ICE-CREAM AND ICES 643 



In summer, as a rule, the bacterial count of ice-cream is higher 

 than in winter. The acid-forming organisms are approximately 

 the same in both seasons, but peptonizing bacteria are relatively 

 numerous at all times. Ayers and Johnson found gas-forming 

 bacteria in tV c.c. in 88.83 per cent, of the samples. 



In Chicago in 1913 the writer found, in examinations of 60 

 samples of ice-cream, figures ranging from 150,000 to 60,000,000 

 bacteria per c.c. As a rule, the higher the number of colonies, 

 the greater was the proportion of acid formers. Similar figures 

 have been reported from a number of cities in this country. 



The bacteria in ice-cream are derived from the different sub- 

 stances which enter its composition. Cream and milk are natu- 

 rally the most important sources of these bacteria. Cream and 

 milk are held at refrigerator temperature from the time they are 

 delivered at the ice-cream factory until they are used. This is 

 a relatively short period and the number of bacteria will, there- 

 fore, not increase materially. The sugar contains but a negli- 

 gible number of micro-organisms, but molds are usually preva- 

 lent. Gelatin may contain several thousand to several million 

 bacteria per gram. However, the gelatin is dissolved in water 

 or milk at boiling temperature, so that the majority of bac- 

 teria are destroyed. Some bacteria are contained in flavoring 

 substances. 



Enumeration of bacteria in finished ice-cream has shown that 

 there are many more bacteria present than in the materials from 

 which the ice-cream has been prepared. This can be explained 

 by the breaking up of clumps of bacteria by the violent agitation 

 to which the mixture is exposed during the freezing process. As 

 a matter of fact, the number of bacteria according to the colony 

 count increases somewhat as the freezing process proceeds. This 

 is shown in the following table (Heinemann and Gordon) : 



GERM CONTENT OF ICE-CREAM IN THE FREEZER 



. Can ■ 



Time of freezing. 123456789 Average. 



37 188 279 237 964 818 710 1010 2050 699 



3 minutes 37 229 160 317 896 958 630 lOEO 2270 732 



7 minutes 61 213 116 324 1003 1063 540 1330 2410 785 



12 minutes 47 310 154 408 1137 1092 590 1800 2420 884 



Average 45 235 177 321 1006 983 617 1307 2287 775 



The distribution of bacteria in a can of ice-cream has, as far 

 as the writer is aware, not been thoroughly studied. It would 

 appear that the whipping in the freezer would effect a very even 

 distribution, although, as previously stated, the number con- 

 stantly increases during the freezing process if the colony count 

 is the index. In a series of counts made by Gordon and the 

 writer there was no evidence of an uneven distribution of bacteria 



