196 AGRICULTURAL BACTERIOLOGY 



dried when it is desired to keep it for long periods. The 

 drink contains about 2 per cent, of alcohol and 1 per cent, 

 of acid. It has been introduced into western Europe and 

 America because of its supposed therapeutic value in the 

 treatment of tuberculosis, and for typhoid-fever conva- 

 lescents. An artificial koumiss is sometimes prepared by 

 adding cane-sugar to cow's milk and seeding it with ordi- 

 nary yeast. 



Kefir is another drink prepared from milk by inoculating 

 the milk with Kefir grains, which consist of masses of 

 yeasts and bacteria. The grains are dried and may be 

 bought as articles of commerce. The composition of kefir 

 is similar to that of koumiss, except that the content of 

 alcohol is less. These drinks have been largely supplanted 

 in this country by yoghurt, which is prepared by the use 

 of the Bact. Eulgaricium. 



Manufacture of vinegar. Ethyl or grain alcohol, as it 

 is often called to distinguish it from methyl or wood alco- 

 hol, furnishes a source of food and energy for a class of 

 bacteria that are known as the acetic acid bacteria, since 

 they oxidize alcohol to acetic acid. If cider, wine, or a 

 similar liquid containing alcohol is exposed to the air, it 

 soon becomes covered with a whitish or gray film, or mem- 

 brane, and the alcohol is gradually changed to acetic acid. 

 The term vinegar is applied to the product thus obtained. 

 If the film, which consists of masses of bacteria, is allowed 

 to remain undisturbed, the liquid remains clear. If it is 

 disturbed, the particles of membrane sink to the bottom, 

 and the surface soon becomes covered with a fresh film. 

 The film, which is known as mother of vinegar, may increase 

 in thickness, and finally present the appearance of a 

 leathery mass. The acetic acid formation can go on only 

 under aerobic conditions, since the organisms concerned in 



