512 MICROBIOLOGY OF MILK AND MILK PRODUCTS 



All of the fermented milk drinks thus far discussed are similar in that 

 each contains a variety of microorganisms, made up of at least one 

 species of yeast with one or more species of bacteria, capable of produc- 

 ing greater or less amounts of acid. In some, as in the case of kefir, the 

 yeast fermentation is allowed to predominate, while in others, like ya- 

 hourth, the action of the yeasts is held in check by the rapid develop- 

 ment of the acid by the Bact. bulgaricum. All of these drinks are com- 

 monly recommended by physicians because of their beneficial effect 

 upon the digestive tract, 



ARTIFICIAL BUTTERMILK. In recent years there has developed an 

 important industry in the manufacture of artificial buttermilk. This 

 is usually made by inoculating skim-milk with a culture of lactic 

 bacteria, either Bact. lactis acidi, or Bact. bulgaricum or a combination 

 of these two types. In making the artificial buttermilk, yeasts are not 

 commonly added. After the milk becomes coagulated, it is then 

 churned in order to give it a smooth, creamy consistency, after which 

 it may be bottled and kept for some time by holding at low tempera- 

 tures. Sometimes a small percentage of whole milk is added at the time 

 of churning to make the finished product more closely resemble natural 

 buttermilk. In making artificial buttermilk, the skim-milk is fre- 

 quently pasteurized in order to get rid of the miscellaneous flora which 

 it contains. The finished product, therefore, differs from ordinary but- 

 termilk in the fact that it contains nearly pure cultures of the lactic 

 organisms while the natural buttermilk will contain a more or less 

 miscellaneous flora in which the acid organisms predominate. It is 

 possible to obtain a more uniform product in the artificial buttermilk 

 than in the natural product, and this is perhaps responsible for the 

 rapid development of this industry. All of these fermented milk 

 drinks contain enormous numbers of microorganisms, usually millions 

 per c.c. 



ICE CREAM 



Ice cream is one of the important manufactured dairy products and 

 its use seems to be increasing steadily. Its bacterial flora varies with 

 the materials used in its manufacture and the conditions under which it 

 is made. It may be made from fresh cream which is only a few hours 

 old and under good sanitary conditions. On the other hand, it may be 

 made from cream which has been produced and handled under unsani- 

 tary conditions, kept in storage for a number of days and finally manu- 



