COMMON COMPOUNDS OF OTHER ELEMENTS 273 



FERMENTED APPLE JUICE 



poisonous compounds of copper may be formed. The 

 most common acids are sulphuric, hydrochloric, and 

 nitric. These are called inorganic or mineral acids, and 

 are manufactured in large quantities for commercial uses. 

 Sulphuric acid is quite cheap and is used in the manu- 

 facture of other acids. Hydrochloric acid contains 

 hydrogen and chlorine and is made by the action of sul- 

 phuric acid on salt (sodium chloride) . 



Another large class of acids is formed in the juices of 

 fruit and vegetables. Among these are acetic acid, 

 citric acid, and tartaric acid. 

 (All of these acids contain 

 carbon, hydrogen, and oxygen.) 

 They are called organic acids. 



When sweet fruit juices fer- 

 ment, their sugar is changed 

 to alcohol and carbon dioxide 

 by the yeast plants. If the 

 alcohol thus formed comes into 

 contact with air, it changes to 

 vinegar. While this change 



from alcohol to vinegar is going on the liquid contains 

 numerous small organisms called acetic acid bacteria. The 

 large numbers of these bacteria form the characteristic 

 mold known as " mother of vinegar." The sour taste of 

 vinegar is due to the acetic acid present. In the vinegar 

 sold for table purposes the amount of acetic acid is usually 

 not more than four or five per cent. In the formation of 

 vinegar the alcohol is really oxidized to acetic acid. This 

 action is usually slow, but may be hastened by the quick 

 vinegar process. This is an arrangement whereby fer- 

 mented apple juice is allowed to drip slowly through beech- 

 wood shavings which have been inoculated with acetic acid 



FIG. 237. 



