indicate that the process of -cooling the mix from a 

 high temperature to a lower one does not necessiarily reduce the number 

 of bacteria as detennined by the plate method. Abundant data, Road- 

 house (13), Marishall (11), Reed and Reynoldis (12), are available, how- 

 ever, to show that cooling is necessary^ to reduce bacterial activity to a 

 minimum. 



THE INFLUENCE OF AGING ON THte BACTERIAL CONTENT OF THE ICE CREAM 



" MIX 



After the ice cream mix had been cooled to 10° C (50° F) it was rum 

 into the holding taid^s where it was held for 24 hours at about 2° C 

 (35.6° F) for aging. The variation in the temperature was slight, not 

 over 2° or 3° C. ' 



Aging, as eveiy ice cream maker knowis, is a very desirahle istep in the 

 process of mlaking ice cream since it influences the viscosity and this in 

 turn, as Mortensen (14) has shown, influences the body and the yield of 

 the finished product. It is, therefore, interesting to study the influence 

 of aging upon the number of bacteria in the mix. The influence of this 

 process on the bacterial count is shown dn Table V. 



Eighteen or 47.36 per cent of the sam,ples in Table V sihowed an in- 

 crease ranging froim 4.35 per cent to as high a:s 409.09 per cent with an 

 average increase of 103.00 percent. The average increase would indicate 

 that the multiplication was considerable, especially siince only three sam- 

 ples show low increases within the limits of experimental error. In 42.10 

 per cent of the cases there was a decrease in bacterial count ranging 

 from 4.54 per cent to as high as 81.81 per cent with an average decrease 

 of 26.98 per cent while 10.52 per cent of the samples showed no change. 



The temperature at which the aging was done, 2° C (35.6° F), would 

 have a tendency to keep down the number of certain bacteria as it is a 

 well known faict that many bacteria found in ice cream grow best only at 

 higher temperatures. If these particular bacteria predominated in the 

 mix, we would expect to have a decrease or ait best uo increase during 

 the period of aging. That there iwas no increase would indicate that 

 these bacteria were either dormant or slowly dying. On the other hand 

 those samples in which there was an increase would indicate that there 

 were present certain types that were capal)le of multiplying at low tern- 



