72 BACTERIOLOGY FOR NURSES 



point ; hence refrigeration is an excellent preservative. Bacterial 

 activity is thus a reason for the existence of the ice industry and 

 the manufacture of refrigerators. 



Occasionally bacteria are intentionally allowed to decompose 

 food up to a certain limit. The so-called gamy flavor of meat 

 is due to the first stage of decomposition. Sauerkraut is another 

 example of food expressly allowed to ferment. The special flavors 

 produced by bacteria in the preparation of butter and cheese are 

 other instances. 



Vinegar Making. The first step in the process of vinegar mak- 

 ing is brought about by yeast cells. By their ability to ferment 

 grape sugar they produce alcohol. The next stage is accomplished 

 by bacteria which cause the alcohol to unite with oxygen, thus 

 producing acetic acid or vinegar. Oxidation of alcohol into vine- 

 gar can be brought about by a chemical process, but it is impracti- 

 cable on a large scale. 



One of the usual methods employed is to add to a weak solution 

 of cider a small quantity of vinegar. After a short time a thick, 

 felted scum forms on the surface of the alcohol. The scum is a 

 mass of bacteria spoken of as the "mother of vinegar," which in 

 some way causes the oxygen of the air to unite with the alcohol. 

 After the amount of acetic acid reaches a certain percentage 

 bacterial action stops and no more acid is produced, even though 

 there be alcohol remaining. It was at first thought that only 

 one species of bacteria was able to produce this fermentation. 

 Later study has shown that several different kinds have the power ; 

 most of them have a common characteristic in that they grow in 

 long filaments without any trace of division. 



A rapid method of vinegar manufacture is carried on by filling 

 high cylinders about three fourths full with wood shavings which 

 have been soaked in warm vinegar. Weak alcohol is then poured 

 in, and as it slowly passes over the shavings it is oxidized into 

 acetic acid. 



Occasionally the fermentation does not proceed in a satisfactory 

 manner ; other species find their way into the fermenting liquid 

 and produce undesirable substances which give it a totally dif- 



